ST72561J4-Auto datasheet(237/324 Pages) STMICROELECTRONICS

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ST72561-Auto

beCAN controller (beCAN)

Doc ID 12370 Rev 8

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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

CAN Bus signal

Sampling of Rx pin

a, CRFR

And

a,#3

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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