- Use two different Mega 2560 Pro boards
- Use two different XM-15B SPP modules
- Use a logic level converter, both ways
- Use a USB-UART module to test if pins 0 and 1 of my board are okay - and they are - I'm able to connect with TS this way
But the bluetooth module won't connect. In the end I was able to figure out that it's about the supply voltage to the Arduino itself. My power supply is based on a LM2940-5 and it outputs exactly 4.95V. When I power the Arduino via a diode or a 10 ohm resistor, so the power supply to the board drops to about 4.3V, the bluetooth suddenly connects and works. But if I power the Arduino with 4.95V and lower the voltage only on the bluetooth module supply to 4.3V it won't work.
The issue does not appear on a full-sized version of Mega 2560 (which is also just a clone). I can connect via that same bluetooth module whether I supply the Arduino with 4.3V or 5.2V.
Any ideas? I understand that I can't just solve this issue by supplying my Arduino with 4.3V because it's rated 4.5-5.5V and I believe it would mess up analog calculations too.