pocketbeagle (PB) with serial port 4 (TX and RX) connected to uart on a Curie nano board. PB I2C1 connected to a MPU6050 breakout board. PD 3.5 volt output (P2 PIn 13) supplies power to MPU6050 board.
This configuration works as I can transfer data between the PB and the Curie Nano via the serial port.
I had power off on the Pocketbeagle and was updating a script on the Curie Nano board. The Curie Nano was powered via the USB connection to my laptop.
I noticed that the power LED on the MPU6050 board was on although it was not very bright. I measured the 3.3 volt input to the MPU6050, it showed 1.7 Volts. I also looked at the 5 volt output from the PB, it was 1.8 volts. As the only device that actually had power was the Curie and the only connection between the two boards was the serial port, I unplugged the serial port wires from the header. The voltages on the PB board power output pins went to zero. Putting the serial wires back brought the voltages back. I measured the voltages on the TX and RX wires. The TX was 3.0 volts and the RX was 0.08 volts. I beleve this is as expected as the TX has a Pull up and the RX has a Pull down.
I checked the boards for shorts and did not find any. I in fact did not expect to find any as the boards run fine when both are powered on. When the PB is powered on, the output voltages are at their correct level.
1: Has anyone else seen this sort of behavour on te power output pins?
2: Is it really an issue or is it just how the OSD335x is designed.
3: Should I be concern about this killing my board?