Panasonic HHC ROM 'capules'

[brouhaha]

The problem with using modern memory chips (flash, FRAM, MRAM, NVSRAM) for an HHC module (whether 4KB, 8KB, or 16KB) is worse than that. It doesn't matter if there are software delays after "activating" the capsule. The power to the capsule sockets is switched by the HHC hardware on _every_ access. If the CPU is executing code from the capsule, it's switched off even between two consecutive reads. AFAICT no modern non-volatile memory chip can handle that; they all have at least 100 microseconds of required startup delay, whereas the HHC needs them to be ready in a few hundred nanoseconds.

AFAICT, Microchip was the last manufacturer of CMOS EPROMs, and they've been obsoleted. They also appear to be the last manufacturer of 5V parallel NOR flash, like the SST39 I mentioned, and those have the 100us startup time requirement.

It's possible that some other flash vendor may make chips that start up faster, but probably not in 5V parts.

Using 3.3V memory in the capsule will require some form of level adaptation, though possibly not full-on level translators. I've had good luck with TI SN74CBTD3861 voltage clamps for interfacing 5V systems to 3.3V logic. That part has ten NMOS pass transistors, biased such that they turn off if one side is over roughly 3.3V. With those between the 5V and 3.3V worlds, the 3.3V device is protected. Unlike level translators, they have basically no propagation delay (less than one ns), but they also don't provide buffering or drive. Anyhow, those would work to use a 3.3V memory part, but of course it's trickier to squeeze a memory part AND three of these voltage clamp chips into a PCB the size of the capsule.

Using new-old-stock OTP 27C128 (or 256, 512) is looking more and more attractive, though a DIP obviously won't fit, and a J-lead PLCC might not either, and those were the most common EPROM packages.

Obviously the 4KB and 8KB modules are easier when using new-old-stock EPROMs, because a 2732 or 68764 should fit your new carrier replacement.