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[1][Return to Main Page]
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
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