Project update. The test board is going into round two. One thing up front: the USB-C connector now works perfectly.
I ordered the second test board and the stencil from Aisler as usual. Here’s the result:
Unfortunately, the two through-holes for the USB-C connector are still in the stencil. But that’s fine — I can work around it by taping over the two holes.
Then I soldered the USB-C connector. Solderability is still good, and the functionality is now correct too. The host/device selection via the two 5.1k resistors works perfectly, the 5V comes in cleanly, and both USB 2.0 data lines arrive as expected.
Then I populated the remaining components. I noticed 2 small mistakes in the process: 1. Diode D2 has the wrong footprint. 2. The ground plane on the board was connected to DC5 instead of GND. But both are quick fixes for next time.
Then I flashed the bootloader using the USBasp programmer. It didn’t work on the first try — I had to set the slow SCK jumper on the programmer.
Then I tried to use my board like an original Arduino Mega. That didn’t work straight away either. I noticed that the two 5V supplies from USB-C and from the board were interfering with each other. The goal was to power the board with the 2560 via external 5V. USB-C is only needed for program uploads and serial debugging, so I power the FT232 from the 5V coming from USB. Unfortunately, I hadn’t considered that the 2560 can slightly back-feed the FT232 through the two UART data lines. The solution was to plug in USB-C first and then apply 5V to the board. That allowed me to establish a connection to the 2560 via USB for the first time. However, it didn’t always work reliably. I then discovered that the serial monitor must be closed during upload. That was never an issue with the original Arduino.
Then I uploaded some test code and got the LED blinking. It worked beautifully!
The next goal is to decouple the two data lines between the 2560 and the FT232, so I can plug in USB-C independently of the board supply voltage. I just ordered these buffers from Reichelt: TEXAS INSTRUMENTS Buffer, Schmitt-Trigger, 1.65 … 5.5 V, SOT-23-5 | Logik-ICs günstig kaufen | reichelt elektronik
I want to put one in each of the TX and RX lines.
The next goal is then to connect existing circuits to my test board — hence all the connectors around the board. These circuits are already running successfully on an Arduino Mega.
The ultimate goal is to combine everything onto a single board.
