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ST72561AR7-Auto Datasheet(PDF) 237 Page - STMicroelectronics |
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ST72561AR7-Auto Datasheet(HTML) 237 Page - STMicroelectronics |
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237 / 324 page  ST72561-Auto beCAN controller (beCAN) Doc ID 12370 Rev 8 237/324 critical period. But the application may lose additional time waiting in the while loop as we are no longer able to guarantee a maximum of 6 CAN bit times spent in the workaround. In this particular case the time the application can spend in the workaround may increase up to a full CAN frame, depending of the frame contents. This case is very rare but happens when a specific sequence is present on in the CAN frame. The example in Figure 113 shows reception at maximum CAN baud rate: In this case tCAN is 8/fCPU and the sampling time is 10/fCPU. If the application is using the maximum baud rate and the possible delay caused by the workaround is not acceptable, there is another workaround which reduces the Rx pin sampling time. Workaround 2 (see Figure 114) first tests that FMP = 2 and the CAN cell is receiving, if not the FIFO can be released immediately. If yes, the program goes through a sequence of test instructions on the RX pin that last longer than the time between the acknowledge dominant bit and the critical time slot. If the Rx pin is in recessive state for more than 8 CAN bit times, it means we are now after the acknowledge and the critical slot. If a dominant bit is read on the bus, we can release the FIFO immediately. This workaround has to be written in assembly language to avoid the compiler optimizing the test sequence. The implementation shown here is for the CAN bus maximum speed (1 Mbaud @ 8 MHz CPU clock). Figure 113. Reception at maximum CAN baud rate Figure 114. Workaround 2 RD CAN Bus signal Sampling of Rx pin DD D D R R R RR R R RR R R RR R R RR R R Ld a, CRFR And a,#3 Cp a,#2 ; test FMP=2 ? Jrne _RELEASE ; if not release Btjf CMSR,#5,_RELEASE ; test if reception on going. ; if not release Btjf CDGR,#3,_RELEASE ; sample RX pin for 8 CAN bit time Btjf CDGR,#3,_RELEASE Btjf CDGR,#3,_RELEASE btjf CDGR,#3,_RELEASE btjf CDGR,#3,_RELEASE btjf CDGR,#3,_RELEASE btjf CDGR,#3,_RELEASE btjf CDGR,#3,_RELEASE btjf CDGR,#3,_RELEASE btjf CDGR,#3,_RELEASE btjf CDGR,#3,_RELEASE btjf CDGR,#3,_RELEASE btjf CDGR,#3,_RELEASE btjf CDGR,#3,_RELEASE _RELEASE: bset CRFR,#5 |
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