piclist 2001\04\09\120455a >
Thread: precise interrupts
www.piclist.com/techref/microchip/ints.htm?key=interrupt
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face BY : Mike Mansheim email (remove spam text)



> Please refresh me on this by copying in or providing  a URL to my old
> message.

> Bob Ammerman
> RAm Systems
> (contract development of high performance, high function, low-level
> software)

First posting:

WARNING: Untested code ahead!

What we really need is a way to ensure that Timer 1 interrupts every 5000
instructions or 1 millisecond.

The problem is that there can be jitter on reaching the interrupt handler
for the timer. This jitter is caused by 2-cycle instructions, other
interrupt handlers, and interrupts being disabled at 'task' level.

We can get around the jitter problem by always adding a constant to the
timer, rather than trying to set it directly. What we are doing is
effectively advancing time forward the rest of the 65536-5000
instructions to get the timer to interrupt again at the right point.

Here is my suggestion (this code would be in the interrupt handler)

fudge = ...number to account for instructions timer is turned off...
magic_value = D'65536'-D'5000'+fudge

   bcf        T1CON,TMR1ON    ; turn off timer
   movlw   low(magic_value)
   addwf   TMR1L,F
   btfsc     STATUS,C
   incf       TMR1H,F
   movlw   high(magic_value)
   addwf   TMR1H,F
   bsf        T1CON,TMR1ON    ; turn it back on

Also, sorry this is in ASM, not C, but the same thing should work in C.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

Second posting:

The maximum jitter on a PIC is one cycle. This is because some
instructions take one cycle and others take two cycles. It is possible to
completely dejitter the PIC by examining the value of the timer register
in the interrupt handler. For example, I use the following code on an 18C
to do exactly that:

org 8

; The next two instructions are used to eliminate jitter on the
; interrupt timing. When the interrupt occurs, it may have to wait
; an extra cycle becuase a two cycle instruction is in progress.
;
; We can tell the difference by looking at the least significant
; bit of TMR2. If the next instruction skips we use 2 cycles, if it
; doesn't skip then we use 3 cycles. The net result is that we add
; a cycle when needed so that we always get to 'dejittered' at the
; exact same time relative to the actual rollover of TMR2!

btfsc TMR2,0,A
bra     dejittered
dejittered:

Note that this technique should work on all PICs.

Also, on an 18C external interrupts are dejittered by the PIC hardware,
as noted by this quote from the last paragraph of section 7.0 of the
18C452 datasheet:

>For external interrupt events, such as the INT pins or the PORTB input
change interrupt, the interrupt latency will be three to four
instruction cycles. The exact latency is the same for one or two cycle
instructions.

I don't believe that external interrupts are dejittered by any prior PIC
families. Please correct me if I am wrong.

So, my conclusion is that for timer and external interrupts the 18C can
process the interrupt with zero jitter. Non 18C chips can do that for
timer interrupts but not external interrupts.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

>>
{Original Message removed}
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