Suggestions on HP21 restoration

[blackjetrock]

You might want to look at the Raspberry Pi Pico, which is a more powerful sort of Arduino. There's more flash, more RAM, more power in the ARM core and two cores. It is also cheap and fairly small. You can also use Eclipse for source level debug out of the box. i used it to replace the insides of the Psion organiser 2:

https://youtu.be/bD-tpxNDMsI

If there's not enough space for a Pico, then you can get the RP2020 chip on it's own and use that in a small form factor (it's a fiddle bit of soldering but you can do it). I did this with a datapack for the Psion:

https://youtu.be/NsxZ0bjGaOU.

there's also the STM32 series of chips, which can do this job. i used the Arduino framework to program these and do a couple of 'insides replacements':

https://youtu.be/IKQccZCiaoQ
https://youtu.be/WuV8q2h9zoU

I stopped using these chips as they became difficult to buy (no such problem with the RP2040, not yet anyway). It's nice having the source level debug too, which is trickier on the Arduino platform, maybe. You can also program the STM32 using 'bare metal'.

Hope that gives you some ideas...

Andrew