TCAN1145-Q1: Unintended wakeup under ESD

Hi Abraham,

Thanks for sharing this information with us on E2E. 

It's interesting to hear that the ESD test is causing the transceiver to wake from sleep. Even with the WUP configuration, the CAN bus signal is still filtered using tWK_FILTER and requires a dominant-recessive-dominant pattern in order to wake the device. I would think this would be uncommon to happen during an ESD strike, however it is certainly possible. 

Because the false wake seems to be related to the lack of termination, this leads me to believe this is related to the CAN bus wake up and not the WAKE pin. However, to be sure, is it possible to read the INT_1 register (51h) and share the value? This would let us see if there was a CAN bus wake (CANINT) or WAKE pin wake (LWU) that caused the device to change modes. 

For the ESD testing itself, which pin is being struck when the device does a false wake up? If CANL is struck and no termination is included, I would think it'd be very unlikely that a positive differential WUP could be generated between CANH and CANL.  If CANH is being struck, this scenario seems more likely. 

Is it possible to capture waveforms of the CAN bus during the ESD strike? It would be helpful to see the differential on the CAN bus during this test to see if the WUP requirements are being met. Knowing what electrical signals are being seen by the transceiver itself may help identify external solutions to this issue. 

As for the test itself, this sounds like an odd case. From a system-level, the CAN bus will include termination in the end application, so it seems odd that the requirement would test the condition without termination. It's also curious that the ESD test would specifically reference wake up functionality. Because ESD events are (hopefully) infrequent in a system, an inadvertent wake up under these conditions would be minimally disruptive as the ECU could simply go back to sleep after identifying there is no need to remain awake. 

Let me know if you have any other questions in the meantime. 

Regards,
Eric Schott

Hi Eric,

Thank you for your comments. 

I have communicated with customer again and they are not able to provide the waveform of CAN bus at the ESD strike because the test was done at a third party lab, INT_1 cannot be read either. Customer is focusing on why wakeup happens without the termination resistor. Could you help analyze the logic behind this? Will termination resistors (R212, R213 in this case) provide a path for the ESD flow to GND?

And will increasing C128, C129 help?

Hi Abraham,

The termination resistor provides a path for any differential between CANH and CANL to discharge. When termination is in place and either signal jumps to a different voltage, the other signal will follow due to this path. This keeps the differential signal seen by the CAN receiver relatively small and short-lived. If termination is not included, there is only a high-impedance discharge path between CANH and CANL. In this case, if one of the signals changes voltage abruptly, the other signal will remain relatively constant and the differential seen by the receiver can be large enough and long enough to be considered a dominant pulse. I expect this relationship is impacting the different results seen with wake up when one signal is struck with or without termination included. 

Bus capacitors will slow the voltage change of either CAN signal. This essentially filters out the high frequencies of any noise or transient strikes. However, this does not have much impact on the differential as seen by the transceiver like I mentioned for termination. If no discharge path exists between CANH and CANL, the differential will still be created. While this may have some impact on the results due to the filtering of the high-frequency contents of the transient, I don't think this would help with the relatively low-frequency portions of the strike that may be interpreted as a dominant pulse. 

Regards,
Eric Schott