33742_08 Datasheet, PDF(45/70 Page) Freescale Semiconductor, Inc – System Basis Chip with Enhanced High Speed CAN Transceiver

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33742_08 Datasheet, PDF (45/70 Pages) Freescale Semiconductor, Inc – System Basis Chip with Enhanced High Speed CAN Transceiver

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FUNCTIONAL DEVICE OPERATION

OPERATIONAL MODES

RXD FAILURE DETECTION

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

5.0V (e.g., to VDD), RXD will be tied to a high level and the

RXD short to 5.0V can be detected at the next LOW-to-HIGH

transition of the differential receiver. Compete detection

requires three samples.

When the error is detected, an error flag is latched and the

CAN driver is disabled. The error is reported through the SPI

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 pulldown 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).

CANL

CANH

Diff Output

Sampling

RXD Output RXD Short to VDD

RXD Flag Latched

RXD Flag

RXD no longer shorted to VDD

Note RXD Flag is neither the RXPR bit in the LPC register

nor the CANF bit in INTR 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.

Analog Integrated Circuit Device Data

Freescale Semiconductor

33742

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