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   Calculation of Clock Frequency by Leo Nechaev
   [2][Up to Source Code Repository]
     __________________________________________________________________

   This program calculates the clock frequency of 6502-based system. The
   result is stored in location 'Ticks' to 'Ticks+3' in BCD representation
   ('Ticks' - 2 symbols before decimal point, 'Ticks+1' to 'Ticks+3' - 6
   symbols after point), then result is sent via UART to the host (usually
   PC with some kind of terminal programm).

   With minimal modifications (deleting 'SED' and 'CLD' instructions,
   changing ticks constants [from BCD to binary format], deleting
   'terminal' subroutines) it may be used to determine real speed of
   processor.
rgConfig        =       $6000   ; write: D6=1 - NMI is off, D6=0 - NMI is on
rgStatus        =       $6000   ; read:  D6=0 - UART is busy
rgTxD           =       $5000   ; write: data to send via UART

vcNMI           =       $FFFA

Refresh         =       450     ; NMI rate in Hz

        ldx     #<NMI           ; installing the NMI vector
        ldy     #>NMI
        stx     vcNMI
        sty     vcNMI+1
        lda     #$40            ; on start NMI is off
        sta     InUse

Again
        lda     #0
        sta     Flag
        sta     Ticks           ; initializing counter
        sta     Ticks+1
        sta     Ticks+2
        sta     Ticks+3
        lda     #$FE            ; initializing NMI counter (zeropoint minus 2 ti
cks)
        sta     Timer
        lda     #$FF
        sta     Timer+1
        lda     InUse           ; turn on NMI
        and     #$BF
        sta     rgConfig
        sta     InUse

-       bit     Flag            ; waiting for zeropoint minus 1 tick
        bpl     -
        lda     #0
        sta     Flag

-       bit     Flag            ; waiting for true zeropoint
        bpl     -
        lda     #0
        sta     Flag

Main                            ; main counting cycle
;number of ticks per command   sum of ticks
;                          v   v
        lda     Ticks     ;4
        clc               ;2   6
        sed               ;2   8
        adc     #$53      ;2  10
        sta     Ticks     ;4  14
        lda     Ticks+1   ;4  18
        adc     #0        ;2  20
        sta     Ticks+1   ;4  24
        lda     Ticks+2   ;4  28
        adc     #0        ;2  30
        sta     Ticks+2   ;4  34
        lda     Ticks+3   ;4  38
        adc     #0        ;2  40
        sta     Ticks+3   ;4  44
        cld               ;2  46
        bit     Flag      ;4  50
        bpl     Main      ;3  53

        lda     #0        ;2
        sta     Flag      ;4   6
        lda     Ticks     ;4  10
        clc               ;2  12
        sed               ;2  14
        adc     #$95      ;2  16
        sta     Ticks     ;4  20
        lda     Ticks+1   ;4  24
        adc     #0        ;2  26
        sta     Ticks+1   ;4  30
        lda     Ticks+2   ;4  34
        adc     #0        ;2  36
        sta     Ticks+2   ;4  40
        lda     Ticks+3   ;4  44
        adc     #0        ;2  46
        sta     Ticks+3   ;4  50
        cld               ;2  52
        lda     Timer     ;4  56
        cmp     #<Refresh ;2  58
        bne     Main      ;3  61 + 34 (from NMI ISR) = 95
        lda     Timer+1   ; 4
        cmp     #>Refresh ; 2
        bne     Main      ; 3

        lda     InUse           ; turn off NMI
        ora     #$40
        sta     rgConfig
        sta     InUse

        ldx     #0              ; send first string to the host
-       lda     Mes1,x
        beq     +
        jsr     Send
        inx
        jmp     -

+       lda     Ticks+3
        pha
        lsr
        lsr
        lsr
        lsr
        beq     +               ; delete non-significant zero (clock < 10MHz)
        jsr     PrintDigit
+       pla
        and     #15
        jsr     PrintDigit
        lda     #"."            ; decimal point
        jsr     Send
        lda     Ticks+2
        jsr     PrintTwoDigits
        lda     Ticks+1
        jsr     PrintTwoDigits
        lda     Ticks
        jsr     PrintTwoDigits

        ldx     #0              ; send second string to the host
-       lda     Mes2,x
        beq     +
        jsr     Send
        inx
        jmp     -
+       jmp     Again           ; repeat process

PrintTwoDigits
        pha
        lsr
        lsr
        lsr
        lsr
        jsr     PrintDigit
        pla
        and     #15
        jsr     PrintDigit
        rts

PrintDigit
        ora     #$30
        jsr     Send
        rts

Send
        bit     rgStatus
        bvc     Send
        sta     rgTxD
        rts

Mes1
        .db     13
        .tx     "Current clock frequency is "
        .db     0

Mes2
        .tx     " MHz"
        .db     0

Ticks   .br     4,0
Timer   .br     2,0
InUse   .db     0
Flag    .db     0

NMI                        ;6
        pha                ;3   9
        inc     Timer      ;6  15
        bne     +          ;3  18
        inc     Timer+1    ; 5
+       lda     #$80       ;2  20
        sta     Flag       ;4  24
        pla                ;4  28
        rti                ;6  34

   Test results:
     * with chip oscillator "1.8432" the result is '1.843266 MHz'
     * with chip oscillator "20.000000M" and divider by 10 the result is
       '2.000040 MHz'

   Last page update: December 27, 2000.

References

   1. http://6502.org/
   2. http://6502.org/source