TCAN1044A-Q1: RXD behavior after the WUP

Hi Hirata-san,

This is a good question, with a not so great answer. 

Originally when the wake feature was first introduced, the behavior was always to reflect the bus state after a tWK_FILTER with no filtered dominant or filtered recessive required. An update to the standard added the filter component to the WUP to increase to noise immunity of standby transceivers.This is why TCAN1044A and other 8-pin CAN transceivers operate this way for WAKE. 

For 14-pin transceivers, industry practice changed again because the transceiver has access to more pins and more modes. Because the MCU may be completely off in this case (INH signal controlling MCU power) it would not be able to identify a transient low on RXD. This is where the latching behavior of the RXD pin during wake comes in.

In the end, all modern CAN transceivers will respond to the same WUP requirements and a high-to-low transition will occur on RXD. If the MCU is active during sleep, it can be configured to wake on a falling edge of this pin. If the MCU was off, it can check the state of the RXD pin at startup to determine the source of the wake.

Let me know if you have any more questions. 

Regards, 
Eric Schott