Has anyone used NVRAM in place of EEPROM? I have one of these that I'm hoping to map to the upper 32K of the address space:

BQ4011YMA

The advantages that I can see are:

• No need to remove the part to flash a new "ROM"

The disadvantages:

• Might accidentally overwrite the "ROM". Maybe I can use a jumper on /WE to "write-protect" the ROM
• Still need some way to program it initially (although don't need a separate programming voltage)
• Might lose its data integrity sooner than other options

What else should I take in to consideration?

I think these have been used, here and there. I think these chips have an integrated battery - they are not cheap, and have a few years life. In our world, it might be worth considering making a battery-backed circuit with a conventional RAM instead.

As you say, you need to get a minimal program loader(*) in there somehow. If you build on a DIP format circuit board with both the battery and the RAM, you may be able to do that using another 8 bit machine. Or you can build some front-panel toggle-in capability for bootstrapping.

Or, these days there's FRAM (FM1608B-SG Cypress Semiconductor F-RAM 64Kb FRAM 5V).

Ref: EEPROM discussion on stardot

(*) Some previous threads on the bootstrapping problem, and minimal program loaders:

Bootstrapping an SBC
ANNOUNCE: C'mon, the Compact MONitor
6502 with a serial port - a minimal monitor?
Demonstration of bootstrapping a ROMless machine (not 6502)

The 27C series of UV-erasable EPROMs are generally faster than the 28C series of EEPROMs, with access times down to 55ns in some cases. For example, I use this EPROM in my POC machines. Also available are one-time programmable (OTP) ROMs, with access times as low as 45ns. I've tested this part in my POC V1.1 unit. It will boot at 15 MHz.


A good soldering iron and some patience, and you're on your way.

_________________
x86?  We ain't got no x86.  We don't NEED no stinking x86!

You could also consider using Flash, something like the 39SF010, which Mouser does stock. It's generally cheaper, and has better performance all round than the 28 series eeprom.

That's a 128k part, but for use as smaller capacities, just ground the upper address lines.