Well, 6500 (and 6800) series processors use the Phi2-low time to prepare for a data transfer and the Phi2-high time to actually perform the transfer. Preparing for the transfer entails the propagation of a new address onto the address bus... and it also entails a potential change of "ownership" of the
data bus. During write cycles the data bus is driven by the CPU, and during read cycles any of several other devices (RAM, ROM, I/O) may take their turn.
Getting the devices to start and stop driving the bus is a somewhat inexact science. One of the reasons is, the devices don't instantly respond to changes on their /CE and /OE inputs -- and, the delay can only be predicted approximately (the margin of error could easily be 10 or 20 ns -- or more). This implies a threat of short-lasting bus collisions (bus contention). For example, in a handover from Device A to Device B, there's a risk Device B may begin driving the bus before Device A has stopped -- and for 10 or 20 ns the computer's Gnd and Vcc network gets blasted with a large current spike.
It's not unusual to see designs which ignore this issue (the collision
is brief, and arguably may be considered tolerable). But the solution is simple. Just arrange your glue logic so no device drives the data bus during the Phi2-low time. This guarantees a big, fat 500 ns timing "cushion" -- enough to soak up any variability and ensure that every bus handover is courteous and gentle.
( I can't specifically explain how a large current spike might produce the symptom we're seeing here, and that's why my concern is only a hunch. But clearly we have a computer that's acting "funny" -- it seemingly executes some code correctly, and other code not. And "funny" behavior is often the result of noise issues such as those caused -- especially on breadboards -- by current spikes. )
I'll be interested to learn the result of the experiment!
-- Jeff
Well moving /OE to /Q didn't do anything. Still doesn't work. I've noticed I thing though. Gareth's loop
works fine ie the probe goes from high to low to high. But what doesn't work is code like this:
which should essentially do the same thing. If I change lda #$00 to lda #$55 and the second lda to lda #$aa which should blink the probe but the VIA gets stuck on $55 and I can't change the value after that no matter what I've tried.
Is my VIA broken in some way? BTW I've double checked that every wire goes where it should go (ie everything is connected as the schema shows).
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Good luck, have fun, and keep us posted!
