Programming:Keyboard scanning

This is the name of the phenomenon when a "ghost" or "phantom" key is generated when you press some combinations of keys. It is caused by the design of the keyboard hardware. It has been tested and confirmed that keyboard clash doesn't occur on an English CPC664 keyboard.

For example, pressing Q,A,P will also generate :.

The ':' key is the "ghost" key.

The problem is much bigger in 2 player games.

It is possible to avoid or minimize the problem by carefully choosing the controls. The problem also occurs when using digital joysticks because these are part of the keyboard matrix.

If you visualize a rectangle in the keyboard matrix above, where 3 of the corners are 3 keys you have pressed. Then the key in the fourth corner will also be pressed this is the "ghost" key.

On Plus it is possible to avoid it mostly by having one player on analogue joystick and the other on keyboard/digital joystick.

This is a routine which scans one line of the keyboard directly. It is approximately 8% faster than the firmware routine and doesn't enable interrupts.

Input: A = line (&40 - &49)

Output: A = hardware keyboard value

Destroyed: BC

LD A,kbdline ; from 0 to 9 with bdir/bc1=01
LD BC,#F782 ; PPI port A out /C out 
OUT (C),C 
LD BC,#F40E ; Select Ay reg 14 on ppi port A 
OUT (C),C 
LD BC,#F6CO ; This value is an AY index (R14) 
OUT (C),C 
OUT (C),0 ; Validate!!
LD BC,#F792 ; PPI port A in/C out 
OUT (C),C 
DEC B 
OUT (C),A ; Send KbdLine on reg 14 AY through ppi port A
LD B,#F4 ; Read ppi port A 
IN A,(C) ; e.g. AY R14 (AY port A) 
LD BC,#F782 ; PPI port A out / C out 
OUT (C),C 
DEC B ; Reset PPI Write 
OUT (C),0


Now check for the value from the table e.g. with''''bit x,A''''and a condition e.g.''''jp z,xxxx''''or''''call z,xxxx''''

Please note:

• Bit = 0: Key is pressed
• Bit = 1: Key is not pressed

This routine scans the complete keyboard and writes all key status bits in a map.

keymap  ds 10  ;map with 10*8 = 80 key status bits (bit=0 key is pressed)

keyscan di              ;1 ##%%## C P C   VERSION ##%%##
        ld hl,keymap    ;3
        ld bc,#f782     ;3
        out (c),c       ;4
        ld bc,#f40e     ;3
        ld e,b          ;1
        out (c),c       ;4
        ld bc,#f6c0     ;3
        ld d,b          ;1
        out (c),c       ;4
        ld c,0          ;2
        out (c),c       ;4
        ld bc,#f792     ;3
        out (c),c       ;4
        ld a,#40        ;2
        ld c,#4a        ;2 44
loop    ld b,d          ;1
        out (c),a       ;4 select line
        ld b,e          ;1
        ini             ;5 read bits and write into KEYMAP
        inc a           ;1
        cp c            ;1
        jr c,loop       ;2/3 9*16+1*15=159
        ld bc,#f782     ;3
        out (c),c       ;4
        ei              ;1 8 =211 microseconds
        ret

In the CPC world it is more common that a game supports both keyboard and digital joystick for those users who don't own joysticks. In addition, it is also nice if the keys could be redefined but that is not necessary.

There are some common keyboard configurations used:

• Cursor keys & SPACE - This configuration is often used and is great for the CPC6128 and Plus and playing using emulators. This is not good for CPC 464 and 664 (which have uncomfortably arranged cursor keys)

• Q,A,O,P,SPACE - (Q up, A down, O left, P right, SPACE fire) This is a good solution, and it's a sort of standard (used by many games). It is best for english QWERTY keyboards, on french AZERTY it'd be A-Q-O-P (but because the decoding of the keys is done in software, normally by the OS, the bits in the keyboard matrix are actually the same).