The TCAN4550-Q1 supports low power sleep mode, and uses a wake-up from the CAN bus mechanism called bus wake via RXD_INT Request (BWRR). Once this pattern is received, the TCAN4550-Q1 automatically switches to standby mode and inserts an interrupt onto the nINT and nWKRQ pins to indicate to a host microprocessor that the bus is active, and it should wake-up and service the TCAN4550-Q1. The low power receiver and bus monitor are enabled in sleep mode to allow for RXD_INT Wake Requests via the CAN bus. A wake-up request is output to the internal RXD_INT (driven low) as shown in Figure 8-7. The wake logic monitors RXD_INT for transitions (high to low) and reactivate the device to standby mode based on the RXD_INT Wake Request. The CAN bus terminals are weakly pulled to GND during this mode, see Figure 7-2.

These devices use the wake-up pattern (WUP) from ISO 11898-2:2016 to qualify bus traffic into a request to wake the host microprocessor. The bus wake request is signaled to the integrated CAN FD controller by a falling edge and low corresponding to a “filtered” bus dominant on the RXD_INT terminal (BWRR).

The wake-up pattern (WUP) consists of

• A filtered dominant bus of at least t_{WK_FILTER} followed by
• A filtered recessive bus time of at least t_{WK_FILTER} followed by
• A second filtered dominant bus time of at least t_{WK_FILTER}

Once the WUP is detected, the device starts issuing wake-up requests (BWRR) on the RXD_INT signal every time a filtered dominant time is received from the bus. The first filtered dominant initiates the WUP and the bus monitor is now waiting on a filtered recessive, other bus traffic does not reset the bus monitor. Once a filtered recessive is received, the bus monitor is now waiting on a filtered dominant and again, other bus traffic does not reset the bus monitor. Immediately upon receiving the second filtered dominant and verification of receiving a WUP, the device transitions the bus monitor into BWRR mode. This indicates all filtered dominant bus times on the RXD_INT internal signal by driving it low for the dominant bus time that is in excess of t_{WK_FILTER.} The RXD_INT output during BWRR matches the classical 8-pin CAN devices that used the single, filtered dominant on the bus as the wake-up request mechanism from ISO 11898-2:2016.

For a dominant or recessive to be considered “filtered”, the bus must be in that state for more than t_{WK_FILTER} time. Due to variability in the t_{WK_FILTER} the following scenarios are applicable.

• Bus state times less than t_{WK_FILTER(MIN)} are never detected as part of a WUP, and thus no BWRR is generated.
• Bus state times between t_{WK_FILTER(MIN)} and t_{WK_FILTER(MAX)} may be detected as part of a WUP and a BWRR may be generated.
• Bus state times more than t_{WK_FILTER(MAX)} is always detected as part of a WUP, and thus, a BWRR is always be generated.

See Figure 8-6 for the timing diagram of the WUP.

The pattern and t_{WK_FILTER} time used for the WUP and BWRR prevents noise and bus stuck dominant faults from causing false wake requests while allowing any CAN or CAN FD message to initiate a BWRR. If the device is switched to normal mode or an under-voltage event occurs on V_CC, the BWRR is lost. The WUP pattern must take place within the t_{WK_TIMEOUT} time; otherwise. the device is in a state waiting for the next recessive and then a valid WUP pattern.

Figure 8-6 Wake-Up Pattern (WUP) and Bus Wake via RXD_INT Request (BWRR)

Figure 8-7 Example timing diagram with TXD_INT DTO