Two 24LC256 wiring thoughts

Derek

I am working with Marty Hebel designing a board using the Propeller, I have been using the Propeller demo board schematic as a guide, and he *cough* i mean WE would like to add another 24LC256 to increase the memory size

but Im not completely possitive on how to wire them up together . I was wondering can SDA and SCL be wired to the same inputs with different addresses for the two 24LC256 or would the SDA and SCL from both memory chips have to be on seperate pins.

If any of that makes no sense just ask for clarifications. Any thoughts would be appreciated greatly.

Thanks,

Derek

Mike Green

You can add up to 3 additional 24LC256 since there are two address lines. SDA and SCL can be wired in parallel for all the EEPROMs. The only issue is that you would have to have pull-ups on both SDA and SCL. The boot EEPROM doesn't need them because the boot sequence uses internal pull-ups, but these are not available after boot (since the pins are intended to be general purpose after boot). These pull-ups do take about 1ma each which is a lot of current if you're trying to run the Propellor off batteries in sleep mode for much of the time, but a trivial amount otherwise.

rokicki

If the pullup current is a problem, simply idle those lines high---then only trace current will flow.

cgracey

Actually, there are no internal pull-ups on P28 and P29. This might have been a good idea, though, if we had thought of it earlier. You will need to add a pull-up to P29/SDA·(10K to VDD is fine). P28/SCL is driven during boot-up, so there's no need for a pull-up there.

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Chip Gracey
Parallax, Inc.

Derek

Thanks guys.