mirror of https://github.com/option8/gp2io.git
29 lines
1.3 KiB
ArmAsm
29 lines
1.3 KiB
ArmAsm
ORG $34A
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ANN2HI EQU $C05D
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ANN2LO EQU $C05C
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BUTT1 EQU $C062
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BYTETRCVD EQU $EF
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BUTT1HILO EQU $EE
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READBYTE LDX #$09 ; reading 8 bits requires 9 transitions.
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LDA #$00 ; clear the accumulator
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STA BYTETRCVD ; $EF is staging for received byte, set 0
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STA BUTT1HILO ; $EE is staging for each bit, set 0
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CTS STA ANN2HI ; set ANN2 HIGH, indicate to AVR "Clear to Send"
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LOOPSTART LDY #$FF ; start wait loop
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LOOPCOUNT INY ; Increment Y - rolls over to 0 on first run (loopcount)
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LDA BUTT1 ; check PB1 status hi/low
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AND #$80 ; clear 0-6 bits (just need bit 7, all others float)
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CMP BUTT1HILO ; compare Accumulator 7 bit with $EE, previous PB2 value
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BNE BITCHANGE ; if PB2 has changed state, store in $EE (bitchange)
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BEQ LOOPCOUNT ; bit hasn't changed yet, return to (loopcount)
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BITCHANGE STA BUTT1HILO ; (bitchange)
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CPY #$44 ; if the loop count is more than 68, bit is one. Bit is set in Carry
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ROL BYTETRCVD ; rotate the new bit into $EF, our result byte.
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DEX ; decrement X, our bit count
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BNE LOOPSTART ; if bit count is not yet full, loop back to (loopcount)
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CTSOFF STA ANN2LO ; if bit count is full, set ANN2 LOW, CTS off
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LDA BYTETRCVD ; puts received byte into Accumulator
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RTS ; return with byte in Accumulator
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