' Interrupt Rate Trick
DEVICE SX28, OSCHS2, TURBO, STACKX, OPTIONX
FREQ   78_750_000

 

' Define pins
UnusedRA  PIN RA INPUT PULLUP
UnusedRB  PIN RB INPUT PULLUP
UnusedRC  PIN RC INPUT PULLUP

 

FreqPin   PIN RB.0 OUTPUT

 

 

' Define variables
' None

 

 

' *** Interrupt rate "trick" ***
' The oscillator is 78.750 MHz or 22 times the colorburst.
' We want exactly 228 colorburst times per line.
' So 228 * 22 = 5016 <- We want to interrupt every 5016 clocks
' We must use a prescaler of 1:32
' 156 * 32 = 4992 clocks
' 157 * 32 = 5024 clocks
'
' Hmmm, how can we get the interrupt rate to be exactly 5016. think...think...think (taps head)
'
' Well the prescaler counts from 0 to 31 then overflows back to zero and increments RTCC,
'  and writing a value to RTCC will reset the prescaler to zero (this is the "trick")
'
' So if we wait until the prescaler is 24, then reset it back to zero,
'  we will add 24 cycles to NEXT interrupt period
'
' If you look at this code, at exactly 23 cycles into the interrupt we do a "CLR RTCC",
'  RTCC is already zero, but this has the effect of clearing the prescaler back to zero.
'

' NOTE: If the prescaler is set to 8,16,32,64,128 or 256, the instruction that modifies RTCC is 1 count

'       If the prescaler is set to 1, the instruction that modifies RTCC is 3 counts

'       If the prescaler is set to 2 or 4, the instruction that modifes RTCC is 5 counts

'

' For low prescale values you will have to either set RTCC to a value, or adjust your RETURNINT value.

'

'

'======================================================================================== 

' Interrupt routine (occurs every 5016 clocks, or 15699.73 times a second)

INTERRUPT NOPRESERVE '(4) <- cycle count
ASM
  JMP $+1         ' (3=7)
  JMP $+1         ' (3=10)
  JMP $+1         ' (3=13)
  JMP $+1         ' (3=16)
  JMP $+1         ' (3=19)
  JMP $+1         ' (3=22)
  NOP             ' (1=23)
  CLR RTCC        ' (1=24) This will reset the prescaler, making the next interrupt take 24 cycles longer

 

  ' Generate a pulse to count
  SETB FreqPin
  MOV FSR,#$FF
  DJNZ FSR,$
  CLRB FreqPin
ENDASM
RETURNINT 156

 

 

PROGRAM Start NOSTARTUP

 

 

Start:
  OPTION = $84 ' Set prescaler to 1:32
  DO
  LOOP
END

 

mov RTCC,#47 ;Sets RTCC Cycles = ((255 - x) +3 +3 +3)
;Where x is (mov RTCC,#x)
;for x = #47, RTCC happens every 217