How to test memory in an HP48 RAM card

[mfleming]

Hi Paul,

Thanks for the pointer to the port memory test program. Just the thing I was looking for! I'll give it a try this morning.

I remember running across and bookmarking your RAM Card project. I was specifically looking to make a 128K RAM card, and chose the Cypress CY62128ELL memory device. My own design is just an update of several found on hpcalc.org in the Miscellaneous section. The same dual-Schottky power circuit but with a 470K pulldown on the chip CE input. This device conveniently has both a positive and negative logic chip enable.

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.

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.

(06-08-2019 01:56 PM)pdo Wrote:  That's interesting! No, I have only the one 48GX. I've been looking around for another to test against, but they're all so expensive these days :-(

Paul

Do as I do and get the "For Parts or Repair" TAS offerings. They're a pain to open but easy to repair, and you don't shell out as much. OTOH, even the high price a pristine model goes for is a fraction of the original retail price! I guess it's the Scot in me that refuses to crack the purse that wide if I don't have to...

~Mark