asm interrupts

[BrianT]

I use a 'soft' interrupt by clearing the PIR1.0 register and preloading TMR1 to a value that gives me 24 mSecs. I start the timer routine and in my software I frequently check if PIR1.0 has been set by the TMR1 overflow. Being a code based 'check it when you can' the timing is a little flakey but it works fine in my application. I wanted to eliminate interrupt based code as far as possible since the overall timing can then get very hard to manage if interrupts get nested.

I do run SWDTEN with a 58 mSec hard reset to keep things in line if the soft check of PIR1.0 should not work as expected.

This is a snip from PIC16F88 code.

MainLoop:
   'Housekeeping 
   clearwdt
   WDTCON = %00001001 ' invoke s/w WDT, 58 mSec timeout, 
'   WDTCON = %00001010  ' has a recovery time about 110 mSec.
   OPTION_REG = %10001010  ' No wpu, prescaler assigned to WDT & /4
   INTCON = %00000000      ' disable all interrupts
   ANSEL =  %00000000      ' all digital
   CMCON  = %00000111      ' Comparators OFF to allow inputs on pins
   ADCON0 = %00000000
   ADCON1 = %00000000
   PIE1 = %00000001        ' enable TMR1 overflow interrupt
   PIE2 = %00000000
   PIR2 = %00000000
   TRISB = %11001111   

   T1CON = %00100000       ' stop TMR1
   PIR1.0 = 0              ' clear bit 0 TMR1 interrupt flag
   TMR1H = $8C
   TMR1L = $A0             ' loads 36000 - gives 24 mS on CRO    
   T1CON = %00100001       ' start TMR1

FindSync:
'high txd
   ' The exits from this loop are via TimeOut or 850 loops of steady '1'
   ' Noise should keep the program stuck in this block until timeout 
   ' but it still finds occasional sync pattterns in pure noise

   if pir1.0 = 1 then timeout
   if radioppm = 0 then findsync

   ' RadioPPM has gone high here - check that it stays high
   ' A byte variable gives a loop time about 2.7 uSecs.
   ' A word variable loops in about 9.2 uS so there are less chances 
   '  to detect a low spike during the sync period.

   for a = 0 to 255  ' measured at 2300 uS 
      if radioppm = 0 then findsync ' noise or channel pulse 
      if pir1.0 = 1 then timeout    ' been here too long 
   next a
   for a = 0 to 255  
      if radioppm = 0 then findsync ' noise or channel pulse 
      if pir1.0 = 1 then timeout    ' been here too long 
   next a
   for a = 0 to 255  
      if radioppm = 0 then findsync ' noise or channel pulse 
      if pir1.0 = 1 then timeout    ' been here too long 
   next a
   for a = 0 to 200 '100  
      if radioppm = 0 then findsync ' noise or channel pulse 
      if pir1.0 = 1 then timeout    ' been here too long 
   next a
   if badch > 2 then timeout
   goto sendpacket ' 250 contiguous 1's - sync or stuck high   
   
TimeOut:  ' arrive here if TMR1 overflow is detected 
          ' OR we were sent here by bad receive frame 
   high txd 'low txd 
   if pir1.0 = 0 then timeout ' wait until timer overflows 
   'Strategy
   ' The failsafe values are predetermined by the FailCh[n] array.
   ' When a failsafe state is detected, the system slowly migrates from 
   ' LastCh[n] count to the FailCh[n] count over a number of frames
   '  using FailLimit as the maximum increment or decrement
   high p1led
   failch[0] = 0     ' throttle - cut to zero
   failch[1] = 125   ' aileron - centralise
   failch[2] = 125   ' elevator - centralise
   failch[3] = 165   ' rudder - a small left rudder
   failch[4] = 125   ' gear - normally 30 or 220.  125 = failsafe 
   failch[5] = 125   ' Aux 1  
   failch[6] = 125   ' Aux 2 
   failch[7] = 125   ' Aux 3

HTH
BrianT