Analog Integrated Circuit Device Data

20

Freescale Semiconductor

33902

FUNCTIONAL DEVICE OPERATION

OPERATIONAL MODES

communication, and thus a proper failure identification

(requires 5 pulses on TXD). The bus dominant clamp circuit

will help to determine such failure situation.

RX Permanent Recessive Failure

The aim of this detection is to diagnose an external

hardware failure at the RX output pin and ensure that a

permanent failure at RX does not disturb the network

communication. If RX is shorted to a logic high signal, the

CAN protocol module within the MCU will not recognize any

incoming message. In addition it will not be able to easily

distinguish the bus idle state and can start communication at

any time. In order to prevent this, an RX failure detection is

necessary.

Implementation for Detection

The proposed implementation is to sense the RXD output

voltage at each low to high transition of the differential

receiver. Excluding the internal propagation delay, the RXD

output should be low when the differential receiver is low. In

case of an external short to VDD at the RXD output, RXD will

be tied to a high level and can be detected at the next low to

high transition of the differential receiver.

As soon as the RXD permanent recessive is detected, the

RXD driver is deactivated.

Once the error is detected, the flag is latched and the

driver is disabled. The error is reported at ERR pin and via

P_SPI.

Recovery Condition

The internal recovery is done by sampling a correct low

level at the Bus as shown in Figure 16.

CANH

CANL

Diff

VDD

Rxsense

RX driver

RXD

TXD

TX driver

60

VIO

Logic

Diag

CANL&H

Diff output

RXD output

RX short to VDD

Prop delay

RX flag

RX flag latched

Sampling

Sampling

The RX flag is not the RXPR bit in the LPC register, and neither is the CANF in the INTR register.

CANL&H

Diff output

RXD output

Rx short to VDD

RX flag (internal signal)

Rx flag latched

Sampling

Sampling

Rx no longer shorted to VDD