ST72521 datasheet(145/211 Pages) STMICROELECTRONICS

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ST72521

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CONTROLLER AREA NETWORK (Cont’d)

10.8.5.3 Unexpected message transmission

Symptom:

The previous message received by pCAN, even if

this message did not pass the receive filter, will be

retransmitted by pCAN with a correct identifier and

DLC but with corrupted data. The data bytes will

be a copy of the identifier bytes IDHR and IDLR in

the following repetitive pattern:

DATA_0 = IDHR

DATA_1 = IDLR

DATA_2 = IDHR

DATA_3 = IDLR

etc.

DATA_7 = IDLR

If no message has been received before the prob-

lem occurs then identifier byte values are random

but the data bytes are in the same repetitive pat-

tern.

Details:

The buffers of the pCAN cell are configurable as

receive or transmit buffers. By default, all buffers

are configured in reception. To use a buffer to

transmit a CAN message the application has to re-

serve this buffer for transmission by setting the

LOCK bit in the BCSR register. So the buffer is

then locked for any further reception and reserved

for transmission.

Once a transmission has been requested by a

write access to data byte 7 of the buffer the appli-

cation might need to abort this transmission re-

quest. To do so, the application can reset the

LOCK bit in the BCSR register.

If the message is pending (RDY bit set) but not

currently being transmitted, then clearing the

LOCK bit will abort it immediately.

If the message is pending (RDY bit set) and cur-

rently being transmitted then the message will not

be interrupted but the CAN core will wait until the

end of this transmission attempt. Then software

must clear the LOCK bit again to abort the trans-

mission.

An unexpected transmission can occur:

IF the application resets the LOCK bit

WHILE

the

CAN

core

preparing

the

transmission1) AND there is no other transmission

pending in another buffer

THEN the LOCK bit is reset but the transmission is

not stopped. Instead the content of the page 0

buffer will be transmitted.

Impact On The Application:

pCAN will echo some messages sent by other

nodes. Identifier and DLC will be correct but data

are corrupted as described previously.

Note 1: The preparation lasts two bit times just be-

fore SOF, this is the critical window during which

the LOCK bit must not be reset by the application.

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