1 course introduction purpose this part-c course covers ways to protect large scale integration...
Post on 03-Jan-2016
215 Views
Preview:
TRANSCRIPT
1
Course Introduction
Purpose This Part-C course covers ways to protect large scale integration
devices against problems caused by external sources of noise.
Objectives Understand the requirement for electromagnetic noise countermeasures.
Learn approaches and design methods for decreasing the electromagnetic susceptibility (EMS) of LSI devices that are operated in noisy environments.
Discover ways to prevent noise from affecting system operation.
Content 28 pages
Learning Time 30 minutes
2
Reducing EMS
EMS reduction is a goal shared by the semiconductor experts who design LSI devices and by the system engineers who apply those devices
It includes techniques for decreasing the electromagnetic susceptibility of a specific system, circuit or device — specifically, those used to minimize problems caused by external sources of noise in the operating environment
3
EMC Electromagnetic Compatibility
EMI Electromagnetic Interference
EMS Electromagnetic Susceptibility
SSCG Spread-Spectrum Clock Generator
WDT Watchdog Timer
PLL Phase Locked Loop
I/O Input/Output Port
Core A microcontroller chip is composed of a core, I/O ports, and power supply circuitry. The core consists of the CPU, ROM, RAM, and blocks implementing timers, communication, and analog functions.
Power supply
Two power supplies are applied to the LSI: Vcc and Vss. The core power supply internal to the LSI is VCL (internal step-down). The Vss-based power supply routed through the LSI is VSL.
Driver buffer
Output circuit transistors as well as output circuits for driving signals with large load capacitance and I/O port output transistors. Clock/bus driver, signals between blocks, etc.
OSC
CPG Clock Pulse Generator
Oscillator
POR/LVD Power-On Reset/Low-Voltage Detect functions
Harness Cables (wires) connecting a board and power supply or connecting one unit in a system to another.
Explanation of Terms
4
Vcc
Powerterminals
Power tointernalcircuits
Vss
Protecting Against Noise, Surges
Implementing protection against noise and surges is a major way to reduce EMS problems
The design shown in this example uses the drain-to-source breakdown voltage (BVDS) of a MOS transistor placed across the device’s Vcc and Vss terminals
5
Level-triggered Bypass Circuit
A control circuit can be used to turn on the bypass transistor when the input conditions on the supply line exceed a preset limit
Power tointernalcircuits
Control circuit
Vcc
Powerterminals
Vss
6
Diode Bridge Circuit
The diode bridge circuit is useful for minimizing EMS problems in devices that have multiple power supply terminals Reduces surge and noise by forcing the
Vss1 and Vss2 voltages to be equal
Vss1 Vss2
Vcc
Diode bridge circuit
I/O-1 I/O-2
7
Note that Vcc1 and Vcc2 have different electric potentials
MOS-transistor OFF circuit
Vss
Vcc1 Vcc2I/O I/O
Vcc1 Vcc2
MOS-transistor Bridge Circuit
A MOS transistor bridge circuit helps to reduce EMS problems in devices with multiple power supply terminals Uses BVDS of MOS transistor to reduce high-level noise voltages Isn’t as effective as diode bridge circuit
8
Vcc
Diode circuitBipolar action circuit
Outputcircuit
I/O terminal
MOS-transistor (OFF) circuit
Vss
Outputcircuit
Inputcircuit
Noise/Surge Bypass Circuits
Various types of bypass circuits can protect LSI devices from noise and surges that enter from the I/O terminals Three basic designs are shown here
9
To internal circuits
Schmitt Trigger circuit
R1 = Input protection resistor D1 and D2 = Input protection diodes
R1
D1
D2T2
T1
T1 and T2 = Input protection transistors or output transistors
Input pinorI/O pin
Vcc Vcc
VIH
Noise spikes
Noise is eliminated
Input Waveform
VIL
Schmitt Output
Schmitt Trigger Circuit
A Schmitt trigger serves as a type of logic-state-dependent amplitude filter, providing hysteresis via preset voltage thresholds that eliminate many noise spikes from its output Turns on when input reaches of VIH; turns off when input falls to VIL Schmitt trigger circuits require a CMOS interface
10
In an actual circuit configuration, the
“High”-state noise is eliminated, as well. “Low”-state noise is eliminated
Analog delay output
Schmitt trigger output
Noise canceller output
Noise
Timedelay Time
delay
Noise canceller circuit
To internal circuits
Schmitt Trigger
Input pinorI/O pin
Analog delay circuit
Analog delay circuit
Noise canceller with analog delay circuit
Noise Canceller (Analog Delay)
A noise canceller provides additional noise/surge filtering by adding a time-delay circuit and logic to the output of the Schmitt trigger circuit Processes signal and time-shifted version
11
Systemclock
Noise canceller circuit
To internal circuits
Schmitt Trigger
Input pinorI/O pin
Digital delay circuit
Digital delay circuit
CLK
CD Q
CLK CLK
CD Q
CD Q
CLK
CD Q
CLOCKOn-chip
Oscillator
Backup system clock
Noise canceller with digital delay circuit
Noise Canceller (Digital Delay)
Noise canceller with digital time-delay circuit doesn’t operate when system clock is stopped Design recommendation: Provide a backup system clock source that
automatically switches in if the main clock fails
12
To internal circuits
Schmitt Trigger circuit
Input pinorI/O pin
Vcc Vcc
C
R
Noise filter
Analog (R-C) Low-pass Filter
R-C LPFs attenuate high-frequency noise and are useful in many places on the LSI device Can be implemented in a small area of the chip This example places it at the input of the Schmitt trigger
13
Vcc
Vss
Judgement output
C
R
Vcc
Vss
Noisefilter
POR & LVDfunctions
R-C low-pass filter
LPF on Internal Power Lines
A low-pass filter on power lines to noise-sensitive functions such as the Power-On Reset (POR) and Low-Voltage Detect (LVD) circuits is a good design precaution
14
Mode pin
Latch D
Q
Mode latchregister
Bus
Typical modes: • Single-chip • Extended • Test
Latching the Mode State
A latch added to the output of the Schmitt trigger prevents unwanted transitions in mode state Captures and holds Mode-pin data when commanded to do so by
software
15
Keep space
Prohibition on Dynamic Circuits
Dynamic latch circuits should NOT be used because noise or surges add charge to circuit’s internal capacitors Causes malfunctions that might disrupt chip operation Renesas uses special CAD tool to prevent
accidental use of dynamic latch circuits
16
High-voltagejudgement
circuit
To high-voltage circuits
External capacitor(mounted on board by user)
Protection circuit Noise filter
C0.1µF
High-voltagejudgement flag
High-voltageterminal
High-voltage Pin Protection
Protection circuits guard against noise and ESD on Vpp terminals and high-breakdown-voltage N-channel open-drain terminals Mount external 0.1µF capacitor on the circuit board near the terminal Integrate an ESD circuit and LPF on the chip close to the terminal
17
OSC1 OSC1 OSC2
Externaltimingnetwork
System Clock Oscillator
Main clock oscillator circuit should have EMS protection to avoid critical logic timing problems Should be designed
so that the external timing network (crystal, etc.) is mounted close to the LSI device
Can include an internal Schmitt trigger for noise filtering
Can be designed with analog noise filtering, too, for additional EMS protection
18
Conventional multiplexed input terminal design
Input terminal design of reduced-noise microcontroller
A/D converterA/D converter
Multiplexed A/D Input Terminals
Input to A/D can use high-isolation switches arranged to reduce leakage and improve converter’s S/N ratio Implements a reduced-noise network design in which both of the series switches
in a channel not selected are open and the shunt switch is closed
19
System clock distribution
CPG
SYSTEM RESET
CLEAR
CPU
To on-chipmodules
Main system oscillato
r circuit
OSD
WDT
OCO
Watchdog timer may include a low-power internal oscillator for use in Sleep mode, etc.
Watchdog Timer, OCO, and OSD
WDT can be used to reset system and resume normal operation if a malfunction causes a stall or runaway condition Initial setting for WDT is “on” OSD monitors main clock
and automatically switches the CPG and WDT to the OCO if necessary
20
Precision oscillation can be obtained by various design techniques: • Frequency trimming • On-chip stabilized power supply • On-chip thermal protection circuit • Synchronization with maser clock on circuit board • Frequency switching • Etc.
System clock distribution
CPU
To on-chipmodules
CPGOn-chip
oscillator circuit
On-chip System Clock
Precision high-speed clock built into LSI device prevents malfunctions caused by noise that otherwise would be applied to external oscillator’s terminals Eliminates need for external
components, OSC pins, and some internal EMS protection components, allowing savings in cost and circuit board space
21
Illegal instruction
detected
Latch
Illegal address
detected
Interrupt flag
CPU instruction decoder
Illegal (unanticipated) address detection
Illegal Instructions/Addresses
Circuit that monitors instructions and addresses generates an interrupt flag when it detects incorrect values
22
Port
Vcc
Write+Address1
Write+Address2
External circuit
Protection register
Important registerbeing protected
D Q
D Q
Register write-enable
switch
Protection for Control Registers
Various methods can be applied to safeguard critical control registers against problems caused by external noise Using a protection register or an external “Register write-enable” switch
prevents erroneous writes to registers such Clock-stop control or Operating-mode control, even if CPU runs away
23
Use SCI or CAN bus when hardware communication is slow
Example: Hardware provides majority processing of data at three strobe points
Data
Strobe
Logic output “L” “H” “L” “L”“H” “H”
“High”“Low”Output from processing
Example: Software initiates retry if data from two reads doesn’t match
Data
Read
Logic output “L” “H” Retry = “L”
Output from processing Result = “L”
No match
Majority Processing
Majority voting can be implemented in hardware or software to supplement other noise countermeasures Reading logic states multiple times instead of only once, then using a
“majority-rules” type of decision-making process, is a good way to filter out noise spikes
24
ROM
RAM
CPU
Logic
Analog circuits
Keeping high-current ports away from analog circuits is highly recommended
Adding an isolation channel at Vss potential further reduces noise coupling
Analog Circuit Separation
Separating the analog circuits from the digital circuits on the chip helps prevent digital noise from degrading analog circuit operation Operating analog circuits in noisy
environments decreases the S/N ratio and reduces performance
25
Port
Vcc
Vcc
Port
R
CVcc to analog circuits
Vcc bus for digital circuits
Noise filter
Analog circuit(A/D converter,
D/A converter, etc.)
Vcc
Power supplies that are used exclusively for analog circuits are often given special names, such as AVcc, Vss, and Vref.
Power Supply Separation
Keeping the digital-circuit power supplies separate from the analog-circuit power supplies is essential for the proper operation of noise-sensitive analog circuits Ensure that the analog
and digital circuits have separate on-chip power traces
Provide separate Vcc pins for the analog and digital circuits
Place low-pass filters near analog circuits to obtain extra EMS protection
26
Power supply offset voltage
Set at 0V
Vcc Residual offset voltage
Vcc drops when a large load is turned on
Vdd Power supply ripple
Offset Voltage and Supply Ripple
Residual offset voltage and power supply ripple cannot be eliminated by low-pass analog filters Eliminate residual offset voltage by
synchronizing the application of the device’s power supply voltages and by properly sizing the capacitancein the Vcc power supply
Eliminate Vdd ripple by routing the power supply paths, including ground paths, as close as possible to the main voltage regulator, and by adding individual regulators for different modules on the chip, while isolating the filter components attached to those regulators
27
Microcontroller operating range
Apply RESET after stabilization
Vcc
Voltage temporarily falls below the minimum allowed
Voltage stabilization time
Vcc(Min.)
Momentary Power Interruptions
When a momentary condition causes the voltage supplied to the LSI device to fall below the lower limit of the guaranteed operating range, apply an external RESET after the voltage stabilization time
28
Course Summary
Preventing noise from entering an LSI
device chip via its terminals
Protecting against runaways and stalls
Safeguarding analog-circuit performance
Minimizing problems due to offset voltage, supply ripple
and power disruptions
For more information on specific devices and related support products and material, please visit our Web site:
http://america.renesas.com
top related