TKT-3500

Microcontroller

systems

Lec 4 – Timers and other peripherals,

pulse-width modulation

Ville Kaseva

Department of Computer Systems

Tampere University of Technology

Fall 2009

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Sources

Original slides by Erno Salminen

Robert Reese, Microprocessors: From Assembly

to C with the PIC18Fxx2, Charles River Media,

2005

PIC18F8722 Family Data Sheet, Microchip

Technology Inc.

Tim Wilmshurst, Designing Embedded Systems

with PIC Microcontrollers – Principles and

applications, Elsevier, 2007.

Wikipedia

TotalPhase Knowledge Base - Article 10045

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Contents

Integrated peripherals of PIC18

Timers

Structure

Usage

Accuracy

Co-operation with capture/compare/PWM module

Pulse-width modulation (PWM)

Short notes about motor control

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Brown-out and hi/low voltage detectors

10-bit AD-converter

Two enhanced USART modules (RS-232 or RS-485)

Two master synchr. serial port modules (I2C+ SPI)

Two 8-bit and three 16-bit timers

Six capture/comparator/PWM modules

Digital IO

General-purpose, bidirectional pins

User can select the direction: input/output

Can be used for parallel communication and for controlling

external devices

data bus

PIC18LF8722

integrated

peripherals

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Brown-out reset (BOR)

Resets microcontroller when voltage is too low

Power lines (wires) have some resistance R

CPU draws current I and hence Vdd = Vs – RI

Current I depends on internal activity of CPU

With large current I, the Vdd drops too much

Logic gets slower resulting in malfunction

Mem write may corrupt mem contents, better reset the

CPU

CPU

Vdd

R > 0+-

Vs

I

voltage

time

Vs

Vdd

Achtung! Reset der

CPU!

V > 0

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BOR (2)

Vdd is allowed to drop momentarily,

i.e. for a duration less than threshold time

theshold TBOR is, for example, 200 us

To detect BOR,

Reset POR to ’1’ immediately after any POR event

Read both status bits BOR and POR (power-on reset)

BOR==0 and POR==1 is indication of brown-out reset

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High-Low voltage detect (HLVD)

Informs when voltage is too high/low

Interrupt flag is set

Programmable circuit

User sets

Trip point (voltage level)

Direction (rising/falling)

Controlled shut-down can be performed before

voltage drops critically low

data bus

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HLVD structure and low-Vdd detection

each node

forms one

trip point

select one

trip point

comparator:

trip point vs. ref voltage

Battery

Start shut-

down if Vdd

drops here

Minimum valid

Vdd

Time

margin

for

shut-

down

Voltage direction

magnitude

brown-out

reset

enable

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HLVD operation

internal ref.

voltage stable

Low voltage

detect

High voltage

detect

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HLVD cntd.

When the module is enabled, the HLVD comparator and voltage divider are enabled and will consume static current To decrease the current requirements, the HLVD

circuitry may only need to be enabled for short periods where the voltage is checked

After doing the check, the HLVD module may be disabled

For example, the HLVD module could be periodically enabled to detect Universal Serial Bus (USB) attach or detach High-voltage detect when device uses, say 3.3V

and receives 5V from USB

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data bus

Analog input/output

Analog-to-digital conversion (ADC)

Requires own reference voltage inputs

10-bit conversion means 1024 distinct signal levels

16 channels, at sample rate ~1 Mhz

Depends on microcontrollers clock frequency

Discussed in more detail in lecture 9

t

v

analog

signal

digital

signal

1

1

2

3

4

5

6

7

8

2 3 4 5 6 7 8 9 10 11 12

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Serial ports

2*EUSART (Enchanced Universal

Synchronous Asynchronous Receiver

Transmitter)

Module for eg. RS-232 or RS-485 transmissions

Lecture 3

2xMSSP (Master Synchronous Serial Port)

i2c and SPI in the same port

Covered in lectures 3+5data bus

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Timers, Watchdog, real-time clock

Timers1. Measuring duration of tasks and waveforms of input signals

2. Generating timed events and output signals

3. Counting input events

Usually several 8-bit and 16-bit timers, 5 in PIC18

The input clock may be selected

Watchdog timer (WDT) Special purpose: reset the CPU unless regularly taken care off

A program gone mad (stuck in infite loop etc) cannot service WDT and CPU will get rebooted

Real-time clock Timer for implementing ”wall clock” or calendar functionality

Needs special power (battery) to keep the timedata bus

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(Enhanced) Capture/Compare/PWM

data bus

Enhanced Capture/Compare/PWM (ECCPx)

Capture Timer value is saved when event (rise, fall) is happened in

the corresponding capture pin

Compare Compares register value to the timer value. When they are

same wanted event occurs

PWM Module for creating Pulse Width Modulation signals

ECCP includes eg. user selectable polarity etc.

Structure of timers

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Timer basics

Timer is a counter

usually count upwards

when enabled, counter_r <= counter+1;

The values are also called timer ticks

Easily converted to wall-clock time when the period

of tick is known

In PIC, the counters are 8-bit and 16-bits wide

The value ranges are [0,255] and [0, 65535] ticks

new value

counter_r++

Counter

enableLoad

enableclk

counter

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Timer basics (2)

Longest delay obtained directly is 2n * (1/f)

Assume f=40 MHz

8-bit: 28 * (1/40) = 255 * 25ns = 6.372 us

16-bit: : 216 * 25 ns = 1638.4 us = 1.6384 ms

Rather short time!

use lower frequency by scaling down the CPU clk

max delay becomes 2n * (1/(f/i))

Scale-factor is typically power of 2

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clk/2

Clock divider

Simple chain of D flip-flops can divide the frequency

Each stage by 2, together by 2stages

DFFs do not use the same clock!

Each stage increases the phase difference between

clk/i and clk

Must synchronize the clocks

clk

...

clk/4 clk/8

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clk

synchronizer

Clock divider – with synchronization

...

clk/2 clk/4 clk/8

clk

syn

hcro

niz

ed, d

ivid

ed

clo

cks

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PIC’s timers

PIC18 has 5 timers

both 8-bit and 16-bit

all have readable register(s) for current value

all have writable register(s) to set the new value

all can generate interrupts

Block diagram of Timer0

operating in 8-bit mode

Clock

Selection

Instruction

clock

Clock Edge

Selection

Pre-scaler

setting

Pre-scaler

assignment

Value

register++

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PIC’s timers

Minor differences in their usage

generate interrupt when

a) overflow occurs

b) value is equal to match register

which clocks can be used

what kind of frequency scaling is allowed

some can be automatically reset due to special

event trigger

Timer2 allows optional use as the shift clock for

the MSSPx module

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Timer1 and 3 subsystems

16-bit susbsystems with identical capabilities

Two 8-bit registers for value

input pin for

on-chip osc.

input for ext.

clk/output pin of

on-chip osc.

TMR1L

May be reset by CCP

module

Timer1

oscillator

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Timer1

Can act in three modes1) Timer – incremented on every instruction cycle

2,3) Synchronous/Asynchronous Counter – incremented on every edge of external clk or Timer1 oscillator

Reading from TMR1L (lower byte) will update the value of TMR1Hbuffer

User gets all the 16-bits accurately

Otherwise, TMR1H could have been incremented before it is read and hence become invalid

Writing 1. Write the new high byte first into buffer

2. Write the TMR1L – this will update also the counter’s high byte from the buffer

Both bytes are written at the same time

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Timer1 Oscillator

An on-chip crystal oscillator circuit between pins T1OSI (in) and T1OSO (amplifier output)

Can be used as CPU’s clk source

Low-power circuit rated for 32.768 kHz crystal Continue to run during all power-managed modes

Often used for implementing real-time clk (RTC) Preload 16-bit counter with 32K = 215 so that the

overflow at 64K will occur with 1 second interval

In ISR, increment the counter variables for seconds, minutes, hours and so on

Asynchronous mode is more accurate

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Timer1 Oscillator (2)

Enabling the amplifier improves noise immunity but increases power

Low-power nature makes the oscillator sensitive to rapidly changing signals in close proximity Oscillator circuit must be located very close to MCU

No circuit should pass within the oscillator circuit boundaries other than VSS or VDD

If a high-speed circuit must be located near the Timer1 oscillator, a grounded guard ring around the oscillator circuit may be helpful

Fig. External components for

oscillator

Capacitor values 27pF are a starting point Check the specs of your

crystal/resonator

Higher C increases the stability but also the start-up time

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Timer2

Always clocked with instruction cycle clock

8-bit counter with both pre- and post scalers

Compares when counter reaches value of period register PR2

Matches are ”post-scaled” before generating an

interrupt

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Timer2 (2)

Counter is reset after each match

Interrupt period becomestTMR2_IF_per = ((tosc * 4) * PRE) * PR2) * POST (10.1)

Common usage of (any) timer is periodic interrupt generation to accomplish certain action at fixed time intervals tTMR2_IF_per

PR2 has largest range of the 3 adjustable parameters, solving it from (10.1) and rounding to integer gives

PR2 = -1

tTMR2_IF_per

((tosc * 4) * PRE) * POST

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Timer2 (2)

Again, rounding

causes errors in

timing

Similar to baud

rate generation

This is still way

better than

implementing

delay with

dummy SW loop

Usage of timers and

capture/compare/PWM

modules

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Shape of things to come in this lecture

Example use cases

1. Time keeping

2. Switch debounce

3. Pulse width measurement

4. Square wave generation

5. (Infrared decoding omitted, see the book)

Accuracy considerations and usage of CCP

modules

Intertwined into examples

Pulse-width modulation (PWM)

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Example1: Time keeping - ISR

Two seconds have

passed when overflow

occurs

Variable secs is also a semaphore

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Ex1: main

Terminal SW on PC:

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Example2: switch debounce

Earlier example used SW delay to implement switch debounce

Here, Timer2 is configured to generate periodic interrupts 5 interrupts with ~6 ms period yield appropriate delay

24ms – 30 ms, since timer’s start value is unknown when INT2 happens!

1

2

4

3 Wait until RB2 high and then 30 ms more

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Ex2: Switch debounce – main()

1

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Ex2: Switch debounce - ISR

{ }

{

}

2

3

4

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Ex3: Pulse width measurement

Basic version of measuring time between two

external events (fall/rise of signal)

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...

Ex3: Pulse width measurement (2)

Pulse width = TMR0 * (tOSC * 4 * PRE)

Accuracy defined by tOSC and PRE

There are few problems, however

1. Timer is reset and read in ISR and entering ISR

takes many cycles

2. What happens if Timer overflows?

RB0/INT0

time

...

...Timer0 w/ PRE=1

latencyISRlatencyISRreset timer

get timer

value

TMR0

0Timer0 w/ PRE=4

12 3

4

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Ex3: min/max pulse widths

Let’s calculate minimum and maximum pulse

widths that can be measured

Pulse width = TMR0 * (tOSC * 4 * PRE)

Assume tOSC = (1/40 MHz) = 25 ns and PRE=1

Maximum: set TMR0=216-1

wpulse,max = 65535 * (25 ns * 4 * 1) = 6 553 500 ns =

6.6 ms

Minimum: set TMR0=1

wpulse,min = 1 * (25 ns * 4 * 1) = 100 ns

This is also the resolution – the minimum

difference between pulses that can be observed

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Ex3: min/max pulse widths

Changing interrupt sensitivity from falling to rising edge requires several instructions

Min pulse width must be larger than 100 ns = 1 instrcution cycle

We could increase prescaler:

For example with PRE=256

min pulse becomes 256*100 ns = 25.6 us

max pulse becomes 256*6.6 ms = 1.68 s

Increasing the prescaler extends maximum pulse width

but reduces precision

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Ex3: min/max pulse widths

Still, previous example is accurate enough for

human-interface such as button

Too inaccurate for sqaure-wave measurement in the

range of ten to hundreds of kHz

Moreover,

the time from falling edge to timer reset is not included

the time after rising edge to timer read is included

These times are likely the almost the same if

interrupts are always enabled

No matter which scheme, you should analyze the

limitations

Next we discuss using CCP modules to enahnce

pulse measurement

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(Enhanced) Capture/Compare/PWM

Two Capture/Compare/PWM (CCP) modules

Three Enhanced Capture/Compare/PWM (ECCP) modules: One, two or four PWM outputs per ECCP

Enhancements: Selectable polarity, Programmable dead time, Auto-Shutdown and Auto-Restart, Half-Bridge and Full-Bridge Output modes

Enhanced modes are not considered on this course

User can of course select which timer is the source

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Capture/Compare/PWM operation

1. Event (edge) occurs in CCCP4

2. Interrupt flag is set

3. Timer value is stored into register

Benefit: value is captured immediately and

not after interrupt latency

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Enhanced Ex3: pulse width with capture

Measured pulse is now fed into pin CCP1

Capture module reads the value of Timer1

There’s no interrupt latency prior to reading

Note that timer is not reset at any point

It is always in operation

We calculate the difference between tic samples

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EEx3: handling timer overflows

In no overflow occurs (short pulse), the

difference between captured values defines

the pulse width directly

In general case, we must record how many

overflows occurs between captures

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EEx3: ISR

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EEX1: main

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Capture/Compare/PWM operation

1. Timer value matches the value of compare register

2. Interrupt flag is set

3. Event (edge) is generated into CCP4 Driven high/low, toggled, driven latched value

4. Special event trigger notifies other modules, e.g. to start AD converter or to reset the base timer

Benefit: event is generated immediately and not after interrupt latency

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Capture/Compare/PWM operation (2)

Periodic interrupt generation

In simple time keeping code, the timer

overflowed once every 2 seconds

To update secs variable every second

load CCPR1 register with 0x8000 = 32K

use special event register to reset Timer1 on

match

trigger interrupt with flag CCP1IF instead of

overflow

increment secs varibale by 1 instead of 2

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EEX3: main

Very small

changes to

dummy version

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Ex4: Square wave generation with compare

mode

1. Let timer run for half period of the desired

square wave

2. Toggle (invert) the output signal’s value

Done automagically by CCP!

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Ex4: approach a) for ISR

Action: Reset timer to zero and wait for new match

Not too complicated but slightly wrong anyway

Reset takes just few cycles but they decrease the wave freq anyway

desired wave

real wave

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Ex4: approach a) for ISR

Modify the compare register

Increment it by half period (HPERIOD)

Never alter Timer1

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Ex4: about false match

Approach b) updates the high byte of compare

register before lower

Why?

First match value 0x010A

Next match value 0x20B5

Updates in wrong order (i.e. lower byte first)

Avoids false match if half period is at least 256

0x010A 0x010B 0x010C

01 0A

Timer1

Compare reg

Calculating new value and

updating take a while

0x10B4 0x10B5

20 B501 B5

0x010D ...time

1 2Match + interrupt

01 B5

False match when compare reg

is in ”intermediate” state. oops3

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Pulse Width Modulation (PWM)

Pulse width modulation is technique that

varies the duty cycle of a square wave

signal while keeping the period fixed

Duty cycle refers to time when signal is ’1’

Duty cycle defines the average current

delivered to external device

Example usages

brightness of a led

controlling a motor

”poor man’s” digital-to-analog conversion

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PWM as DAC

Shown RC network does low-pass filtering for signal.

since PIC’s output pin can source only few milliamps.

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PWM as DAC (2)

External operational amplifier (’opari’)

provides large current drive capability than

PIC’s output

Series resistor + parallel capacitor slows

down the changes in signal level

Low-pass filtering - only the low frequency

component are let pass

Larger the time constant (RC product), lower

the cut-off frequency

RC defines the amount of ripple – voltage

variation

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Capture/Compare/PWM operation

CCP module has built-in capability for

constructing PWM modulated signals

Period register

defines the period

This register sets the

duty cycle

Next duty cycle is

written here to avoid

glitches

When duty cycle

ends, new val is

loaded from low reg

into hi reg

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PWM for LED

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PWM for DC motor

PWM regulates the current through power-

FET

Diode protects FET against voltage spikes

due large inductance of a motor

FET is voltage-controlled, practically no current flows into gate

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H-bridge for DC motor

Motor’s rotation direction depends on the direction of electrical current

FETs in opposite corners conduct at the same time

Pulse ratio defines the direction

The longer PWM phase defines directions

Motor keeps rotating but slows down during the other phase

50%- 50% keeps the motor still

Current I when

PWM signal is ’1’

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Servo motor

A servo motor does revolve freely but

between two extreme positions

Here they are marked as

CW - clock-wise

CCW -counter clk-wisemid

CWCCW

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clock-wise

counter-

clock-wise

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Conclusions

Timers are integral part of most

microcontroller applications

Timers are used for

Measure external events (pulse-width, frequency)

Trigger timed actions (task execution, wave

generation, wakeup)

Pulse-width modulation

PIC has capture-compare-PWM modules that

increases the accuracy and convenience of

these tasks