ST72521 datasheet(147/211 Pages) STMICROELECTRONICS

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ST72521

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CONTROLLER AREA NETWORK (Cont’d)

Software Work-around - Devices without Hard-

ware Fix:

To implement a transmission abort under safe

conditions, any reset of the LOCK bit during the

critical window (2 bit times) must be avoided. Two

different cases have to be considered, either the

pCAN enters standby mode after the abort, or the

abort is performed and pCAN keeps running.

Abort followed by STANDBY mode (RUN=0)

In this case, aborting the pending transmissions

can safely be done by first entering STANDBY

mode and then releasing the transmit buffers.

STANDBY mode is entered by resetting the RUN

bit in the CSR register and once the current trans-

mission attempt, even if it fails due to error or lost

arbitration, has been performed, pCAN enters

STANDBY mode (RUN=0). Once in STANDBY

mode the application can abort all pending trans-

missions by resetting the corresponding LOCK bit.

Abort while staying in RUN mode (RUN=1)

Contrary to the STANDBY case described previ-

ously, in the RUN case the application has to han-

dle the error or arbitration lost conditions. In case

of transmission errors, causing the frame to be

transmitted again and again, the application must

set the NRTX bit in the CSR register. This will

cause pCAN to abort the transmission at the end

of the current attempt.

In case of arbitration lost, setting the NRTX bit

does not abort the transmission, therefore the ap-

plication must reset the LOCK bit to abort the

transmission. To avoid resetting the LOCK bit dur-

ing the critical time window, leading to the problem

described at the start of this section, the applica-

tion must monitor the BUSY bit in the BCSR regis-

ter and reset the LOCK bit just after the falling

edge of the BUSY bit. The time between the falling

edge of the BUSY bit and the SOF of the next

transmission attempt is in any case long enough to

guarantee that the LOCK bit is reset before the

critical time window.

The “C” code sequence below shows the software

work-around for both the error and arbitration lost

cases.

_asm("SIM\n");

// Mask interrupts

CANCSR |= NRTX;

// Set non automatic retransmission bit

while(!(CANBCSR & BUSY) &&// Wait till BUSY bit is set

(CANBCSR & RDY) ); // or transmission done

while( CANBCSR & BUSY ); // Wait till BUSY bit is reset (falling edge)

if( CANBCSR & RDY )

{ // transmission still pending -> must be aborted

CANBCSR &= ~LOCK; //Arbitration lost => cancel transmission safel

while( CANBCSR & RDY );// Wait for unlock confirmed

CANCSR &= ~NRTX;// Reset NRTX bit once abort sequence done

_asm("RIM\n");

}

else

{ // No more abort required as RDY bit already reset

CANCSR &= ~NRTX;// Reset NRTX bit once abort sequence done

_asm("RIM\n"); // Enable interrupts

}

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