The U3A project is based on a (FT232) that ist used as USB (Full Speed) to serial UART transceiver. Most signals of the (FT232) are routed to the connector pins J2 and J4. Whithin the DIP switches the adapter can be switched from UART to UPDI mode to program microcontrollers. It is possible to select between two voltage sources. Either 5V from USB or 3V3 from the internal LDO on the board.

The LDO itself can drive around 800 mA. Therefore it is necessary to install a cooling unit inside the housing. The mount for this is already provided in the housing.

Experience	Level
Soldering
Mechanical

There are two parts of the hardware. The pcb and the housing of the U3A. The pcb is created with KiCAD and the housing with FreeCAD. All files are built with github actions so that they are ready for a production environment. The housing is printed with a 3D-printer (Dremel 3D40).

The circuit board is populated on both sides (Top, Bottom). The best way for soldering the SMD components is within a vapor phase soldering system and for the THT components with a standard soldering system.

The housing has a tolerance of 0.2mm on each side of the case. So the pcb should fit perfectly in the housing. The tolerance can be modified with FreeCAD in the Parameter Spreadsheet.

The U3A is powerd over USB with 5V. The voltage of the pins can be setup through jumper J1. Either 5V or 3V3. The output of the system is driven by CBUS4. It needs to be configured through FT_PROG. Mostly pins of the IC are routed to the connectors J2 and J4

#
#               +-----------------------------------+      +---------------------+
#               |J2     U                       +---+---+  |                     |
#            +--+--+    S                       |  USB  |--+ +-----------------+ |
#            | VCC |    B      UPDI             +---+---+  | | TeraTerm        | |
#            | GND |    /    +---+---+              |      | | ~~~~~~~~        | |
#            | PDI |    U    |   |   |              |      | | ~~~~~           | |
#            | CTS |    A    |   |   |              |      | | ~~~~~~~~~~~~    | |
#      L     | RTS |    R    | X | X |              |      | |                 | |
#      O +---+ TXD |    T    +---+---+     +-+      |      | +-----------------+ |
#      O +---+ RXD |    /                 / /       |      |                     |
#      P     +-----+    U               +-+-+-+-+-+-+      | Computer            |
#      B        |       P               | 5V  | 3V3 |      |                     |
#      A        |       D               +-----+-----+      +---------------------+
#      C        |  J4   I                           |
#      K        | +---+---+---+---+---+---+---+---+ |
#               | | V | C | C | D | D | D | R | G | |
#               +-+ C | B | B | C | T | C | I | N +-+
#                 | C | 3 | 2 | D | R | R |   | D |
#                 +---+---+---+---+---+---+---+---+

R. GAECHTER