Simon Frankau's blog

Playing with SIMMs

I've been playing about with SIMMs, making them do stupid things. No, not Sims, SIMMs.

After something of a hiatus, I've finished (for now) another Teensy-based project, to read and write to a 30-pin SIMM from the '90s. The background is that I'm interested in eventually building a 68K Mac compatible, but it's a project I've never found the time for. However, warm-up experiments, like playing with the SIMMs I'll use, are very doable. :)

All the code and notes are up on Github, but really what I've been doing is investigating how quickly SIMM data decays if you don't run the required refresh cycle. The summary of what I found was:

While the required refresh cycle is 128ms, most cells can hold the charge for much longer. Each cell has its own decay characteristics, with the effective RC value being roughly log-normal, as far as I can tell. The median cell takes 3 minutes to decay at room temperature.
I'd read online that there's a strong temperature component to the decay rate. I'd tested the SIMM at 20-40 degrees, and could see a decent kink up in decay rate at 40 degrees, hinting that decay accelerates with temperature. Sadly, lacking decent tooling to generate stable temperatures beyond 40 degrees, this side is rather more qualitative than quantiative.

In any case, fun, fun, fun! Now to find the next toy microcontroller project - probably a good opportunity to get that RPi Pico doing something interesting....

Posted 2021-07-31.

Full index