ST92F124 datasheet(418/429 Pages) STMICROELECTRONICS

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ST92F124/F150/F250 - KNOWN LIMITATIONS

KNOWN LIMITATIONS (Cont’d)

Workaround 2

Workaround 2 (see Figure 7) first tests that

FMP=2 and the CAN cell is receiving, if not the

FIFO can be released immediately. If yes, the pro-

gram goes through a sequence of test instructions

on the RX pin that last longer than the time be-

tween the acknowledge dominant bit and the crit-

ical 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 (1MBd @ 8MHz CPU clock).

Figure 6. Reception with TCAN=12/fCPU and sampling time is 16/fCPU

Figure 7. Workaround 2 in Assembler

RR R

R RR

RR R

R RR

RR R

CAN Bus signal

Sampling of Rx pin

asm (“

Bytes/cycles

spp #48

set CAN0_CTRL page

2/4

Use spp #36 for CAN1

r0, R244

For FIFO 02/4

NB: Replace R244 with R245 for FIFO 1*/

and

r0, #3

3/6

r0, #2

3/6

jxnz

_release

(JRNE instruction)

2/6

if FMP is not 2 then FIFO

release can be done

pushw

RR232

push working group

2/8 or 10

srp

#31

set group F as working group

2/4

btjf

r1.5, _release

REC bit of CMSR register

3/6 or 10 if jmp

btjf

r12.3, _release

sample RX bit for 8 bit time

3/6 or 10 if jmp

btjf

r12.3, _release

ie. 11 btjf instructions

3/6 or 10 if jmp

btjf

r12.3, _release

3/6 or 10 if jmp

btjf

r12.3, _release

3/6 or 10 if jmp

btjf

r12.3, _release

3/6 or 10 if jmp

btjf

r12.3, _release

3/6 or 10 if jmp

btjf

r12.3, _release

3/6 or 10 if jmp

btjf

r12.3, _release

3/6 or 10 if jmp

btjf

r12.3, _release

3/6 or 10 if jmp

btjf

r12.3, _release

3/6 or 10 if jmp

btjf

r12.3, _release

3/6 or 10 if jmp

_release:

or R244, #32

set RFOM bit of CRFR0 register

3/6

NB: Replace R244 with R245 for FIFO 1*/

popw

RR232

restore previous working group

2/10

“);

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