builtin serial/jtag - unable to flash via serial

[MabezDev]

The builtin serial/jtag peripheral of the c3 & s3 also exposes a serial port.

With recent versions of esptool it is possible to flash via the serial port, however it is not possible to flash with espflash currently (it hangs).

To reproduce:

1. Wire up a USB cable to the serial/jtag peripheral pins.
2. lsusb should report something like this:

   Bus 000 Device 010: ID 303a:1001 303a USB JTAG/serial debug unit  Serial: 7C:DF:A1:61:E4:80
   

3. A serial port should also show up, on mac/linux it comes up under /dev/ttyX (windows untested)
4. Try and use espflash over this port

[jessebraham]

Referencing esptool.py it seems there are a number of procedures which differ between the standard flashing process and using JTAG. As we do not presently support flashing over JTAG (see #23) this behaviour is mostly expected.

Thank you for bringing this (back) to attention though, we'll get to this eventually I'm sure :)

[igrr]

I think esptool doesn't implement flashing over JTAG, it flashes over the "serial" part of the USB-SERIAL-JTAG peripheral. The difference between that and flashing over UART is in the reset / download mode entry procedure. It still uses DTR/RTS pins but the sequence in which they have to be set is slightly different compared to normal case.

The changes required are in this commit: espressif/esptool@5f60add