FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
RXD FAILURE DETECTION
TXD PERMANENT DOMINANT FAILURE
PRINCIPLE
The SBC senses the RXD output voltage at each LOW-to-
HIGH transition of the differential receiver. Excluding internal
propagation delay, RXD output should be LOW when the
differential receiver is LOW. In the event RXD is shorted to
5.0 V (e.g., to VDD), RXD will be tied to a high level and the
RXD short to 5.0 V can be detected at the next LOW-to-HIGH
transition of the differential receiver. Compete detection
requires three samples.
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 time-out detector.
After time-out, 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.
When the error is detected, an error flag is latched and the
CAN driver is disabled. The error is reported through the SPI
register LPC, bit RXPR.
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.
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.
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).
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).
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.
CANL
CANH
Diff Output
Sampling
Sampling
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.
RXD Short to V
DD
RXD Output
RXD no longer shorted to V
DD
RXD Flag Latched
RXD Flag
Note RXD Flag is neither the RXPR bit in the LPC register
nor the CAN-F bit in INTR register.
INTERNAL ERROR OUTPUT FLAGS
Figure 26. RXD Recovery Conditions
There are internal error flags to signal whenever thermal
protection is activated or overcurrent detection occurs on the
CANL or CANH pins (THERM-CUR bit). The errors are
reported in the CAN register.
33742
Analog Integrated Circuit Device Data
Freescale Semiconductor
40
FREESCALE [ Freescale ]
