Recently acquired some FM18W08-SG FRAM chips, which are 5V compatible and proven drop-in replacements for SRAM chips in various Nintendo cartridges. Will try them out on one of my calcuators at some point, but this memo is a bit concerning:

Every F-RAM memory access requires a falling edge of CE̅, and therefore the CE̅ pin cannot be held LOW as in an SRAM device. The CE̅ must be toggled HIGH to LOW to latch each new address and it must be HIGH for the specified pre-charge period after the access. (Source)

I'm curious if anyone here knows whether TI graphing calculators access memory in a compatible manner. If not, would there be workarounds short of reprogramming the ROM?
I don't know of any work that's been done to characterize the precise memory bus behavior on a calculator, but even modifying the ROM probably wouldn't help if it's not compatible with FRAM requirements because those signals would probably be generated by the ASIC rather than anything in software.

That said, it wouldn't make much sense to replace a calculator's SRAM with FRAM because it's used as working memory in addition to being used as medium-term persistent storage, whereas battery-backed SRAM in game carts is traditionally only used for long-term persistent storage which is rarely written. You'd be much happier replacing the Flash chip.

If used as working memory (in place of regular RAM) the endurance of FRAM could become a concern: FM18W08 is rated for at least 1014 erase cycles, which at one million writes per second (probably near the upper limit of a Z80 operating at typical speeds, but within the realm of possibility) you'd exceed the rated endurance of the FRAM in not much more than a day of operation.
Good to know. Will save the chips for the GB carts then, rather than an otherwise perfectly functional calculator. I'd been thinking about why and how they got to 10^14 cycles during testing, but those numbers definitely put it into perspective.

Assuming I don't break anything, I'll have extras at the end, will report back if I do try the transplant on a beater calculator for fun.
  
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