FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
In the CAN register, bits D2 and D1 (CAN-F and CAN-UF,
respectively) are used to signal bus failure. Bit D2 reports a
bus failure and bit D1 indicates if the failure is identified or not
(bit D1 is set to logic [1} if the error is not identified).
DETECTION PRINCIPLE
In the recessive state, if one of the two bus lines is shorted
to GND, VDD, or VSUP, then voltage at the other line follows
the shorted line due to bus termination resistance and the
high impedance of the driver. For example, if CANL is shorted
to GND, CANL voltage is zero, and CANH voltage, as
measured by the Hg comparator, is also close to zero.
When the detection mechanism is fully operating any bus
error will be detected and reported in the TIM1/2 and LPC
registers and bit D1 will be reset to logic [0].
In the recessive state the failure detection to GND or
VSUP is possible. However, it is impossible to distinguish
which bus line, CANL or CANH, is shorted to GND or VSUP.
In the dominant state, the complete diagnostic is possible
once the driver is turned on.
NUMBER OF SAMPLES FOR PROPER FAILURE
DETECTION
The failure detector requires at least one cycle of
recessive and dominant state to properly recognize the bus
failure. The error will be fully detected after five cycles of
recessive-dominant states. As long as the failure detection
circuitry has not detected the same error for five recessive-
dominant cycles, the bit “non-identified failure” (CAN-UF) will
be set.
CAN BUS FAILURE REPORTING
CANL bus line failures (for example, CANL short to GND)
is reported in the SPI register TIM1/2. CANH bus line (for
example, CANH short to VSUP) is reported in the LPC
register.
RXD PERMANENT RECESSIVE FAILURE
In addition CAN-F and CAN-UF bits in the CAN register
indicate that a CAN bus failure has been detected.
The purpose of this detection mechanism is to diagnose
an external hardware failure at the RXD output pin and to
ensure that a permanent failure at the RXD pin does not
disturb network communication.In the event RXD is shorted
to a permanent high level signal (i.e., 5.0 V), the CAN
protocol module within the MCU cannot receive any incoming
message. Additionally, the CAN protocol module cannot
distinguish the bus idle state and could start communication
at any time. To prevent this, an RXD failure detection, as
illustrated in Figure 25 and explained below, is necessary.
NON-IDENTIFIED AND FULLY IDENTIFIED BUS
FAILURES
As indicated in Table 11, page 38, when the bus is in a
recessive state it is possible to detect an error condition;
however, is it not possible to fully identify the specific error.
This is called “non-identified” or “under-acquisition” bus
failure. If there is no communication (i.e., bus idle), it is still
possible to warn the MCU that the SBC has started to detect
a bus failure.
TXD
CANL
CANH
Diag
TXD
Driver
Logic
Diff Output
Sampling
Sampling
Sampling
Sampling
2.0 V
V1
VDD
CANH
RXD Sense
RXD Output
RXD Short to V1
RXD
RXD Flag Latched
RXD
Driver
Diff
60Ω
RXD Flag
CANL
Prop Delay
Note RXD Flag is neither the RXPR bit in the LPC register nor the CAN-F bit in the INTR register.
Figure 25. RXD Path and RXD Permanent Recessive Detection Principle
33742
Analog Integrated Circuit Device Data
Freescale Semiconductor
39
FREESCALE [ Freescale ]
