How to test memory in an HP48 RAM card

[pdo]

(06-08-2019 04:41 PM)mfleming Wrote:  Have a look at the Cypress Application Note for designing battery-backed systems. It contains some useful information, and refers to the dual-Shottky diode approach we both use as a "legacy design" technique - a nice way of saying old school. One problem the AN identifies is a path to ground from any pull-up connected to the SRAM Vcc through an unpowered processor pin via an input protection diode. You have two pullups on OE/ and WE/, and at 47K they could sink tens of uA. Perhaps that's the source of your battery life conundrum! They shouldn't be needed anyway, since they are don't-care when chip enable is deasserted.

Thanks for pointing out that application note, I hadn't seen it before. The idea that the OE and WE pullups could be draining the battery is certainly worth investigating -- I'll do some experimenting (they're only there as a "just-in-case" measure anyway).

Quote:I do drive the CdDet input from Vcc rather than VccOn, but that could be a problem given the AN warning. Although the other card designs do the same thing, I may do one more design change to address that. I've been monitoring card and calculator battery levels but haven't seen anything of note to worry about.

If you do make the mod I'd be very interested to know if you see any change in behaviour!

Happy hardware hacking :-)
Paul