HP41C with Rp Pico attached

[ThomasF]

Hi Andrew,

I used your code as a base, but added another OLED driver.
I also implemented my own font (14-segment lookalike with original charset).

I thought I could use a single bi-directional levelshifter for ISA, but maybe changing the direction of the gpio on the fly makes things hickup.
The Tiny2040 doesn't have that many gpios, but it is small and easy to use.

I use the SDK, compile, pressing boot+reset and copy the file from the PC to the RPI, so turnaround time is very short when developing (and I don't need a second Pico for flashing).

Cheers,
Thomas