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 EMIElectromagnetic Interference EMSElectromagnetic Susceptibility SSCGSpread-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 V CL (internal step-down). The Vss-based power supply routed through the LSI is V SL. 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/LVDPower-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 Power terminals Power to internal circuits 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 to internal circuits Control circuit Vcc Power terminals 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 Vss1Vss2 Vcc Diode bridge circuit I/O-1I/O-2

7 Note that Vcc1 and Vcc2 have different electric potentials MOS-transistor OFF circuit Vss Vcc1Vcc2I/O Vcc1Vcc2 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 circuit Bipolar action circuit Output circuit I/O terminal MOS-transistor (OFF) circuit Vss Output circuit Input circuit 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 D2 T2 T1 T1 and T2 = Input protection transistors or output transistors Input pin or I/O pin Vcc V IH Noise spikes Noise is eliminated Input Waveform V IL 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 Time delay Noise canceller circuit To internal circuits Schmitt Trigger Input pin or I/O pin 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 System clock Noise canceller circuit To internal circuits Schmitt Trigger Input pin or I/O pin Digital delay circuit CLK C D Q CLK C D Q CLK C D Q CLOCK On-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 pin or I/O pin 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 Noise filter POR & LVD functions 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 latch register  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-voltage judgement circuit To high-voltage circuits External capacitor (mounted on board by user) Protection circuitNoise filter C 0.1µF High-voltage judgement flag High-voltage terminal 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 OSC2 External timing network 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 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 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-chip modules CPG On-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 register being protected DQ DQ 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” “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 Port R C Vcc 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 capacitance in 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: