Bitbanged Serial
Program is working well. Prints some sample text, then an endless string of ascii '5' for use when calibrating the exact value of 'SECU' (see code).
8.68 uSec basic pulse width (115,200 bps)
ItsyBitsyM4 (ATSAMD51G19A) from Adafruit.
Older version of this README follows.
Bitbanged Serial
This seems to be a variant of ainsuForth-d51, privately held as expA (experiment 'A' iirc).
https://github.com/wa1tnr/ainsuForth-d51.git
If that's the case, it's an Atmel Start based experiment on serial output, using only software.
Hardware: Probably Feather M4 Express.
At this writing (November 2020) there's just the one C source file, distinguished by 'bit_CLR' and 'bit_SET' utterances
(That will probably identify the correct file uniquely).