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| Electronics » the Z80 project » keyboard interface A computer without a keyboard is about as useful as a.. marshmallow crowbar? Anyway, as I didn't want to build one from scratch this time the natural solution was to connect a standard PC PS/2 keyboard.. The internet again obliged with the necessary specifications for the PS/2 signals/protocol and the interface itself ended up consisting of another PIC microcontroller, a 74HC245 buffer and a 74HC74 flipflop to act as the "keycode ready" interrupt line. Minimal code was required for the PIC, it only needed to read the serial clock and data lines from the keyboard, convert the data to a scan code byte, present it to the 245 buffer and pulse a line to the 74HC74 to set the interrupt flag to get the CPU's attention. 16f628 Souce code: Here.
I didn't need to send any data to the keyboard which simplified the design drastically (but it does mean the caps lock LED doesn't work - no biggie.) I was also able to cut down on the traffic from the keyboard interface too - newer PS/2 keyboards seem to default to "Set 2" scancodes and send extra bytes for certain keys (at least two bytes are sent when keys are released as well). A bit of optimizing in the PIC's code and I'd converted all this to a custom system whereby just a single byte is sent for every key (I care about) that is pressed or released (in my system, bits 0-6 hold the "set 2 scancode" AND $7F and bit 7 signifies press/release. The only key that could spoon this up is "F7" which has the "make" scancode $83.. To stop any press / release confusion that code can be trapped by the PIC's software and converted to some other unused scancode before output). The PCB shown on the right was an early version with a 5 volt power 'supply' on the same board (mainly to use up the space). The keyboard interface is now on the same PCB as the IDE interface. |
