Assembly: waitcnt - trouble

T. Salewsky

Hi all,

i have a little problem with waitcnt in assembly.
The following assembly code runs good.
It Toggles with a period of 1,0 s.


con
_clkmode = xtal1 + pll16x
_xinfreq = 5_000_000
var
long stack[noparse][[/noparse]60]
pub Toggle_Main
cognew (@Toggle, stack)
dat
org 0
Toggle
mov dira, Pin ' P0 als output
:loop
mov Time, cnt
add Time, #9

waitcnt Time, Delay
waitcnt Time, Delay
xor outa, Pin
jmp :loop

'

---

Pin long |<0 | |<16
Delay long 40_000_000 'wait 0,5 s
Time res 1
'##########################################

But if i use waitcnt only 1 time -- it will never wait. It Toggles with a Preiode of 2,1 micro sec.
see next.


con
_clkmode = xtal1 + pll16x
_xinfreq = 5_000_000
var
long stack[noparse][[/noparse]60]
pub Toggle_Main
cognew (@Toggle, stack)
dat
org 0
Toggle
mov dira, Pin ' P0 als output
:loop
mov Time, cnt
add Time, #9


waitcnt Time, Delay
xor outa, Pin
jmp :loop

'

---

Pin long |<0 | |<16
Delay long 40_000_000 'wait 0,5 s
Time res 1
'##########################################
-- WHY? I dont understand, but i want to understand.

Is there anyone on earth who can help me?

rokicki

You need to examine how the assembly waitcnt instructions works some more.

waitcnt dest, src

waits until the time in dest, *then* adds src to dest.

So your code should be more like:

(head of loop):
mov Time,cnt
add Time,Delay

loop:
...
waitcnt Time,Delay ' this waits, and updates the time for the next time
xor outa, Pin
jmp :loop

Mike Green

The Delay applies to the second waitcnt. The first waits until cnt+9 occurs (not much time), then adds Delay to the Time. The second waitcnt waits for cnt+9+Delay to occur and leaves Time set to cnt+9+Delay+Delay. I think what you want is to move ":loop" to after the "add".

T. Salewsky

Thank you, now i understand.