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DTACK Issue #5
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Hi, I assume then there might be an error on the current GAL routing and that fix you applied would be required if using 4 SRAM chips? Thanks |
Thanks for posting this fix. |
Driving DTACK push-pull is likely to prevent other expansion cards from working that are installed alongside the RAM expansion. (It seems that this is currently accidentally being avoided by the DTACK line not being driven at all). |
On the schematic DTACK is Pin 1 on the GAL where as in the PLD file its is Pin 23. Since it is an output it can't be on Pin 1.
I noticed this after assembling a board with 3 SRAM chips and testing in an ACE with a Sharp 1 MB expansion on the internal memory expansion header. I worked around the issue by cutting the link on the "1 MB" jumper and adding a wire between the middle pad of the jumper and Pin 1 of the GAL. I tested with a program found somewhere called "mtest.r" which passed.
I also felt that the GAL was getting warm from driving DTACK as push-pull rather than open drain so I added an open drain buffer. It is maybe less warm, but I don't have a way to measure well. It worked with and without the buffer so this may not be an issue. I think using the tri-state outputs on the GAL should be able get the same effect.
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