Re: 6522 VIA Questions

From: Ruud Baltissen (
Date: 2001-08-28 11:35:03

Hallo Nick,

> Did I understand mode 2 incorrectly?

No. I was wrong. I completely forgot about this mode.

> None of the documents I've read describe the "shift" bug, though
> I have heard of it. It would be good to know what it exactly is
> (will save pulling out many hairs at a later stage!)

I found this using GOOGLE with "6522 bug":

My conclusion is that the 6522 works fine as long as you turn of interrupts.

> 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 valid
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, connect
PHI2 to the data- and CLR-input, tie the PRESET-input to +5 Volt. The
Q-output now delivers the modified PHI2.

  / __|__
 / /  |_/     Groetjes, Ruud
 \ \__|_\

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