Rabbit 2000 - Open-Drain Outputs Used for Key Scan

44 Rabbit 2000 Microprocessor
push af ;10
push hl
ld hl,(ptr) ;11
ld a,(hl) ;5
ioi ld (port),a ; 13 output data
inc hl
ld a,0x0f ;4
and l ; see if hl at end of cycle
jr z,step2
ld (ptr),hl
pop hl
pop af
reti
step2:
ld a,(beginptr)
ld l,a
ld (ptr),hl ;13
pop hl ;7
pop af
reti
; 103 clocks total
4.2 Open-Drain Outputs Used for Key Scan
The parallel port D outputs can be individually programmed to be open drain. This is use-
ful for scanning a switch matrix, as shown in Figure 4-2. A row is driven low, then the col-
umns are scanned for a low input line, which indicates a key is closed. This is repeated for
each row. The advantage of using open-drain outputs is that if two keys in the same col-
umn are depressed, there will not be a fight between a driver driving the line high and
another driver driving it low.
Figure 4-2. Using Open-Drain Outputs for Key Scan
o.d.
o.d.
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