Pico-100BASE-TX: Bit-Banged 100 MBit/s Ethernet and UDP Framer for RP2040/RP2350 (opens in new tab)

That's the power of the Pico - the bit-banging happens in the PIOs, not the main cores. So idling takes no CPU (the idle symbols are pre-calculated and DMAed into the PIOs, which do the bit banging), and transmission only needs the CPU for framing and encoding, not the timing sensitive / interrupt driven bit-banging stuff.

The ADC example in the README is pretty fun; being able to stream data out to a PC at a high rate over a standard interface is always useful in some niche use case, and I don't think anyone has managed High Speed USB over PIO (yet?) so this is likely to be the fastest way.

Bit-banging isn’t the right term because the toggling isn’t done by the main CPU. The RP series has programmable PIO units which handle the low level timing and line toggling. The CPU communicates with the small program running on the PIO.

As others have already mentioned, the bit banging part is mostly handled by the PIO, so you mostly just spend CPU cycles on 4b5b encoding and scrambling. The more immediate practical problem though is that this is transmit only, no receive.

Combined with RMII ethernet phys only costing around 30 cents even at single quantities definitely makes it just a fun project, though definitely an impressive one at that.

Yeah the first thing that came to mind when I started reading the readme file was "that's not bit-banging -- this is the whole point of PIO."

The pico has programs that run independently to set the pins, so actually it does leave a lot of CPU cycles.