Setting TMR1 for 24 mSecs
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.
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
