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ST72521 数据表(PDF) 149 Page - STMicroelectronics |
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ST72521 数据表(HTML) 149 Page - STMicroelectronics |
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149 / 211 page  ST72521 149/211 CONTROLLER AREA NETWORK (Cont’d) The figures below show the abort behaviour in the four possible cases. Figure 75. Abort and successful transmission In this case the abort request performed during the transmission has no effect, as the first transmis- sion is successful. Figure 76. Abort and transmission delayed by busy CAN bus In this case the NRTX bit is set to abort the trans- mission after the first attempt. As the first attempt is successful the READY and BUSY bits are reset by pCAN and the transmit buffer becomes empty. An abort is no longer required. Figure 77. Abort and error during transmission In this case NRTX (abort request) is set before the error, thus pCAN resets READY and BUSY after the error (the first attempt). The abort has been successful and the transmit buffer is empty. Figure 78. Abort and arbitration lost In this case the the NRTX bit is set but has no ef- fect, as the previous transmission attempt failed due to an arbitration lost. The application waits for the falling edge of BUSY bit and checks that READY is still set. This is the case, this means pCAN has lost the arbitration and LOCK bit can be safely reset. Abort is immediate and pCAN resets the READY and BUSY bits. Timing Considerations As no interrupt signals that an abort has been suc- cessful, the application has to wait until the trans- mit buffer is empty (transmission has been aborted or transmitted successfully). This time can vary depending on the case in which the abort is per- formed (arbitration lost, error or successful trans- mission). To show the impact of the software work- around on this timing behaviour Figure 79 and Fig- ure 80 compare the reference behaviour (worst case when abort is done by LOCK only) with the behaviour when NRTX, BUSY and LOCK bits are used. Figure 79. Abort by LOCK only - Reference behaviour CAN TX CAN RX LOCK READY BUSY TX RQST NRTX ABORT RQST CAN TX CAN RX LOCK READY BUSY TX RQST NRTX ABORT RQST CAN TX CAN RX LOCK READY BUSY TX RQST NRTX ABORT RQST Error CAN TX CAN RX LOCK READY BUSY TX RQST NRTX ABORT RQST CAN TX CAN RX LOCK READY BUSY TX RQST NRTX ABORT RQST |
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