
Analog Integrated Circuit Device Data
44
Freescale Semiconductor
33742
FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
RECOVERY CONDITION
The SBC will try to recover from a bus fault condition by sampling for a correct low level at TXD, as illustrated in
Figure 26.As soon as an RXD permanent recessive is detected, the RXD driver is deactivated and a weak pull-down current source is
activated in order to allow recovery conditions. The driver stays disabled until the failure is cleared (RXD no longer permanent
recessive) and the bus driver is activated by an SPI register command (write 1 to the CANCLR bit in the CAN register).
Figure 26. RXD Recovery Conditions
TXD PERMANENT DOMINANT FAILURE
PRINCIPLE
In the event TXD is set to a permanent low level, the CAN bus is set into dominant level, and no communication is possible.
The SBC has a TXD permanent timeout detector. After timeout, the bus driver is disabled and the bus is released in a recessive
state. The TXD permanent dominant failure is reported in the TIM1 register.
RECOVERY
The TXD permanent dominant is used and activated also in case of TXD short to RXD. The recovery condition for TXD
permanent dominant (recovery means the reactivation of the CAN drivers) is done by an SPI command and is controlled by the
MCU.
The driver stays disabled until the failure is cleared (TXD no longer permanent dominant) and the bus driver is activated by
an SPI register command (write logic [1] to CANCLR bit in the CAN register).
TXD TO RXD SHORT CIRCUIT FAILURE
PRINCIPLE
In the event the TXD is shorted to RXD when an incoming CAN message is received, the RXD will be at a LOW. Consequently,
the TXD pin is LOW and drives CANH and CANL into the dominant state. The bus is stuck in dominant mode and no further
communication is possible.
DETECTION AND RECOVERY
The TXD permanent dominant timeout will be activated and release the CANL and CANH drivers. However, at the next
incoming dominant bit, the bus will be stuck again in dominant. In order to avoid this situation, the recovery from a failure
(recovery means the reactivation of the CAN drivers) is done by an SPI command and controlled by the MCU.
INTERNAL ERROR OUTPUT FLAGS
There are internal error flags to signal whenever thermal protection is activated or over-current detection occurs on the CANL
or CANH pins (THERM-CUR bit). The errors are reported in the CAN register.
CANL
Diff Output
RXD Output
RXD Short to VDD
RXD Flag
RXD Flag Latched
Sampling
Note RXD Flag is neither the RXPR bit in the LPC register
RXD no longer shorted to VDD
CANH
nor the CANF bit in INTR register.