CH340s are cheap Chinese USB-to-UART chips that are, well… cheap and easy to solder. They come in various variants, where the last letter indicates which exact variant it is (CH340B, CH340C, CH340G, CH340E, CH340R, CH340T). I bought one to make an onboard programmer for an ESP chip here. 20 pieces for 7 euro was a good deal!.

To make a programmer fully automated (no need to push buttons), we need UART and two pins to drive ENA and GPIO0 of the ESP module. The trick is that UART chips have RST and DTR lines that can be connected to those pins on the ESP and then driven by the flashing tool on the host PC.

So far, so cool - except that I bought a CH340E, and it’s the only one in the family that doesn’t have a DTR pin :P

So I have to push a pin on the board manually to flash the device (at least only one, because the RST pin is there and that part works fine).

In the next revision I will use CH340X and follow this tutorial  - it seems that with this version there is no need for extra transistors logic (but I will left place for them on the PCB anyway)

Nevertheless, the programmer is working - just not in a fully automated way. So for a first try, I’m kind of okay with it.