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Hercules™ Safety Microcontrollers
Hercules™ Safety Microcontrollers Forum
Protection circuit for ESD/EMC immunity on TMS470MF06607
For reset pin: (user can no-pop the cap/res for nRST, cap for the nPORRST at the beginning, and assemble them if the ECU fails ESD/immunity test).
For TEST and ENZ pin (Please do not rely on the internal pull down):
CAN: No protection is needed in the path from MCU to CAN transceiver. If this trace is long (>5cm), please consider reserve the footprint of a SMT capacitor to ground (in the driver end). This capacitor can slow down the edge and mitigate the emission in case it turns to a problem. In the transceiver side, usually, there are a TVS diode and a high voltage capacitor (around 100pF) together for EMC/ESD protection. They should placed as close to the connector as possible. The characteristic of the TVS diode should refer to their customer’s requirement for ESD.
GPIO pins: if they served as external interrupt purpose, a low pass filter is needed in front of the pin. If they go the connector, please consider the ESD protection. Depending to the requirement, it could be a resistor + a high voltage capacitor, or a TVS diode + a high voltage capacitor.
I took you advice and used a 1nF cap placed near the nPORRST pin and it helped my ESD immunity. I am curious though, I tried a .1uF can in the same configuration but it did not help. Can you explain why this might be? I am using the 1K pull-up resistor. I don't however know what the internal circuitry of the nRST and nPORRST are inside the micro.
Any help you could offer would be greatly appreciated. I am just curious now...can't figure out why .1uF wouldn't do....
I don't know exactly why 0.1uF does not help in your case. What is the type of this test, contact discharge or air-discharge? What is the schematic and layout of this pin? Is it possible to get the waveform on this pin during ESD test? Anyway, I don't recommend 0.1uF due to other reasons:
1. The startup ramp (from 20% to 80%) of PORRST should be less than 10us. Therefore, a 0.1uF with a 1k resistor is not suitable.
2. nRST sometimes is an output pin, 4mA IO, driving an 0.uF capacitor might cause damage on this pin after thousands of powercycling.
The internal circuit:
1. PORRST: input with 100uA pull down.
2. nRST: input with 100uA pull up or Open drain output (during normal operation, it is input but it is opendrain output when PORRST is low or reset occurs)
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