[cctalk] Re: RS232 then and now (bitbanging)

Bit banging also works if you have devices that specialize in it. A nice example is the Programmable I/O (PIO) engines in the Raspberry Pico microcontroller. I did a DDCMP implementation with those, including the "integral modem" feature. That includes clock recovery and demodulation of the FM signal, at speeds up to 10 Mb/s or so though of course no DEC DDCMP device ever ran nearly that fast. It looks like it could also do 10 Mb/s Ethernet directly (handling AUI waveforms in software) -- I haven't tried that but I might just for fun. It would also be an easy way to do the 10 bit UART and 19 bit USRT needed to drive a PLATO terminal. I also used this machinery to drive a software-defined radio chip from Harris/Intersil, on a board I originally designed for EPP mode parallel printer port interfacing. It has a 16 bit serial port that just runs continously, and the PIO handles that nicely. Similar but different is the bit-serial interface you'll find on audio DAC devices, that too would be a very simple case. As for SPI, the standard driver for the CYW43439 WiFi chip on the Pico uses PIO to deal with the not quite standard SPI used by that device. Or at least the mismatch in expectations. SPI is only barely standardized and it suffers very badly from the Dave Clark effect ("so many standards to choose from"). For example, the ENC28J60 Ethernet MAC chip has a SPI interface that does multi-byte transfers with the CS signal asserted for the entire time, while the "mode 0" SPI standard apparently says it's dropped in between every byte. One solution is to ignore the built-in SPI blocks and use the PIO; another is to use the standard SPI block but override the CS pin in software so it remains asserted even though the SPI block dropped the assert. Ugh. This sort of bit banging design is very effective as well as cost-effective, considering that a Pico sells for $4 (quantity one, a whole module -- the chip alone is under a dollar if I remember right). paul