27c256 as CHR on SKROM question
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27c256 as CHR on SKROM question
I was wondering, when using a 32kB eprom on an SKROM board, where there are inputs on MMC1 for chrA15 and chrA16, the eprom has only A0-A14, so how to handle those two pins on MMC1? Should they be grounded or logical 1? Or just left floating?
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infiniteneslives
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Re: 27c256 as CHR on SKROM question
I think you mean outputs. And the rule your thinking about only applies to inputs.jpx72 wrote:I was wondering, when using a 32kB eprom on an SKROM board, where there are inputs on MMC1 for chrA15 and chrA16
The MMC1 is driving those pins low or high depending on what the NES is writing the to MMC1. You don't really know what those values are. So shorting them to gnd/vcc would create high current in that pin (bad).
If you're not using an output just leave it floating, which is what you should do in this case.
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infiniteneslives
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No problem, hopefully that explanation will aid in remembering what to do in the future.jpx72 wrote:Oh .. OH!
I understand now those are outputs (pin 10 an 11 on MMC1).
Thank you so much for the answer, I'll leave them floating!!!!
It's a little more difficult to explain why you tie unused inputs to ground/Vcc. The reason is basically that all of the IC's inputs are 'somewhat' coupled together. While that may sound wrong, it's because they are all connected to ground and Vcc in some way. Leaving the pin floating can cause it's circuit to go unstable. You would think, "who cares I'm not using that part of the chip?!" However this effect gets coupled to the other portions of the circuit that you're trying to use. This results in crazy behavior and erratic problems with your circuit/cart.