ππ° COMiX-35: Part 11 (PAL key auto-repeat issue)
32Noticed this issue on my homebrew COMiX-35 when set to PAL video mode.
COMiX-35 homebrew clone:
https://github.com/schlae/comix-35
There are three jumpers that should be set depending on whether it is PAL or NTSC video output. I have my board configured for PAL video output as the Y1 crystal is 17.73MHz.
However this causes the key auto-repeat to be triggered incorrectly, when a key isn't pressed. Switching to NTSC (specifically JP3) solves this issue.
There is an output flag on the CDP1871 keyboard encoder (incorrectly labelled RTP) for the key auto-repeat called RPT (pin 35):
"The repeat output flag which is used to indicate that a key is still closed after data has been read from the CDP1871A (DA = high). It remains low as long as the key is closed and is used for an autorepeat function, under CPU control. This output is normally connected to a flag input (EF1 - EF4) of the CDP1800-series microprocessor."
This pin is connected (via a 10KΞ© resistor, R15) to the EF2 input pin (p23) on the CDP1802 CPU. JP3 is also connected to this line, switching between the Q or inverted Q output of a D-type flip flop. There is also a diode between the flip flop output and the EF2 line ... I'm wondering if the PAL one is the wrong way around ... ?
An interesting issue I thought.
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