In SX Microcontrollers, SX/B Compiler and SX-Key Tool, John Kauffman wrote:

File is self-explanatory, I think. 
Symptom: LED stays on.

 

 

' =========================================================================
'
'   File...... INT-RTCC-01.SXB
'   Purpose... Demonstrate simple use of interupt triggered by RTCC
'   Author.... John Kauffman
'   E-mail.... 
'   Started...
'   Updated... 2006 04Apr 12 - 17:42 
'
' =========================================================================


' -------------------------------------------------------------------------
' Program Description
' -------------------------------------------------------------------------
' Purpose = Demonstrate simple use of interupt triggered by RTCC
' objective: divide down RTCC interupts until one flash per 4 seconds.
' 4,000,000 / 256 / 256 / 256 = 0.25 = once per 4 seconds.
' RTCC prescaler set to 256
' INterupt increments ISRCOunter by 1
' In MAIN program when ISRCOUnter = 255 then increment ISRMultiplier1
' In MAIN program when ISRMultiplier1 = 255 then increment ISRMultiplier2
' When ISRMultiplier2 gets to 255 LED flashes 

' -------------------------------------------------------------------------
' Device Settings
' -------------------------------------------------------------------------

DEVICE          SX28, OSC4MHZ, TURBO, STACKX, OPTIONX
FREQ            4_000_000

' -------------------------------------------------------------------------
' IO Pins
' -------------------------------------------------------------------------
LED  VAR RC.0   
' LED circuit:
' RC.0 - 470ohm - LEDcath - LEDanode - Vdd

' -------------------------------------------------------------------------
' Constants
' -------------------------------------------------------------------------


' -------------------------------------------------------------------------
' Variables
' -------------------------------------------------------------------------
ISRCounter VAR byte
ISRMultiplier1 VAR byte
ISRMultiplier2 VAR byte


' =========================================================================
INTERRUPT
ISR_Start:
ISRCounter = ISRCounter + 1
ISR_Exit:
  RETURNINT ' {cycles}                                 

' =========================================================================
PROGRAM Start

Start:
'Pin      76543210
' Following line sets only RC.0 (LED) to be an output
TRIS_C = $11111110 'Alternate = $FE
OPTION = $100000111
'bit      76543210
' option bits (left-7 to right-0)
' RTW 1 = RTCC ( not W)
' RTI 0 = RTCC roll-over interupt is enabled
' RTS 0 = Increment is form internal cycle (not expternal pin) 
' RTE 0 = Increments on low to high transition (should have no effect since not on pin
' PSA 0 = Prescaler assigned to RTCC (not W)
' 1 = Prescaler = 1:256
' 1 = Prescaler = 1:256
' 1 = Prescaler = 1:256

Main:
' 4_000_000 / 256 / 256 /256 = 0.238 = once per 4 seconds
' first divide by 256 is in prescaler
IF ISRCOUnter = 255 Then
  ISRMultiplier1 = ISRMultiplier1+1
  IF ISRMultiplier1 = 255 THen
      ISRMultiplier2 = ISRMultiplier2+1
      IF ISRMultiplier2 = 255 THen
 HIGH LED
 Pause 500
 LOW LED
      Endif
  Endif
Endif

---------- End of Message ----------


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