ST72521 datasheet(151/211 Pages) STMICROELECTRONICS

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ST72521

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CONTROLLER AREA NETWORK (Cont’d)

Workaround Description

The bus-off entry works correctly in almost all cas-

es, only when REC is greater than 127 a bus-off

will not be recognized by pCAN. Therefore the

pCAN bus-off signalling (BOFF) is still used but it

needs to be complemented by monitoring TEC by

software.

To detect the bus-off condition by software the ap-

plication has to monitor the value of the TEC reg-

ister periodically. An overflow signals a bus-off

condition. When a bus-off condition has been de-

tected the application must execute the following

sequence to recover from bus-off properly: the ap-

plication stops pCAN by clearing the RUN bit in the

CANCSR register resets all pending transmission

by clearing the LOCK bit in the BCSR register and

starts it again by setting the RUN bit.

To detect the bus-off condition properly, the TEC

monitoring period must be lower than the time be-

tween two overflows. As the problem only occurs

when pCAN is in Error Passive State (REC > 127)

pCAN will continuously try to send a SOF followed

by an Error Passive Flag and a Suspend Trans-

mission. This leads to 26 (1 + 6 + 8 + 3 + 8) bit

times. Each time TEC is incremented by 8, hence

to reach 256 the sequence must be executed 32

times. Under these conditions the shortest se-

quence leading to a TEC overflow lasts 832 bit

times.

Depending on the baudrate the application will

have to adapt the monitoring period, for example

at 500kbps the period must be less than 1600us.

The ‘C’ code below shows an implementation ex-

ample of the monitoring sequence. This code is

called periodically as described above.

To detect the overflow, the test condition must

take into account that TEC might also have been

decremented due to a successful transmission. So

an overflow condition is detected:

IF the current TEC value is lower than the previous

TEC value

AND the difference is greater than the number of

possible successful transmissions during the mon-

itoring period.

In the example above, one message can be sent,

therefore one is added to CANTECR.

************************************************/

/* INITIALISATION

/************************************************/

unsigned char TECReg=0; //Previous value of TEC

unsigned char BusOffFlag=0; //Set to one if bus-off

/************************************************/

/* BUS-OFF MONITORING SEQUENCE

/************************************************/

if( (CANCSR & BOFF) || ( CANTECR+1 < TECReg) )

{

BusOffFlag = 1;

}

else

{

TECReg = CANTECR;

}

ST72521 Datasheet(PDF) 151 Page - STMicroelectronics

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