In my experience with parallel FRAM, it’s as fast as SRAM and is a drop in replacement with the same timing.

A lot of folks have replaced battery backed SRAM with FRAM on game consoles.

For the specific chip I used (MSP430FR2355), you could read and execute code at max 8MHz without wait states. Which means it's reading at most 4MHz since each MSP430 instruction that accesses memory requires at least 2 cycles. So latency is at least 1/4MHz=0.25µs

The metal ions from the ferroelectric material can contaminate the silicon production line. I read that they would manufacture the silicon die at one facility (i.e. make the transistors) and then do the rest of the fabrication (the ferroelectric material, top metal, etc.) at another facility to avoid contamination. Maybe that's the process incompatibility that you mentioned. I don't know if FRAM is on a decline or will hold on as a niche product.

Thanks, I've fixed those!

I did my PhD on a purported successor to this - rather than using magnetic bubbles the idea was to use things called Skyrmions which emerge naturally in some materials as either information carriers or to store data in place. In practice it’s never really been taken up by industry for a few different reasons (density, can’t move the skyrmions in a straight line with a current, difficult to read whether one exists or not in a specific place).

Someone gave me a board with bubble memory chips to examine, but when I opened up the chip it turned out to be regular DRAM; they were mistaken about the type of memory.

Oh wow I’d never heard of bubble memory before. That’s crazy.

The (in retrospect) strange and complicated things people came up with to store bits before magnetic core got popular and then later silicon wiped the field are so much more interesting than what we’ve got now.

I get why silicon won. But it’s just nowhere near as fun as bubble memory, delay lines, or CRTs.

It wouldn't make sense to use FRAM for ROM, since the big feature of FRAM is fast write speed. I found one DEC document that says the Flash ROM on one product is the Intel 28F008SA, an 8Mb flash chip. So I expect the VAXstation uses boring flash too, rather than costly FRAM.

Link: https://bitsavers.org/pdf/dec/semiconductor/arm/EC-QU5KA-TE_...

There are multiple factors that limit the number of writes that FRAMs can handle: changes in crystal structure as Ti ions replace O, mobile ions collecting at grain boundaries, and something to do with 90º domains.

I’ve replaced battery backed SRAM in several game consoles and other devices with FRAM (Neo Geo CD, Sega Saturn, an HP oscilloscope) and for some it’s drop-in, and in a few you have to bodge some lines.

The FM1808B and FM1608B might work for you. These are 32Kx8 and 8K×8 FRAM chips with standard SRAM pinouts, in DIP packages. If you don't need a DIP, you have a lot more choices.

From looking at various patents, I believe they put down a layer of PZT and then etch it into cubes with photolithography. Look at the process diagram at the bottom of my article, step 1128.

Ok in looking into this for https://github.com/bkw777/WP-2_IC-Card?tab=readme-ov-file#m-...

Everspin claims to be internally shielded for between 25 to 125 gauss depending on the temperature rating and package type (pdf included above).

In my case using the parallel interface and TSOP-II package, the Industrial version is the best and is supposed to resist up to 125 gauss.

And a fridge magnet is around 100 gauss (obviously they vary widely but I found a random chart that placed fridge magnet at 100).

Also magnetic fields drop off with an inverse-cube law so even a small distance drastically weakens the field. So in many applications with a chip inside of some product like a vintage computer, the chip is likely to be at least an inch away from any exterior surface, and often a magnet or coil will also be inside of some other enclosure like a speaker, adding yet more inches of distance. So even the commercial version at only 25 gauss shielding is probably fine in most cases.

In my application above it's a thin card with essentially no distance from the exterior to the chip, so any bare exposed magnets like a fridge magnet, or the pretty strong magnet in the base of a flash light right on my desk here, can easily come to within 1mm from the chip.

If the shielding claims are true then at 125 gauss the card should be ok even with a mild magnet laying right on the card. Setting on top of something like a speaker should be fine since the speaker magnet is suspended in the center of a box. But I still have to generally avoid magnets or electromagnets. Maybe it can stand up to my fridge magnets, but that flashlight is quite a bit stronger.

If you are designing a pcb and can make changes and have room, a common steel rf shield should help a lot as long as it's made of mostly iron or nickel. mu-metal is best but a common plain steel shield should be almost as good since steel is mostly iron. They do sell them in generic form not just custom made: https://www.digikey.com/short/zm42wjmc

Wow that is a biggie, than you.

And they are expensive. I have an external memory card for a vintage portable word processor that the full bom ends up about $100 for 512k to build one, and something like over $30 of that is just the mram.

I did not know that. That's a showstopper for...a lot of use cases. Thanks for the warning.