RE: 6522 VIA Questions
Date: 2001-08-29 06:06:28

Hello Ruud

>My conclusion is that the 6522 works fine as long as you turn of
hmmm... thanks for the info. I won't be needing interrupts for this
application. The serial port was to save a 7474 chip, but since I need one
flip-flop anyway, I can use the other.

>> Can you describe this? Aren't all 65xx chips bus /timing compatible?

>Yes, they are. But the C64 isn't. The addresslines of the 6522 must be
>before the ChipSelect signals and PHI2 are activated. With the C64/128 the
>VIC-II still commands the addressbus the first xx nanoseconds of the
>positive half of PHI2. The trick is to slow down the rising edge of PHI2
>using the DOT-clock. Connect the DOT-clock to the CLK-input of a 74,
>PHI2 to the data- and CLR-input, tie the PRESET-input to +5 Volt. The
>Q-output now delivers the modified PHI2.

This explains why my attempts to interface an LCD controller directly to the
bus using the 6502 method in the datasheets was such a failure :(

Had I known, I would have implemented this simple circuit! Is this part of
one of your schematics? Should be part of the hackers-bible.

- Nick


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