Quote:
Originally Posted by Timbucus
Just plugged in my Memory expansion and the changing character in F27 -(I am writing to B27 remember) is back on CHARSET. So it seems the 100pF will not help once we have a more complex bus. I will see what I have in smaller caps, not sure I have four off of any smaller and i must start digging in the code and see what can be determined on behavior.
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I think the most likely time for the corruption is when NENIN is raised, and I think it was already confirmed that the address is driven later by the PIC so it seems unlikely the PIC is driving the address too soon.
My suspicion is that the address lines are pulling high before the NWDS is pulled high. The capacitors on the address lines would help in that case, but so would making sure there are no pull ups enabled on the PIC inputs on the address lines.
In most cases enabling pull ups on inputs would be a good thing, to avoid them floating and self bias both high and low transistors of the input. In this case they would only float until driven by the PIC a short time later so they shouldn’t stay at an indeterminate level.