ST72F521 datasheet(153/215 Pages) STMICROELECTRONICS

153 / 215 page

ST72F521, ST72521B

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CONTROLLER AREA NETWORK (Cont’d)

10.8.5.5 Bus-off state not entered

Symptom:

pCAN does not enter bus-off state under certain

conditions. This is fixed in FLASH version of

ST72F521 starting from silicon Rev R and in ROM

version ST72521B starting from silicon Rev Y.

Details:

According to the CAN standard, pCAN is expected

to enter bus-off state when TEC (Transmit Error

Counter) is greater than 255.

But if REC (Receive Error Counter) is greater than

127 (Error Passive State) pCAN does not enter

bus-off and the BOFF bit of the CSR register is not

set. To enter bus-off, REC must decrease to a val-

ue lower than 128, this is the case with any correct

reception even if the message is filtered out.

As bus-off state is not entered and pCAN still at-

tempts to transmit its message, after the overflow

the TEC register continues to increment as long as

transmission errors occur.

Impact on the application:

The application will not stop attempting to transmit

CAN messages, even when the bus-off conditions

have been reached, until the transmission has

been successful or the value of REC becomes

lower than 128. However the application will not

disturb the communication of the other nodes on

the CAN network as pCAN is in Error Passive

State.

Figure 82. CAN Error State Diagram showing “BUSOFF not entered” limitation

ERROR PASSIVE

When TECR or RECR > 127, the EPSV bit gets set

When TECR and RECR < 128,

ERROR ACTIVE

BUS OFF

When TECR > 255 and RECR < 128 the BOFF bit

When 128 * 11 recessive bits occur:

- the BOFF bit gets cleared

- the TECR register gets cleared

- the RECR register gets cleared

the EPSV bit gets cleared

gets set and the EPSV bit gets cleared

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