Part Number: TCAN4550
We have design using 2 TCAN4550
When testing the circuit one board has expected CAN H and CAN L signal like below
Same design but different board has unexpected CAN signals like below
-CAN H stays High (Yellow)
-CAN L has signal rising above 2.5V and below 2.5V (Blue)
What could cause this?
Hello James,
The CANH and CANL signals are certainly unexpected and the recessive voltage level should remain around 2.5V during the recessive bits which is set by a weak internal resistor divider in the transceiver. The TCAN4550 CANH and CANL pins each have a 40k resistor to Vcc/2 or 2.5V. During a Dominant bit, the CANH signal is driven above 2.5V, and the CANL signal is driven below 2.5V. During the Recessive bits, the resistance on the bus pulls the CANH and CANL voltage levels back to 2.5V which I will also refer to as the common mode voltage.
In your waveforms, it appears that there is some other voltage source holding the common mode voltage to a higher potential and skewing the recessive voltage level for each signal. The CANH signal drives High during the dominant bit, and then is pulled down to the common mode voltage level as expected. The CANL signal likewise drives low during the dominant bits, and then is pulled to the common mode voltage during the recessive bits as expected. But because the common mode voltage is higher than 2.5V, the CANL signal is pulled above 2.5V.
This new common mode voltage looks pretty close to 3.3V which is another commonly used voltage rail in applications. I don't know if you have 3.3V on your board, but I would suggest checking if there is anything that could cause a higher voltage to couple onto the CAN signals. Also check that all components on the bus such as termination resistors, caps, etc. are good.
Does the recessive level return to 2.5V after the CAN packet is completed? Your scope plot doesn't show the idle time after the packet is complete. If the higher common mode voltage is only present during the CAN message, and it returns to 2.5V after the message is complete, then there may be a physical problem with the transceiver.
You said it works on one board, but not another. Does this issue occur on multiple boards or TCAN4550 devices, or is the fault isolated to this one board or device?
Regards,
Jonathan
Hi Jonathan
We have seen passing on multiple boards and failing on multiple boards of same design.
Yes, recessive level will eventually return to 2.5V as shown below
We see there is transceiver only test mode
Please confirm if the following will enable Transceiver Test mode
SPI issues following same time or individually?
Is there specific logic sequence sent on GPIO1 or can any data, i.e. 500kHz square wave, be used?
