3V <> 5 V thoughts

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exxos
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3V <> 5 V thoughts

Post by exxos » Sat Mar 23, 2019 8:26 pm

I was half thinking if the CPU bus pullups were on 3.3V rail then the CPU would actually still run on 5V, but all the IO's would be on 3.3V. So it could interface directly to 3.3V stuff without bus shifters. Though it wouldn't actually work as things like the 244 buffers etc would be driving the bus at a 5V level.

So I am thinking of a actual 3.3V bus on likely the next revision (or maybe this one depending on time) . Where the motherboard will have the translation buffers and then be linked to 2 or 3 IO expansion ports. I'm thinking of the space under the keyboard for these. So things like flash,fast ram, ide etc which can run on 3.3V can use those expansion ports. There is a little bit more selection on 3.3V parts so may make things easier in that respect.

Also as there is a 3.3V bus, FPGA could just use 3.3V devices and be plugged into the 3.3V expansion ports. I am thinking of things like the blitter here, rather than trying to hack everything onto a small board, just have 3.3V FPGA boards which plug in with stuff.

Mostly just thinking out loud with all this...
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MasterOfGizmo
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Re: 3V <> 5 V thoughts

Post by MasterOfGizmo » Mon Oct 14, 2019 10:13 am

exxos wrote:
Sat Mar 23, 2019 8:26 pm
I was half thinking if the CPU bus pullups were on 3.3V rail then the CPU would actually still run on 5V, but all the IO's would be on 3.3V.
You'd still have 5V on the bus whenever the 5V CPU drives it. The 3.3V pullups would only pull it to 3.3V when no 5V part is driving against them.

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