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WIP documenting interfacing idea

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Daniele Verducci (ZenPenguin) 2021-01-10 07:22:55 +01:00
parent 8959c7add8
commit b64ed1839a
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pat80-io-devices/composite-pal-adapter/software/avr-assembly/main.asm
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@ -3,6 +3,15 @@
; Every line, for 52 times, it loads a byte from memory into PORTA register and then shifts the byte to the left to show another bit (do it 7 times)
; This also displays byte's MSB pixel "for free", as the video pin is PD7 (last bit of PORTA).
;
; INTERFACING WITH PAT80:
; Use PortB as data port. Before writing anything, issue a read (pin RW HIGH) and check the busy pin on the data port.
; If the busy pin is high, retry reading until goes low. When the busy pin goes low, we have
;
; ELECTRONICALLY:
; The data port D0 (= PB0) is tied to ground with a 1KOhm resistance. When the MC is busy drawing the screen, the data port is in
; high impedance state, so that avoids causing bus contention, but when read returns a 0bXXXXXXX0 byte. When the MC starts vsync,
; begins checking the port for data... TODO
;
; PINS:
; Video pin: PA0 (pin 1)
; Sync pin: PC0 (pin 22)