ST92F124 datasheet(416/429 Pages) STMICROELECTRONICS

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ST92F124/F150/F250 - KNOWN LIMITATIONS

KNOWN LIMITATIONS (Cont’d)

Figure 3. Workaround 1 in Assembler

We can assume a time quantum number between

8 and 25. The worst case is when the baud rate

prescaler is 0 (BRP=0) and the time quantum is 8,

ie. TS1+TS2=5. This means a CPU frequency of

8MHz and 1 Mbits/sec for the CAN communica-

tion. In this case the minimum time between the

end of the acknowledge and the critical period is

52 CPU cycles (48 for the 6 bit times + 4 for the

(PROP SEG + TSeg 1). According to the previous

code timing, we need less than 22 cycles from the

time we see the dominant state to the time we per-

form the FIFO release (one full loop + the actual

release) therefore the application will never re-

lease the FIFO at the critical time when this work-

around is implemented.

Timing analysis

- Time spent in the workaround

Inside a CAN frame, the longest period that the Rx

pin stays in recessive state is 5 bits. At the end of

the frame, the time between the acknowledge

dominant bit and the end of reception (signaled by

REC bit status) is 8TCANbit, therefore the max-

im um tim e spe n t in t he wor k ar ou nd is:

8TCANbit+Tloop+Ttest+Trelease in this case or

8TCANbit+68TCPU.

At low speed, this time could represent a long

delay for the application, therefore it makes sense

to evaluate how frequently this delay occurs.

In order to reach the critical FMP=2, the CAN node

needs to receive 2 messages without servicing

them. Then in order to reach the critical window,

the cell has to receive a third one and the applica-

tion has to release the mailbox at the same time, at

the end of the reception.

In the application, messages are not processed

only if either the interrupt are disabled or higher

level interrupts are being serviced.

Therefore if:

TIT higher level + TIT disable + TIT CAN < 2 x T CAN

frame

the application will never wait in the workaround

TIT higher level: This the sum of the duration of all the

interrupts with a level strictly higher than the CAN

interrupt level

TIT disable: This is the longest time the application

disables the CAN interrupt (or all interrupts)

TIT CAN: This is the maximum duration between

the beginning of the CAN interrupt and the actual

location of the workaround

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

_whileloop: btjf r1.5, _release /*

REC bit of CMSR register

3/6 or 10 if jmp

btjf

r12.3, _whileloop /*

RX bit of CDGR register

3/6 or 10 if jmp

_release:

or R244, #32

set RFOM bit of CRFR register

3/6

NB: Replace R244 with R245 for FIFO 1*/

popw

RR232

restore previous working group

2/10

“);

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