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I have run into a bit of trouble with a sewable electronics project. I am trying to power a SparkFun 7-Segment Serial Display from one of the analog ports of a ATmega328 board. I realize this is unconventional, but I designed the circuit to not require any crossed connections to make the sewing a bit easier, and that was the only way to do it. The display wouldn't light up, and after some debugging with a multi-meter it turned out that the analog pin I was using (A3) was only at 1.3V despite a digitalWrite command. I tested the same command on one of the other analog pins (A0) and it produced 5V as expeced. When I bridge the VCC pin and A3, everything works fine. When I set A0 to 5V , and bridge it with A3, the output voltage drops to 1.8V relative to GND.

The really weird thing is that just running the example code from the sparkfun website without any modifications still somehow results in a \~1.2V reading from A3 to GND. I have no clue what is going on here. Usually I would disconnect the display from A3 and keep testing, but I am trying to avoid it. Troubleshooting is difficult as I had to clip the excess tails to prevent short circuits making me extremely reluctant to disconnect anything until I have figured out what the problem is, as the thread is very hard to work with, and it took the project on the order of 60 man-hours to get where it is. I was hoping someone here might know what is going on, or point me in the right direction.

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    You should measure required current first. There is possibility the A3 output driver is already destroyed. I'd expect it needs over 100mA if you lit everything on.
    – KIIV
    Jun 6, 2020 at 8:24
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    The doc says it should not draw more than 14.1 mA @ 5 V. Jun 6, 2020 at 10:52
  • The sample code you mention uses Hardware SPI, with fixed pins. Check the desired digitalWrite separately and if it shows your atmega328 still alive, you can post your actual problem here. Jun 6, 2020 at 11:07
  • I am using the I2C example code. What are you looking for me to check? I can get the atmega in the lillipad to control the other pins just fine. Jun 6, 2020 at 12:23
  • The supply current is without including the LEDs. That's just for the ATMega328P on the board. You can add up to 24mA for the LEDs (with 1kΩ resistors on the board and 2V forward voltage for the LEDs, and multiplexed in groups of 8) depending on the number of LEDs turned on at any one time. Yo A3 pin is daid! You should never try and power anything like that from an IO pin.
    – Majenko
    Jun 6, 2020 at 14:01

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