6502 computer: The Keyboard

Entry posted by bluejay April 7, 2021

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Remember how I made that PS/2 keyboard interface? Yeah, I decided to get rid of it. Too complex on the software side. Now I have this 8x8 keyboard matrix. It has every key I thought I might need for the computer.

I know the drawing of the matrix is messy, but who cares. This is how the matrix works: the computer outputs a 1 to all 8 PA bits upon initialization. The other side of the switch matrix is ORed to CB1. If any key is pressed, CB1 will go high and trigger an interrupt. The computer then outputs a 1 to each of the PA bits, and reads from PB to figure out which column the pressed key is on. Then, the computer jumps to a bit of code that figures out exactly which key was pressed. Then it outputs a value into video RAM that the MC6847 can process. (at least, this is what the computer will do. I haven't written the entire keyboard routine yet because I don't know what kind of ASCII the MC6847 wants. Once I've figured that bit out I'll write the rest of the routine.)

Drawing the keyboard layout was surprisingly difficult. I wanted it to be easy to convert into an 8x8 matrix and still be easy to use. Also, I didn't want to deal with having a SHIFT key, so each symbol has its own dedicated key. I don't imagine this would be terribly easy to use, but whatever. You may have noticed the lack of a space bar; I didn't notice it until writing this blog entry. I'll get rid of the CLS key and replace it with a space bar later. The keyboard is 4 rows by 16 columns, which is easily split in half to fit into an 8x8 matrix. The character matrix is shown to the left of the switch matrix. It's gonna be a pain in the ass to decode that.

So, here's the entire BIOS code so far. It's obviously incomplete, and I'll continue to work on it in the future.

Spoiler

.org $c000

VIDEO = $B000
JOYSTICK = $B001
DDRB1 = $B002
DDRA1 = $B003
PCR1 = $B00C
IFR1 = $B00D
IER1 = $B00E
KBIN = $B010
KBOUT = $B011
DDRB2 = $B012
DDRA2 = $B013
PCR2 = $B01C
IFR2 = $B01D
IER2 = $B01E

sysinit:

JSR viainit
   JSR videoinit
   JSR kbinit
   JSR videotest

system:

JSR kbinput

viainit:

   LDA #$FF
   STA DDRB1   ; Output to MC6847
   LDA #$00
   STA DDRA1   ; Input from joystick
   LDA #$00
   STA DDRB2   ; Input from keyboard
   LDA #$FF
   STA DDRA2   ; Output to keyboard
   RTS

videoinit:

   LDA #$00
   STA VIDEO   ; set all control bits of MC6847 to low
   LDA #$00
   STA $9FFF   ; set video counter to zero
   RTS

kbinit:

   LDA #%10010000
   STA IER2
   LDA #%00010000
   STA PCR2
   LDA #$FF
   STA KBOUT
   RTS

videotest:

   LDA #$13
   STA $8000   ; print T
   jsr videocount
   LDA #$04
   STA $8001   ; print E
   jsr videocount
   LDA #$12
   STA $8002   ; print S
   jsr videocount
   LDA #$13
   STA $8003   ; print T
   jsr videocount
   RTS

videocount:
   LDX $9FFF   ; load value of video counter to X
   INX       ; increment video counter
   STX $9FFF    ; store X to video counter
   RTS

kbinput:
   LDA #%00000001
   JSR keysub
   BNE kb1
   LDA #%00000010
   JSR keysub
   BNE kb2
   LDA #%00000100
   JSR keysub
   BNE kb3
   LDA #%00001000
   JSR keysub
   BNE kb4
   LDA #%00010000
   JSR keysub
   BNE kb5
   LDA #%00100000
   JSR keysub
   BNE kb6
   LDA #%01000000
   JSR keysub
   BNE kb7
   LDA #%10000000
   JSR keysub
   BNE kb8
   PLA
   RTI