Hi Lao,
The issue you are observing with the TXD pin being pulled low for 4 microseconds could be due to several factors. Here are a few possibilities to investigate:
1. Dominant Bit on CAN Bus: The CAN bus protocol uses a dominant and recessive state. When the TXD pin is pulled low, it is driving a dominant bit on the bus. This behavior could be normal during data transmission, where a dominant bit (logical 0) is transmitted. The 4 microsecond duration may correspond to the time it takes to transmit a bit on the CAN bus.
2. Bus Load or Termination: Ensure that the CAN bus is correctly terminated with 120 Ohm resistors at each end of the bus. Incorrect termination or excessive capacitance on the bus can affect the waveform and cause irregularities.
3. Noise or Transient Interference: External noise or power supply instability could momentarily affect the TXD signal. Check for proper decoupling capacitors on the supply pins and ensure that the layout minimizes noise.
4. Driver or MCU Issue: Ensure that the MCU driving the TJA1057 is functioning correctly and is not causing unintended signaling on TXD due to misconfiguration or timing issues in the software.
To troubleshoot further, you may want to capture a complete CAN bus frame to see if this behavior is part of normal communication or if it is due to an anomaly in the system.
Hope it helps.
BRs, Tomas
