What you are describing is typical behavior for electronics and is something you need to pay attention to. According to the OSD3358 vendor, if power is not applied to the chip, voltage should not be applied to any of its pins. You should either design a circuit to prevent this, or design your system to ensure that if one board has power, both do.
Jim On Mon, May 25, 2020, 1:08 AM Graham Stott <gbcsto...@comcast.net> wrote: > Configuration: > > PocketBeagle (PB) with an I2C connection to a MPU6050 breakout board. > Power to the MPU6050 board is 3.3 volts from the PB. > > The PB is connected to a CurieNano using uart TX and RX. This is serial > port 4. The two boards have their own power supplies. There is a common > ground wire between the two boards. > > This configuration works well and I can send messages between the two > boards. The PB has no problem "talking" to the MPU6050. > > I had the power to the PB disconnected and was just using the CurieNano. I > was uploading a script. > > I noticed that the power LED on the MPU6050 board was on (not very > bright). I measured the 3.3 volts line going into the board. It measured > 1.7 volts. I also measured the 5 volts output pin on the PB. It measured > 1.8 volts. As there was no power to the PB and the only other connection > was the serial port, I removed the two wires. The LED went off and the > voltages on the PB went to zero. Put the cable back on and voltages come > back. I measured the TX and RX line from/to the CurieNano. The TX was 3.0 > volts and the RX was 0.08. Removing the I2C cable had no effect. > > Questions: > > 1: Had anyone else seen this behavour? How is a voltage on a serial port > on a PB with no power showing up (as a lower voltage) on the 5.0 volt and > 3.3 volt output pins!! > > 2: Is this an issue I need to be concerned about. Could it damage the > boards? > > 3: Is this just a design artifact of the OSD3358 when it is powered off > the voltage on the RX pin is reflect via some substrate to the voltage > output pins? > > 4: Am I missing something? > > I have look for any shorts and found none. And anyway the voltage output > pads on the IC are nowhere near the serial port 4 pads. > > Graham > > > -- > For more options, visit http://beagleboard.org/discuss > --- > You received this message because you are subscribed to the Google Groups > "BeagleBoard" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to beagleboard+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/beagleboard/9eab48a6-867b-473a-b913-872bc88a4022%40googlegroups.com > <https://groups.google.com/d/msgid/beagleboard/9eab48a6-867b-473a-b913-872bc88a4022%40googlegroups.com?utm_medium=email&utm_source=footer> > . > -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. To unsubscribe from this group and stop receiving emails from it, send an email to beagleboard+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/beagleboard/CAGS%2B2h_AmY5_P3hmY0qd2SwtFyzqJMHOM3twa6M7eWRE5BTSOA%40mail.gmail.com.
