What did I forget?
Posted: Fri Dec 26, 2025 9:57 pm
It's been oh, about fifty years since I used a 6522 in anger, so a recent attempt to use one ended in slight disaster.
Obviously I've forgotten something obvious (though my code looks very similar to Garth's helpful page) and I can't see what it is.
and of course
In a test loop, it appears to trigger the first interrupt and then never return; attempting to print a continuous output of time values stops at the same point every time I try it, partway through an output word (but by counting the characters output, after close to 20ms).
Help?
Neil
Obviously I've forgotten something obvious (though my code looks very similar to Garth's helpful page) and I can't see what it is.
Code: Select all
;-----------------------------------------------------------------------
; real time clock timer operations
; Ph2i clock is 1.8432MHz; we want a 50Hz interrupt. Set up timer one
; in free run mode to give that frequency and start it running.
; The timer interval is 1843200/50 = 36864; subtract two for timer
; overhead
start_tick:
stz fiftieths
stz seconds
stz minutes
stz hours ; reset the clock to midnight (at the oasis)
lda # lo 36862
sta VIAT1CL ; register 4
lda # hi 36862
sta VIAT1CH ; register 5; load counter with full byte
; and reset IFR6 (timer 1)
lda VIAACR ; register $b
and #%01111111
ora #%01000000 ; repeated interrupts required, no portb output
sta VIAACR ; started
lda #%11000000 ; set interrupt enable for timer one
sta VIAIER ; register $0e
cli ; enable interrupts
rts
;-----------------------------------------------------------------------
; real time clock update on irq
; We do not allow further interrupts in this atomic operation
; There are no interrupt sources other than this (at the moment).
rtc_update:
bit VIAT1CL ; reenable interrupts
pha ; acc is the only register used
inc fiftieths
lda fiftieths
cmp #50
bne rtc_u_x ; not yet fifty?
stz fiftieths
inc seconds
lda seconds
cmp #60
bne rtc_u_x
stz seconds
inc minutes
lda minutes
cmp #60
bne rtc_u_x
stz minutes
inc hours
lda hours
cmp #24
bne rtc_u_x
stz hours
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; FIXME extend to date
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
rtc_u_x:
pla ; restore acc
rti ; restores binary mode etc
Code: Select all
;-----------------------------------------------------------------------
;
; reset and interrupt vectors
code
org 0xfffa
dw init ; nmi
dw init ; reset
dw rtc_update ; irq/brk
Help?
Neil
