SKC - простой контроллер PC/AT-клавиатуры на ATmega88

[m.d.]

After press ZX key combined with shift (extend mode, cursor keys, caps lock, etc..) you must:
1) active only shift in time min. 1/50 sec
2) active next key parallel with shift

After release this combined ZX key:
1) release key only (shift is active)
2) after time min. 1/50 sec also release shift

This is function of original multilayer ZX membrane.
ZX BASIC also work better with this. KEY routine need shift before active key.

Thanks for more precise information on original Speccy keyboard! But is Spectrum+ have an additional circuitry for emulating combo key presses with a single 'extended' key? Exteneded keys in many russian clones consist of 2 standard keys which are pressed simulatenously.