capacitivetouchsensetechnologysilabs

8/8/2019 CapacitiveTouchSenseTechnologySiLabs

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Capacitive Sensing Solutions from

Silicon Labs

In this section, we are going to cover the Silicon Labs Capacitive Sensing solutions.


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Agenda

What are we measuring?

Capacitance measurement techniques

Relaxation oscillator operation

SAR operation

CapSense system considerations

C8051F93x-92x implementation

C8051F700 implementation

Tools to aid in capacitive sensing development

Where to learn more

We are going to cover Capacitive Sensing technology starting with an overview of

how CapSense works. Next we will cover how the technology is implemented in

the C8051F900 family and the different pad topologies. We will also cover some of

the tools available for the technology.


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What Are We Measuring?

Capacitance If two metal plates are placed with a gap between them and a voltage is applied to one of

the plates, an electric field will exist between the plates

Any two conductive objects can be used for the plates of the capacitor

When using capacitive sensing for touch applications the capacitor is typically formed bya PCB element and the conductive object in proximity

Capacitor parameters Area capacitance is directly proportional to the area of the plates

Dielectric capacitance is directly proportional to the relative permittivity of the materialbetween the plates (air, glass, plastic, etc.)

Distance capacitance is indirectly proportional to the distance between the two plates

The capacitance of a parallel-plate capacitor is given by:

where:

0 is the permittivity of free spacer is the relative permittivity of the dielectric material

A is the area of the plates

D is the distance between the plates

dDielectric

Capacitor

plates

A

V

When using capacitive sensing for touch applications the PCB trace typically acts as

one plate of the capacitor. When a conductive object (such as a finger for touch

sensing) comes into proximity it acts as the other plate and air is the dielectric. As

the object moves closer to the pad the capacitance changes based on the equation

for capacitance shown. The constants in the equation represent the value for the

dielectric constant of the material, i.e. air, glass etc. The value for A is a measure ofthe area of the pad used for the sensor capacitor and d represents the distance

between the two plates. So you can see, for touch applications, as the finger moves

closer to a pad the value for d is reduced which then increases the capacitance.

ris the relative static permittivity (sometimes called the dielectric constant) of the

material between the plates

A is the area of each plate

D is the separation between the plates.


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Capacitive Sensing Technology

What are capacitive sensing switches?

They are external capacitors that are influenced bythe presence of conductive objects such as a finger

They are also known as contact-less switches

Why are they useful?

Make it possible to revolutionize a products userinterface and industrial design

Low-cost because the switch is as simple as a traceon a PCB

No moving components means long-term reliability

Target applications

Handsets

Notebooks

Personal media players

White goods

Automotive Medical

Industrial

Touch sensitive switches

Traditional push-button switches

Touch sensitive switches are found in a variety of consumer products including home

appliances, MP3 players and cell phones. A touch sensitive switch is a switch that is

implemented as a trace on a printed circuit board. The architecture of the trace creates a

capacitive element. Touching this trace with a finger creates a change in capacitance, which

is detectable using a variety of techniques. Capacitive sensing has received a lot of attention

lately due to the fact that it can replace mechanical switches that have a tendency to wearout and break. Capacitive Sensing switches can be as simple as a trace on a PCB that forms

one plate of a capacitor and an object that is in close proximity to the pad represents the

other plate. With respect to human interface it is often a persons finger that becomes the

second plate of the capacitor. When a persons finger moves closer to the pad the

capacitance changes inversely with the distance. As the distance between the finger and the

pad decreases the capacitance increases. We can use several measurement techniques to

quantify the capacitance and provide a pressed or not pressed button state.

There are many applications for this technology, not just switches although that is probably

the most common. Industrial Applications can take advantage of capacitive sensing. Thereare many industrial applications where detection can be done using a capacitive sensor, such

as detection of the water level in a tank.


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The Relaxation Oscillator (RO)

How a relaxation oscillator works Charge a capacitor until it reaches a certain voltage threshold

Once the threshold is reached, discharge the capacitor until itreaches a lower threshold

The frequency of the relaxation oscillator is indirectly proportional tothe value of the capacitor

C > Inactive Capacitance

C = Inactive Capacitance

Here we will take a closer look at the relaxation oscillator method. As mentioned on the

previous slide, the relaxation oscillator method builds an oscillator which uses the sensor

capacitor as a timing element. The diagram shows the relationship of the comparator

reference voltages and the output waveform. The reference voltages used for the

comparisons in this example are the 2/3 and 1/3 VDD points. The exponential voltage rise

occurs from the 1/3VDD point and continues to the 2/3VDD point. At that point thecomparator output toggles and causes a change in current flow. This change causes the

voltage to decay back down to the 1/3VDD point and then the cycle repeats. We will take a

look at this cycle in more detail in some of the following slides. Using this method the

oscillation period is inversely proportional to the sensor capacitor value. As capacitance

increases the oscillator frequency decreases.


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Charge Timing Using SAR

How the successive approximation technique works Drive a current through the sensor capacitor and compare the

generated voltage potential to the voltage of a reference capacitorwith a known ramp rate

Successive comparisons modify the current value in order toprovide equivalent ramp rates for the ISENSOR and the IREF

Silicon Labs patent pending approach

ISENSORIREF

MSB Bit Decision

(Bit 15)

LSB Bit Decision

(Bit 0)

V

t

The SAR implementation is based on the principle of capacitance that as you source a

constant current then the voltage output will increase over time. This is based on the

equation dV = I/C dt. The comparison point of the generated voltage is set to a constant

voltage over all cycles. This constant voltage is also used as the reference point for the

charge time of a reference capacitor. When all bit values have been determined the ramp

rate of the sensor capacitor will be equal to that of the reference capacitor.


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Relaxation Oscillator (RO)

Implementation


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RO Capacitive Sensing Implementation (1 of 3)

When the positive input is configured to generate the

threshold voltage levels, the comparator output is 1 when

the positive input terminal (CP+) is greater than the negativeinput terminal (CP-) and 0 when CP- is greater than CP+

The next several slides show how the relaxation oscillator actually works. The slides

describe the inactive state behavior of the relaxation oscillator. By inactive state we mean

that a finger is not moved on or off the switch, and so the capacitance of the switch is the

same.

In this configuration, the output of the comparator is 1 when the positive input is greater

than the negative input. In the initial state, we can assume the capacitor is at 0V or less than

2/3 VDD. The comparator output is then logic 1 or VDD. This has two effects: 1) the

resistor network gets simplified to what is shown on the right and this sets the threshold to

2/3 VDD, which is shown in blue in the graph. 2) The switch capacitor is charged by the

output of the comparator through the feedback resistor. This is shown by the red line in the

graph.


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CP+ > CP-, and 0 when CP- > CP+

Once the voltage on the capacitor gets charged to 2/3 VDD, the output comparator becomes

logic 0, which has two effects. 1) The resistor network gets simplified to what is shown on

the top right and the positive input is changed to 1/3 VDD. 2) The capacitor, which is

charged to 2/3 VDD, now starts discharging through the feedback resistor.


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CP+ > CP-, and 0 when CP- > CP+

The final step of the cycle is the same as the first. Once the voltage on the switch capacitor

reaches 1/3 VDD, the output of the comparator switches to logic 1 or VDD, which starts the

charge process again. This repetitive scenario gives us our oscillation frequency based on

the value of the RC time constant of the feedback resistor and the sensor capacitor we are

trying to measure.


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Putting Your Finger on the Switch

By touching the switch directly with a finger the capacitance increases

The rise and fall time of a parallel-plate capacitor is given by:

The additional capacitance increases the time constant, thereby

reducing the output frequency

RC

When using Capacitive Sensing the PCB trace acts as one plate of the capacitor.

When a finger comes into proximity it acts as the other plate and air is the dielectric.

As the finger moves closer to the pad the capacitance changes based on the equation

for capacitance shown. The constants in the equation represent the value for the

dielectric constant of the material, i.e. air, glass etc. The value for A is a measure of

the area of the pad used for the sensor capacitor and d represents the distancebetween the two plates. So you can see as the finger moves closer the value for d is

reduced which then increases the capacitance. Keeping in mind the relaxation

oscillator operation from the previous slides we noted that as capacitance increases

the frequency of oscillation decreases.

ris the relative static permittivity (sometimes called the dielectric constant) of the

material between the plates

A is the area of each plate

D is the separation between the plates.


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How to Detect the Change

There are two ways to measure the change in frequency

Measure the frequency

Count the number of relaxation oscillator cycles over a fixed period of time

Measure the period

Count the number of system clock cycles over a fixed number relaxation cycles

The example code installed with the IDE counts the system clock cycles during one

relaxation oscillator cycleInactive State

Finger On the Pad

There are two ways to measure the change in the relaxation oscillator frequency. There is a calibration step that needs to be

performed first in order to measure a count value with no finger present on the pad. This would be considered the idle, steady

or inactive state. The first measurement technique uses two timers. The first timer is clocked by the relaxation oscillator (RO

timer) and the second is used to generate the fixed time base (Fixed Timer). At the overflow of the Fixed Timer the value of

the RO Timer value is read. If the RO Timer value is less than the inactive state value from a calibration then it is determined

that there is a valid button press. When measuring theperiod we use a timer that is clocked by the system clock (System

Timer). The comparator output in this case is used to provide a capture event. The captured value is compared to a calibrated

value and if the system clock counts are higher then the capacitance is greater causing the capture period to be longer, thus

allowing more system clocks. When this condition exists it is considered to be a valid button press. With the relaxation

oscillator solution, this is the minimum amount of cycles we can count and still get an accurate results. We cannot use a

sampling period shorter than 1 relaxation oscillator cycle and achieve repeatable measurements.


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Charge Timing Implementation


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Successive Approximation (CS Module)

Two current DACs used

First is the variable DAC for the current through the sensor capacitor

The second is a constant current source for the internal referencecapacitor

Two comparators are used

First monitors the voltage of the sensor capacitor

The second monitors the voltage of the internal reference capacitor

The outputs of the comparators determine the logic level of the

associated bit

ISENSORIREF

CSENSOR

VREF SARCREF

The successive approximation (SAR) method uses a pair of current sources to drive current

into separate capacitors. The first is the reference capacitor denoted as CREF and the other is

CSENSOR. The current source denoted as IREF is a fixed current used to drive the CREFcapacitor to provide a constant ramp voltage. The second current source denoted ISENSORsources a variable current to the sensor capacitor. After each measurement cycle the SAR

logic will change the value of ISENSOR, attempting to get the ramp rate the same as thereference capacitor.


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Putting Your Finger on the Switch

By touching the switch directly with a finger the capacitance increases

The change in voltage of a parallel-plate sensor capacitor is derived from the

current of the capacitor given by:

This additional capacitance reduces the voltage potential of the sensor capacitor

at a given current

Inactive Switch

Active Switch

Charge Time

dtC

IdV

SENSOR

SENSOR

dt

dVCI SENSORSENSOR

or

As with the relaxation oscillator, when using Capacitive Sensing the PCB trace acts

as one plate of the capacitor. When a finger comes into proximity it acts as the

other plate and air is the dielectric. As the finger moves closer to the pad the

capacitance changes based on the equation for capacitance from the earlier slides.

Keeping in mind that as a finger moves closer the value for d is reduced which then

increases the capacitance. Keeping in mind the SAR operation from the previousslides we note that as capacitance increases the comparison point for the threshold

voltage decreases.


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Successive Approximation

Decision tree based on binary weighted voltages. Start the first comparison at the mid-scale voltage IREF/2 (for 16 bit

conversion IREF=0x8000)

Determine MSB value based on the timing of the capacitors VSENSOR > VREF then MSB = 0

VSENSOR < VREF then MSB = 1

Set the next comparison voltage to the next comparison value If the bit was a 1 IREF=0xC000 If the bit was a 0 IREF=0x4000

Continue process through all 16 bits

ISENSOR

IREF

Bit 15 = 0

IREF Charge Time

Current value for

ISENSOR = 0x8000 Current value for

ISENSOR = 0x4000

Bit 14 = 1

Current value for

ISENSOR= 0x6000

Bit 13 = 0Bit 0 = 0

Total Sample Time < 50us

Lets take a closer look at the successive approximation steps in determining the value of

the ISENSOR source. The SAR will set the current source to the mid-scale and drive the

ramp until the timeout period expires as governed by the reference capacitor charge time.

Mid-scale for the 16 bit current source is 0x8000. At this point the voltages are compared.

If the sensor capacitor has charged to a value that is greater than the reference as shown in

the figure then there was too much current and the MSB value is then set as a zero(conversely, if the current did not reach the threshold then that means there wasnt enough

current and the MSB is then set to a one). Since the previous bit was set low the next

weighted bit is set to the a value lower and half of the setting or at 0x4000. The charge

cycle is started again and the voltages are once again compared. In the example above we

see that with this new value the voltage didnt make it to the threshold point meaning we

need to supply more current to the sensor capacitor. This bit is then set as a one and the

next value is again the halfway point to the mid-scale voltage or 0x6000. This process

continues all the way to the last bit at which time the ramp rates of the two capacitors are

equal.


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CS Module Advantages

No external components required

Differential measurements reduce noise susceptibility

SAR sampling immune to DC offsets and low frequency noise DC offset voltages have no effect on bit determination algorithm

Fast conversion time Low frequency coupled AC noise susceptibility reduced (50/60 Hz)

High frequency susceptibility reduced by design

Can be configured for autonomous operation

ISENSORIREF

CSENSOR

VREF

SARCREF

The SAR implementation eliminates the need to provide any external components

and can monitor capacitance just by connecting to the pad of the MCU. All of the

components necessary to implement CapSense are integrated on chip. Another

benefit is the fact that the implementation uses a differential measurement for the

sensor capacitor and the reference capacitor thus providing very high rejection to

power supply noise. The SAR sampling method is not affected by DC offsets as therelative change in the voltage and the time period for that change is dependent on

the capacitance. Also, the sampling time is very short minimizing the affects of low

frequency noise. The design provides filtering for any high frequency interference

that may cause inadvertent switch actuation. Since the peripheral is self timed it can

run autonomously from the CPU which means the CPU can be in a low power state

and be woken up by a switch actuation event.


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Designing Touch Sensitive Switches

Since we are detecting a change in capacitance,

it follows that we want to maximize the change as

much as possible

On the PCB there are two main factors that affect the switch

capacitance and how much it changes

Size, shape and placement of the switch pattern on the PCB

The characteristics of the trace that connects the switch to the MCU

Now that we understand how Capacitive Sensing works, lets take a look at the

system implementation. Our main goal in designing CapSense switches is to

maximize the change in capacitance between the inactive state and a finger press.

In order to accomplish our goal we have to keep in mind the equation for

capacitance that we discussed earlier and is shown here for reference. When

looking at this we see that the areas we can control are the pad dimensions whichrelate to the quantity A in the equation. This directly relates to the size and shape of

the switch pattern. We also control d which is the distance from the pad of our

assembly and the dielectric constant relating to the material we use to cover the pad.

One thing that tends to get overlooked is the characteristics of the trace that

connects the switch to the MCU. Being connected to the switch pad, all capacitance

we add in the form of trace capacitance increases our inactive state capacitance.


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Initial Configuration

Performed for each switch individually Even if all the switches are the same size and shape, their location on

the board will affect their inactive state capacitance

Performed during development or production test The results should be programmed to Flash

Three step procedure1. Measure the sensing method output with nothing on the switch (inactive state count)

2. Measure the sensing method output with the switch active (active state count)

3. Set the threshold to a value between the two measurements

The threshold value determines the sensitivity

Sensitiv

ity

Least

MostInactive State Counts

Active State Counts

Single count threshold

Inactive to Active Count

Active to Inactive Count

The purpose of the initial configuration is to determine the operating parameters of

the switches. These values can be used as reference points for subsequent runtime

operation of the switch. Measuring the inactive and the active state of the switches

provides us with the information necessary to generate initial threshold values for

each of the switches. One element of setting the threshold levels shown in the

diagram is hysteresis. We can separate the inactive to active state threshold to onecount value and the switch active to inactive state to a lower value. When we are in

the inactive state we compare the difference to the upper threshold. When the

button has been considered active, a release of the button would occur after the

lower threshold is crossed. An important note with these thresholding techniques is

that the closer to the inactive state we set the compare value the more sensitive the

switch as it would take less capacitance to cause the switch to be active the lower

we set the threshold value. See application note AN367 for threshold

recommendations as well as the capacitive sensing API for firmware

implementations.


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Traditional System Signal to Noise

SNR is affected by different system level parameters

Capacitive sensing pad layout

Proximity to other conductive objects

Ground planes Dielectric material and thickness

Calculating SNR traditional method

1

0

)(1 n

tBn

B

1

0

)(1 n

tAn

A

BtBNS ))(max(

1. Calculate the mean for the inactive and the active state

2. Calculate the peak noise value for the inactive state.

3. Determine the number of counts for the signal

BAS

4. Divide the signal (S) by the noise (NS)

SN

SSNR

External stimuli

Electrical interference

Environmental affects like temperature

and contaminants on the switch

There are many system level considerations when implementing Capacitive sensing.

The responsiveness of a switch in the application is dependent on the pad size, the

dielectric material type and thickness as well as the layout and how the PCB is

oriented and what is in proximity to the pad. Once all of these factors are in place,

only characterization of the switch can really provide the information required for

the firmware to make the determination as to whether or not the switch is in theinactive state or the active state. In order to provide adequate margin between the

two states all of the design considerations mentioned must be structured such that

the maximum signal to noise ratio can be realized. This is required so suitable

threshold levels can be identified. This slide explains the traditional signaling

details required to characterize the system. In literature you can find that a suitable

signal to noise ratio for Capacitive sensing for touch applications is 5 to 1. The

equations listed above outline the traditional method to calculate the system level

signal to noise ration of the switches.


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Can be conservative based on increased SNR capability of the C to D converter

Calculate the inactive and active baselines

Calculate the standard deviation of the noise for the inactive and active state and

determine the safe zone

Determine the thresholds of the signal

System Characterization (1 of 3)

1

0)(

1 n

BB tIn

I

1

0)(

1 n

BB tAnA

BI

BABItoA xIASafeZone 3.3

IBItoA IxSafeZoneActiveInactiveTo 3.375.0

IBABAtoI IxASafeZone 3.3

AtoIAB xSafeZoneAactiveActiveToIn 75.03.3

With the traditional approach to switch characterization no consideration is given to

the noise in the active state. In reality, the human body is a good antenna and

injects quite a bit of noise into the system. It makes sense that the noise level of the

active state should be considered in setting the appropriate threshold values.

Additionally, the traditional method only uses peak values of noise in the

calculation of SNR. Here is a different approach where the noise level of both theinactive and active state are used to determine the threshold levels. We break this

up into two different conditions, the first is when going from the inactive to the

active state and the second is from the active to the inactive state. In order to

calculate thresholds we first calculate the mean of both the inactive and active

states. Next, a safe zone is determined which takes into account 3.3 standard

deviations about the mean of the current state and 1 standard deviation about the

mean of the next state. For example, when going from the inactive state to the

active state we calculate 3.3 standard deviations about the inactive state mean and 1

standard deviation about the active state mean. Combining these values with the

means determines the safe zone. As a first approximation the thresholds are set at

75% of these values as shown in the last set of equations.


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Initial Configuration

Even if all the switches are the same size and shape, their location on the

board will affect their inactive and active state capacitance

Inactive State noise

3.3

Active State noise

1

Inactive State MEAN

SafeZoneItoA

Active State MEAN

75% * SafeZoneItoA

Inactive to Active Threshold Setting

Here we see the preceding equations represented graphically for the inactive to

active state transition. From the graph you can see that the standard deviations for

both steady state values are used for determining the safe zone. The threshold is set

to 75% delineated by the green hash mark.


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Store the operating parameters to the flash

The threshold values determine the sensitivity

Inactive State noise

1

Active State noise3.3

Inactive State MEAN

SafeZoneItoA

Active State MEAN

75% * SafeZoneAtoI

Active to Inactive Threshold Setting

Here we see the preceding equations represented graphically for the active to

inactive state transition. This graph differs from the one on the preceding slide in

that the active state noise being considered is now 3.3 standard deviations about the

mean instead of 1 standard deviation when going in the reverse direction. The

threshold is set to 75% of the safe zone this time delineated by the red hash mark.


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Periodic Configuration

Perform similar procedure as initial configuration during normal operation toaccount for a dynamic operating environment Some relevant factors are:

Water, oil, or other materials on the switch

Changes in supply voltage Change in humidity and temperature

These factors can affect the capacitance of the switch or the charge/dischargeprofile If the effects are large enough, the incorrect measurements can lead to false switch

events

Once a system is deployed, there may be environmental effects that change the characteristics of the

switch. An example would be a touch panel outside or an industrial environment where some oil or

other material can get on the switch. When it rains the water would change the properties by altering

the dielectric constant thereby changing the capacitance we would measure. Another effect could be

as simple as differences in finger sizes. To compensate for these problems, periodic configuration (or

baselining) compares runtime baseline values to calibrated reference baseline values and scales the

thresholds appropriately. The inactive baselines are a way to determine the offset between inactivecapacitance measured during calibration and inactive capacitance measured at runtime. The active

baselines are a way to determine how much the channel's gain has changed. Without periodic

configuration, calibrated active and inactive threshold values can become invalid as conditions

around the device change.

Determining the threshold values is an important part of the Capacitive sensing system. Following

the guidelines ensures reliable switch actuation when the system is deployed. All of these

measurements should be tailored to the system. For example, the value of the noise should be

determined in the environment that the system will be used. The active region should be set to a

value that takes into account the distance of the finger to the pad due to dielectric gap as well as

possible offset positions (which would reduce the output). These values are generated during the

development phase of the project and can be stored in flash. The sensitivity margins provide thecapability to enhance the detection algorithms in order to adjust for environmental affects such as

temperature drift. The inactive measurement can be continually updated as long as it is below the

noise threshold. Keep in mind this is only a recommendation for standard switch implementations.

There are many applications that may require deviating from these guidelines.


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Different Switch Arrangements

Solid pads

Square pads and round pads

Round pads have uniform field lines around the pad and are preferred

Have similar sensitivity

Highest sensitivity

Interleaved pads

Reduces pad area and increases parasitic capacitance

Not recommended

Effects of grounding

Grounding around switches reduces sensitivity by absorbing some of the field lines

The smaller the gap the more parasitic capacitance

The gap should be sized according to the thickness of the dielectric material

Necessary if no ground reference to the object in proximity

Hatched ground planes minimize desensitizing parasitic capacitance while providing

good noise reduction

Now lets put what we learned to use. There are different topologies to consider

when generating the Capacitive Sensing pads. When designing the pads we have to

keep in mind that the field currents are going to try and find their way to ground

whether it is a ground connection or a virtual ground represented by a finger. The

most common type of pad is the solid circular pad. This provides the highest

sensitivity because of the area based on our equation for capacitance. Without anygrounding, all of the field lines would be directed from the positive pad to the lower

potential represented by the finger. There are also designs that interleave pads.

These methods reduce the flux density to the finger because of reduced area and

parasitic capacitance, thereby reducing sensitivity, and are not recommended.

Another topology is to surround the pad with a ground or guard. The size of the gap

between the pad and the ground changes the field lines which affect the parasitic

capacitance. This shows up as a smaller net change in capacitance.


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Effects of the Switch Size and Shape

The inactive state capacitance (in pF) for each switch

without any material on top is provided below

It is easy to see that the bigger the switch and the more

traces there are within a certain area, the more inactive

state capacitance the switch has

Both of these factors make the switch more robust

1.91.12.24

3.12.53.93

6.44.88.42

6.44.88.41

CBApF

A B C

1

2

3

4

Lets take a look at some of these using real examples. The table shows

measurements taken from the pad topologies shown. The values in the table are

marked according to the letter and number designations from the figure. For

example, A4 is the largest circular pad with a small gap to the ground ring. The

values represented in the table show the inactive state capacitance (capacitance with

no finger present). Since we are trying to measure the relative change incapacitance we would like to start with a low inactive state capacitance which

provides greater dynamic range. You can see the coupling of the interleaved pads

increases the inactive state capacitance and the solid pad enables us to get a lower

inactive state capacitance.


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Effects of Trace Length

In order to achieve reliable switch measurements,

the percentage change in capacitance between

an inactive switch and an active switch should be atleast 0.5%

The trace that connects the switch to the MCU input pin

should be as short as possible to minimize the total parasitic

capacitance added to the switch

With lower parasitic capacitances, any change in the switch

capacitance is more significant and more easily detected

Parasitic capacitance in the PCB design affects the measurement capability. When

we talk about parasitic capacitance we are referring to the voltage potential that

exists along the length of the trace, therefore, longer traces have a higher

susceptibility to parasitic capacitance. This is important in reaching our goal of

increasing the relative change in capacitance when using this method for switches

and is often an area overlooked. The value of trace capacitance adds to the overallinactive state capacitance of the Capacitive Sensing switch. In order to reduce the

effects of trace capacitance shorter, narrower traces, which exhibit lower overall

capacitance, are recommended. In addition to the inactive capacitance value of the

trace, its proximity to noisy signals and grounds affect the overall performance of

the system. Long traces are no big deal unless they are surrounded by conductive

objects (like traces or grounds) at a different voltage potential (parasitic) or, and just

as important, neighbored by other conductive objects with changing voltage

potential (noise), such as traces from other nearby capacitive sensors. Why are

these factors important? It is desirable to have at least a 0.5% change in the

capacitance between the inactive state and the active state. For example, if the total

capacitance of the switch and parasitic capacitances is 200 pF, the change in

capacitance when you put your finger on the switch should be at least 1 pF.

Overall, we want to reduce parasitic capacitance on the trace and at the sensor to

maximize the sensitivity.


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Switch ArrangementsSize and Shields

Switch size

Same size as the expected conductive object recommended

Shields

Plane layer at the same voltage potential as the pad

Introduces no parasitic capacitance

Increase sensitivity by forcing all field lines to the approaching

conductive object

Shield

No voltage potential exists

Pad area larger than needed

Pad

Conductive

object

When designing the Capacitive sensing pad the overall dimensions of the object you

are trying to sense should be considered. For example, the conductive object shown

in the slide is much smaller than the Capacitive sensing pad shown. With the

conductive object of this size we would have unnecessarily oversized the pad. The

same guidelines hold true when the conductive object is the human finger. Another

aspect of switch design is the concept of shielding. In most instances we arefamiliar with using grounds as shields in order to block unwanted noise. However,

when we are driving the pad capacitance with our excitation source (like the

relaxation oscillator) and if we add a ground under the pad we will induce a voltage

potential difference and thus a parasitic capacitance. There will be field lines from

our pad that are attracted to the lower voltage potential. But what happens if we use

a plane that is at the exact same potential as the pad we are exciting as shown in the

figure? Since we now have the same voltage potential between the two plates there

is no capacitance. All field lines that are generated from the pad are directed to the

conductive object we place near the pad while blocking the unwanted parasitic

capacitance.


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Effects of Different Materials for the Dielectric

The type of material on top of the switch affects the inactive state

capacitance and the active change in capacitance

The materials covered in the application note AN338 are:

Glass (3.2, and 5.9 mm)

Plexiglass (1.6, 5.0, and 9.8 mm)

Mylar (0.35 and 0.70 mm)

ABS plastic (2.0 and 4.0 mm)

FR4 (1.6 mm)

Recommendations

Use the thinnest material possible to maximize the change in capacitance

Use materials with a higher dielectric, such as glass, to increase the

absolute capacitance of the switch

This increases the relative change in capacitance when compared to the trace

and other parasitic capacitances

We discussed different areas that we can control in the design of the pads in order to

increase the sensitivity and robustness of the switch. The first area we covered was

the pad area and we saw that larger solid pads increased our sensitivity. The next

area we can control is the dielectric material and the distance d from the pad. Most

Capacitive Sensing pads have an overlay of some sort, like glass. The thicker the

glass the farther away from the pad we are for the touch. Remember thatcapacitance goes down with an increase in distance so the material we place on top

of the switch should be as thin as possible and still meet the application

requirements. Also, the material chosen influences the capacitance via its dielectric

constant. Using materials with a high dielectric constant will increase the

sensitivity. Here are some materials that were tested in application note AN338

from Silicon Labs. A link to the appnote can be found at the end of this

presentation.


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Design Considerations Summary

The circular switches have the lowest inactive

capacitance and also exhibit the largest relative

change in capacitance when the switch is active

Use the thinnest possible material with the highest dielectric constant

Ensure that there is no gap between the board and the covering material

Application notes are available to provide more detailed design

guidelines including switch spacing and trace routing

The advantage of having the lowest inactive capacitance is that it takes a shorter

amount of time to measure one period when considering the relaxation oscillator

method. The SAR technique conversion time is constant regardless of the value of

the capacitance. This provides a consistent measurement from pad to pad. The

advantage of having the largest change in capacitance is that it is easier to detect.

The solid circular pad met both design goals, lower inactive capacitance and a highsensitivity. Using the thinnest possible material to meet the design goals enables the

largest change in capacitance to be detected and therefore makes the design more

robust. By maximizing the change in capacitance that can be detected the threshold

and calibration algorithms can potentially be simplified reducing code space. When

adding the material over the switches it is important to make sure there is no air gap.

Air has a low dielectric constant compared to materials such as glass. Ensuring that

there is no gap will ensure that the highest change possible will occur when the

switch is active. Certain adhesives from 3M (such as 467 or 468) provide good

bonding of the dielectric material while providing a uniform area and high dielectric

constant. See application note AN338 for more information regarding capacitive

sensing and measurement results of different materials.


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C8051F93x Implementation


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Capacitive Sensing RO Implementation

How this is implemented on the C8051F93x-92x

devices

The relaxation oscillator is implemented using an on-chipcomparator

The switch is the capacitor that is charged and discharged

Measure the output frequency using a capture mode of the timer

Lets take a look at how to implement the Capacitance Sensing using the C8051F9xx family

from Silicon Labs. The relaxation oscillator itself is implemented using only the

comparator. Remember from our earlier discussion about how the relaxation oscillator

works. We have the resistor network used to set the comparator threshold and the feedback

resistor to provide the current path to charge and discharge the capacitor. The comparators

on the C8051F93x/92x family of devices include the charging mechanism for the switchcapacitors that are connected to the comparator multiplexer as well as automatically

configuring the varying voltage threshold on chip. All that is required is to connect the pad

to the pin of the MCU. The comparator output also can be used as the capture source for

the timer.


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C8051F93x/92x Comparator Block Diagram

Comparator block diagram Comparator configured

to monitor one touch

sensitive switch

The left side shows a simplified block diagram of the comparator peripheral when

configured for Capacitive Sensing mode. The positive input multiplexer is

configured to set the voltage threshold. The negative input multiplexer can select

between the different GPIO pins that are connected to touch sensitive switches. The

output of the comparator (CP OUT) has three functions. The first is to help set the

voltage threshold, which is the resistor network in the top left. The second is tocharge and discharge the capacitor through the Rfeedback resistor. The third

function is the the output frequency of the relaxation oscillator.

The simplified diagram on the left shows one configuration where the positive input

sets the threshold and the negative input is connected to the switches. The

comparator multiplexers can switch roles and the positive input can be connected to

the switches and the negative input can set the threshold. The positive input

multiplexer can connect to 12 different GPIO pins and the negative input

multiplexer can connect to 11 different GPIO pins for a total of 23 possible inputs.

The right side shows the block diagram simplified even further to show the

configuration for one touch sensitive switch.


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RO Capacitive Sensing with Other MCUs

The capacitive sensing solution can be implemented on otherSilicon Labs MCU families using only passive components

The only extra requirements compared to the C8051F93x/F92x solutionare the (3 + N) resistors, where N is the number of switches and 3 extraport pins for the feedback

Silicon Labs MCU families other than the C8051F93x/92x family caninterface up to 12 switches directly or more with an externalanalog multiplexer

External ResistorsNo External Components

The C8051F9xx family integrates all of the components required to implement

Capacitance sensing. However, all of the active components are available in other

Silicon Labs MCUs so this technology can be used with those as well. The only

requirement is that the passive components are needed external to the device.


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Adding Multiple Switches

Adding multiple capacitive switches only requires

switching the comparator multiplexer input

The active switch on the multiplexer will start the charge-

discharge cycle as soon as the multiplexer is switched over This enables rapid measurement of all switches

There is an input multiplexor to the comparator that enables multiple switches to be

used. All that is required is to switch the register value to enable the next pad and

we can scan all of the switches. The comparator immediately starts the

charge/discharge cycle once the multiplexor changes state.


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The Comparator Module

Here is a diagram of the actual implementation in the C8051F9xx family. Notice

that the input multiplexors exist for both the positive and negative inputs to the

comparator and that there are up to 23 pads that can be connected to the inputs. All

of the resistors are integrated on chip as well so that the Capacitive Sensing pad is

the only required external connection.


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The Timer Module

Comparator 0 is used as the capture event for the timer module.

Allows for measuring the period of the relaxation oscillator using the systemclock (SYSCLK)

Here is a diagram of the actual timer implementation in the C8051F9xx family.

Notice that the comparator output is used to capture the number of system clock

counts at the end of each relaxation oscillator period. Earlier we discussed how this

measurement technique is accomplished. The value read from the timer capture

register (TMR2RLL/TMR2RLH) is the value we read and subtract our inactive

measurement. This difference calculated is the representation of the period of therelaxation oscillator.


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Benefits Of The Implementation

Low MCU overhead

443 bytes code space for one switch and only one additional byte foreach additional switch

Only requires one comparator and one timer Efficient algorithm allows the MCU to go into low-power mode and wake up

periodically to detect a switch event

Less than 0.05% CPU utilization

No external hardware overhead

Directly connect the switch traces to the MCU port pins

No other external feedback resistors or capacitors are needed

Simple configuration

Easy to perform with any material on the switch

Insensitive to noise and supply voltage Insensitive to 50/60 Hz noise

Does not require precise source voltage (VDD)

What are some of the benefits of this implementation?

Low MCU overhead

1) Most of the code space is used for initializing the peripherals and performing the

configuration. Adding additional switches is trivial because all that has to be done is to

increase the array size to store the mux settings and the configuration threshold.2) Typically requires only 1 comparator and 1 timer. A second timer can be used to

schedule the measurements, but this is not necessary and the function can just be called as

often as possible or whenever the firmware requires it.

3) While the MCU is not measuring the switches, it can go into sleep mode.

4) All circuitry required to implement the excitation to the pad is integrated on chip. This

enable a direct connection to the pads from the MCU.

Simple Configuration

All configurable parameters are obtained the same whether the dielectric is plastic, glass,just the PCB trace alone, or anything else that typically covers the switch.

Insensitive to noise and supply voltage

There are a couple of aspects that affect the supply sensitivity. Sensitivity to noise and

VDD drift. If there are transient glitches they would not be apparent across multiple

samples and we can remove there affects. Also, because of the reference threshold

tolerances and set up of the comparator circuitry, supply voltage variation does not affect

the value of the time constant of the measured capacitance. Thus we can have the same

period across temperature and dont need a precise VDD. The system clock is also stable

across temperature which is the time base used for the measurement of the oscillator period.


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ToolStick Evaluation Kit

Four capacitive sensing pads

Four LEDs

Light sequence game

Source code available

Sample configurations for timers and the comparator modules

available

Wake on touch

Capacitive Sensing

Daughter CardToolStick Base Adapter

Part Numbers:

CAPTOUCHSENSESK Capacitive Sensing Starter Kit

CAPTOUCHSENSEDC Capacitive Sensing Daughter Card

Lets take a look at some of the hardware available to check out Capacitive Sensing.

The ToolStick base adapter can connect to the Capacitive Sensing daughter card.

This card implements the touch pads as 4 solid round pads with a ground guard.

The sample code provided allows customers to play with the sensitivity of the

switches and see how Capacitive Sensing might work in their application.


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What Can You Do With the Kit?

Check out the effect of differing sensitivities

Play with the polling speed in firmware to see

responsiveness and that it is easily fast enough that you canpress multiple switches simultaneously and not recognize

any delay

Look at environmental effects to verify sensitivities

The moisture in your breath increases the dielectric of the capacitor

This is an example of configuring the switches to be too sensitive

Sensiti

vity

Least

MostInactive Counts

Active Counts

Single count threshold

Inactive to Active Threshold

Active to Inactive Threshold

The development tool is a good starting point for implementing your own system.

You can play with the values in the source code provided in order to see how

varying the threshold value affects the trip point of the switch. You can also take

this a step further to see how environmental affects change the capacitance values.

Even by blowing on the board, the humidity of your breath can change the dielectric

constant and cause false positives depending on sensitivity settings.


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C8051F700 CS Implementation

16-bit SAR converter

32 channel input multiplexor

Single channel or multi-channel scanning using auto scan

Hardware accumulator

Window logic threshold to trigger an event when an active state is detected

Lets take a look at how to implement the Capacitive Sensing using the C8051F700 family

from Silicon Labs. The 16 bit SAR block itself is implemented as a complete stand alone

block and generates its own time base. This allows it to run autonomously from the CPU

and provides the capability to wake the CPU from a low power state. Also integrated is a

20 bit accumulator that can add 1, 4, 8 or 16 scans and then provide the division using a

simple shift function. Using this hardware accumulator and simple shift function provides alow overhead averaging function to reduce the affects of noise in the system. The window

comparator is software programmable and is set based on the system level performance of

the pad configurations. After the inactive and active signal levels are determined and a

suitable threshold is obtained the window comparator can be set to trigger an event when

the active switch threshold is met. This is useful to reduce CPU overhead and also for low

power modes. The conversion of the CS0 module is capable of being generated from

several sources including software trigger, all of the timers and auto-scanning. All of the

required components for Capacitive sensing are integrated on chip. Therefore, we can

connect the Capacitive Sense pad directly to the pin of the MCU.


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Benefits of the CS Implementation

Increased sensitivity to change in capacitance

Hardware accumulator

Decreases system noise

Reduces CPU overhead

Insensitive to noise and supply voltage changes

Insensitive to 50/60 Hz noise due to sample t ime

Differential sampling doesnt require precise supply voltage

No external hardware overhead

Directly connect the switch traces to the MCU port pins

No other external feedback resistors or capacitors are needed

Low MCU overhead

Autonomous peripheral allows the MCU to go into low-power mode and wake up on a

switch event

Simple configuration

Easy to perform with any material on the switch


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Results


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CS0 vs. RO Set-up

Set-up C8051F700 and C8051F930 daughter cards used

Similar layout

Similar pad dimensions. The pads were simple as they were designed just for demo purposes.

1/8 dielectric material

Measured parameters Inactive state counts

Active state data

Calculated parameters Determine the mean of the inactive data

Determine the noise of the inactive data

Determine the mean of the switched active state data

Calculate the Signal level

Calculate the SNR and thresholds

Capacitive Sensing

Daughter CardToolStick Base Adapter

1/8 overlay

The hardware used to compare the relaxation oscillator method and the Capacitive

Sensing Module (CS0) was the ToolStick daughter cards. The overlay material was

placed on the switches and the data was accumulated viat the ToolStick Base

Adapter. The inactive measurements were taken and then a finger press on the

overlay material for each of the boards. The results are found on the next slides.


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C8051F930 daughter card with 1/8 overlay results

Sensitivity much less than the CS0 module

Not a lot of SNR margin in this set-up

Would need to revisit pad design, overlay material and software

RO System Signal to Noise Example

Inactive State Average = 413.12

Active State Average = 419.87

**Inactive state noise peak = 0.799

*Inactive state noise = 1.3Signal = 6.75

SNR = 6.75/.799 = 8.44

SNR 3.3sigma = 5.02

*The value of noise used is 3.3 standard deviations from the mean

**The value of noise used is the peak value from the mean

The RO method did not perform as well as the CS0 module. In order to use the RO

method the pad would have to be redesigned and special consideration would have

to be given to the overlay material and the thickness as well as the layout of the

board. These factors were not even really considered on the F700 board as it was

also just designed as a demo where a finger can be placed directly on the PCB.


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CS0 System Signal to Noise Example

Inactive state ave. = 19942

Active state ave. = 20353

*Inactive state noise1 = 26.7

**Inactive state noise2 = 17.2Signal = 20353-19942 = 411

*SNR1 = 411/26.7 = 15.4

**SNR2 = 411/17.2 = 23.5

Noise threshold = 164

OFF threshold = 246

ON threshold = 369

Signal(S)

Inactive State noise (N)

Active State MEAN

**SNR= 23.5

Inactive State MEAN

Inactive to Active Threshold

Active to Inactive Threshold



C8051F700 daughter card with 1/8 overlay (raw data)

Here is an example using simulated data on a finger press using the CapSense

module. The inactive measurement yields an average value of 19942. The active

state measurement yields a average value of 20353. The difference of the average

values gives us our signal level and in this case is 411. Once the signal is obtained

the threshold values can be determined. For example, to set the value for the ON

threshold we calculated 90% of the signal (.9 * 411). This value is then added tothe inactive mean to give is the actual threshold value in either counts (for the

relaxation oscillator method) or 16 bit sample (using the SAR method). The other

values are found in the same manner. There are two ways to measure the noise in

the system. The first s using the standard deviation and the statistical value to

encompass a percentage of the data points. The next way is to take the maximum

value of the of the inactive data and subtract the mean to find the peak. Both values

are shown above.


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CS0 Actual System Design Example

Inactive state ave. = 16436.7

Active state ave. = 16736.57


**Inactive state noise2 = 11.3

Signal = 16736.57 - 16436.7 = 299.87

*SNR1 = 299.87/16.86 = 17.8

**SNR2 = 411/17.2 = 26.5


OFF threshold = 180

ON threshold = 270

Signal(S)


Active State MEAN

**SNR= 26.5

Inactive State MEAN



Front panel display and touch pad input PCB with 1/8 overlay

Raw data usedno oversample

Here is an example using actual data from a completed system. The CS0 module

was used in this example to accumulate single data points. The mean for the

inactive counts and the active state were calculated and are shown on the side bar.

The overall SNR in this system just using raw samples was 26.5 if using the

standard method to calculate SNR. When using the more conservative approach our

SNR measured 17.8 which is still 3.56 times better than what has been consideredthe accepted SNR for touch sense applications.


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CS0 Actual System Design Example (Averaged)

Inactive state ave. = 16387.5

Active state ave. = 16736.69


**Inactive state noise2 = 3.5

Signal = 16736 - 16387 = 349*SNR1 = 349/4.78 = 73

**SNR2 = 349/3.5 = 99.7


OFF threshold = 170

ON threshold = 256

Signal(S)


Active State MEAN

**SNR= 99.7

Inactive State MEAN



Front panel display and touch pad input PCB with 1/8 overlay

16x hardware-based oversample

Here is an example using actual data from a completed system. The samples in this

example were derived using a hardware averaging function. Sixteen samples were

accumulated and averaged using hardware. This approach requires no CPU

overhead to sum the sample and then perform the averaging function.

Oversampling by 16 should decrease the noise by a factor of 4. The overall SNR

will increase as a result.


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CS0 vs. RO Conclusions

CS0 sensitivity much greater than the RO module Reduces software overhead

Provides margin in systems with changing environments

Allows the use of various overlay thicknesses and materials

Tests did not make use of the hardware accumulator

Can increase the SNR even more

CS0 scan time is extremely small Reduces noise sensitivity

Increases number of switches that can be monitored

RO method can achieve these good results with added software overhead

and scan time

CS0 provides more SNR margin 4x the SNR margin over the RO method

16x the SNR margin using the hardware averaging

.


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Visit the Silicon Labs website to get more information on Silicon Labs products,technologies and tools.

The Education Resource Center training modules are designed to get designers upand running quickly on the peripherals and tools needed to get the design done.

To provide feedback on this or any other training go to:

Learn More at the Education Resource Center

http://www.silabs.com/ERC

http://www.silabs.com/mcu

http://www.silabs.com/CapTouchSense

AN338: Capacitive Sensing Solution

AN367: Understanding Capacitive Sensing Signal To Noise Ratios and SettingReliable Thresholds

AN366: QUICKSENSE Firmware API

AN418: Baselining in the QUICKSENSE Firmware API

http://www.silabs.com/ERC

Visit the Silicon Labs Education Resource Center to learn more about the MCU

products.


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