Elapsed_Int-18 Alteration

[Rob]

I've added a slight alteration to Darrel's Elapsed_Int-18 to get 48MHz oscillator. Could somebody please check if this is correct?

' ------------------------------------------------------------------------------
' To calculate a constant for a different crystal frequency - see this web page
' http://www.picbasic.co.uk/forum/showthread.php?t=2031
' ------------------------------------------------------------------------------
Asm
  IF OSC == 4                       ; Constants for 100hz interrupt from Timer1
TimerConst = 0D8F7h                 ; Executed at compile time only
  EndIF
  If OSC == 8
TimerConst = 0B1E7h
  EndIF
  If OSC == 10
TimerConst = 09E5Fh
  EndIF
  If OSC == 16
TimerConst = 063C7h
  EndIF
  If OSC == 20
TimerConst = 03CB7h
  EndIF
  If OSC == 48                      ; If an oscillator of 48MHz is used e.g.
TimerConst = 015A4h                 ; in USB PICs, set TimerConst to 015A4h.
BSF T1CON,T1CKPS0, 0              ; MUST also set to prescale of 2 NOT 1  
  EndIF
; -----------------  ADD TimerConst to TMR1H:TMR1L
ADD2_TIMER   macro
;    CHK?RP  T1CON
    BCF     T1CON,TMR1ON, 0        ; Turn off timer
    MOVLW   LOW(TimerConst)        ;  1
    ADDWF   TMR1L,F, 0             ;  1    ; reload timer with correct value
    BTFSC   STATUS,C               ;  1/2
    INCF    TMR1H,F, 0             ;  1
    MOVLW   HIGH(TimerConst)       ;  1
    ADDWF   TMR1H,F, 0             ;  1
    endm

The timer seems to run doubly quick like the prescale setting is not being set correctly. If I set the prescale setting in the ADD2_TIMER macro though it works correctly but will increase the amount of instructions required therefore altering the amount of time a tick takes:

; -----------------  ADD TimerConst to TMR1H:TMR1L
ADD2_TIMER   macro
;    CHK?RP  T1CON
    BCF     T1CON,TMR1ON, 0        ; Turn off timer
    If OSC == 48                   ; IF OSCILLATOR IS 48MHz THEN
        BSF T1CON,T1CKPS0, 0      ; SET PRESCALE VALUE TO 2
    EndIF
    MOVLW   LOW(TimerConst)        ;  1
    ADDWF   TMR1L,F, 0             ;  1    ; reload timer with correct value
    BTFSC   STATUS,C               ;  1/2
    INCF    TMR1H,F, 0             ;  1
    MOVLW   HIGH(TimerConst)       ;  1
    ADDWF   TMR1H,F, 0             ;  1
    endm

Thanks in advance

Rob