Not sure whether to post this here or in the LED and multiplexing area, so I'm doing both. I hope that's not a violation of the rules.
I have a working LED cube with working software - I've been modifying the software this evening and when I stopped everything was working fine. Experience has shown that when I power it all down, I have a hell of a time getting it going again and now I can't. So I'm looking for thoughts on what to try.
Until this evening, I had a USB from my computer to the Arduino (a MEGA2560), which sends SPI signals to a circuit board that interfaces with the cube. I have a separate 5V power supply on the circuit board to power the LEDs. With the Arduino powered by the USB, on startup I usually get junk on the cube, but after some time and switching the circuit board power on / off a few times, it seems to sort itself out. Still, I would prefer that that doesn't happen.
Recently, in response to a different post of mine, I was advised by one of the experts here that I should really power my Arduino from my board and not via a separate power supply. So tonight I made a special USB cable that is the same as a normal USB cable, except I cut the USB 5V supply wire and instead connected it to a 5V supply from my circuit board. When I turned it on, I get junk again on the LED cube and it doesn't seem to be going away.
I found this on the troubleshooting forum.
Why doesn't my sketch start when I'm powering the board with an external power supply?
Because the RX pin is unconnected, the bootloader on the board may be seeing garbage data coming in, meaning that it never times out and starts your sketch. Try tying the RX pin to ground with a 10K resistor (or connecting it to the TX pin).
I understand the part about connecting the RX pin to ground. Does the other part mean I should try connecting the RX pin directly to the TX pin without a resistor?
Any other suggestions to get this going again?