This is a tiny (Bare PCB dimension: 45mm(W) x 20mm(H)) USB to serial converter using the low cost CH340c chip with a few surrounding parts. The advanctage of CH340c is no external crystal needed that could cut down the cost. The mainly used pin at the tail of the board is compatible with FTDI breakout board, so you can use it to program Arduino Pro mini by temporary touching the pins of the boards without the need of dupont cable. It supports both 3.3V and 5V by switching the jumper connector. In addition, almost all pins from the chip have been broken out except pin 15 (R232). Also I have added an additional Micro-USB port to provide extra connection flexiblity. **Please make sure you don't connect both USB port at the same time since there is no protection circuit for that case.** Most SMD components are 0805 except for C9 (10uF tantalum capacitor) for easy hand soldering. Please be reminded that the thickness of the 2 layer PCB is recommended to be 1.0mm, the standard 1.6mm thickness for most PCB manufacturers such as JLCPCB is a bit too thick for the Micro-USB port. **Important notes: Rev 2.1 is the same as Rev 2.0 but it fixed the USB-A port fitting problem but not tested yet. ** These are the assembled Rev2.0 board and I'm quite happy with it :)