Using the PIO you can get some serious speed out of it. Someone made an arbitrary waveform generator[1] reaching 125Msps stock, for example. But you can overclock[2] the Pico up to 420MHz apparently so maybe you can get even more, though not sure if the GPIO drivers can handle that though.
If you want to communicate with standard Ethernet gear, that would be either 10Mbit/s exactly (10BASE-T standard) and 100Mbit/s exactly (100BASE-TX standard).
OP implemented 10BASE-T here. Next step up is 100BASE-TX, which nobody did in software/bit-bang as of yet, as far as I'm aware - part of the reason it's fairly large jump in complexity. Namely, you'd need 3 analog voltage levels, as well as pretty hairy digital scrambling/encoding, to output 100BASE-TX. RPi-pico might just be able to eek that out with few external resistors and enough elbow grease.
Other reason nobody bothered with bit-bang 100BASE-TX - virtually all switches will happily down-negotiage to 10BASE-T, which gives you ~1.2MB/s, and that's plenty fast enough for a microcontroller with mere kilobytes of RAM.
Comments
Using the PIO you can get some serious speed out of it. Someone made an arbitrary waveform generator[1] reaching 125Msps stock, for example. But you can overclock[2] the Pico up to 420MHz apparently so maybe you can get even more, though not sure if the GPIO drivers can handle that though.
[1]: https://www.instructables.com/Arbitrary-Wave-Generator-With-...
[2]: https://www.youtube.com/watch?v=G2BuoFNLoDM
If you want to communicate with standard Ethernet gear, that would be either 10Mbit/s exactly (10BASE-T standard) and 100Mbit/s exactly (100BASE-TX standard).
OP implemented 10BASE-T here. Next step up is 100BASE-TX, which nobody did in software/bit-bang as of yet, as far as I'm aware - part of the reason it's fairly large jump in complexity. Namely, you'd need 3 analog voltage levels, as well as pretty hairy digital scrambling/encoding, to output 100BASE-TX. RPi-pico might just be able to eek that out with few external resistors and enough elbow grease.
Other reason nobody bothered with bit-bang 100BASE-TX - virtually all switches will happily down-negotiage to 10BASE-T, which gives you ~1.2MB/s, and that's plenty fast enough for a microcontroller with mere kilobytes of RAM.