ItsyBitsy M0 Express - BAT pin output 5V??

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grue42
 
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ItsyBitsy M0 Express - BAT pin output 5V??

Post by grue42 »

Hello,
I have an ItsyBitsy M0 Express brand new from the bag direct from Adafruit.
https://learn.adafruit.com/introducing- ... m0/pinouts

Is the BAT pin supposed to have an output on it?
I'm measuring 5V across the BAT and G pin when the board is plugged into the USB port. I expected it to be a one-way input from an external supply. Is there a different pin I should use for my battery input?
I was hoping to read the battery status over serial (to test) - but it's always 5V when USB is plugged in, Battery or no. Do you have any suggestions on how I can solve this?
Thanks!

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Franklin97355
 
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Re: ItsyBitsy M0 Express - BAT pin output 5V??

Post by Franklin97355 »

The vbat is connected to vbus through two diodes.It may be the diodes were compromised or bad.
2022-11-03_19-07-51.png
2022-11-03_19-07-51.png (4.71 KiB) Viewed 153 times

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grue42
 
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Re: ItsyBitsy M0 Express - BAT pin output 5V??

Post by grue42 »

Thanks Franklin!

I measured 3 boards and they all behave the same way. I checked the diodes with the power off, and they seem to be ok.
The diodes are MBR120s:
https://www.smc-diodes.com/propdf/MBR12 ... 0REV.B.pdf
Is it possible there's reverse leakage current, or that my DVM is affecting the measurement?
I'm not sure if this is helpful, I lifted the anode end of the VBAT diode from the board, and measured current between the floating anode end and the VBAT pin with the USB plugged in. It was about 0.02mA.

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Zetopan
 
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Re: ItsyBitsy M0 Express - BAT pin output 5V??

Post by Zetopan »

Note that the diodes used for this function are Schottky for minimal forward voltage drop. Schottky diodes have higher reverse leakage currents than silicon diodes. As a result, a high input impedance multimeter will "see" the voltage that the diode is blocking. Connect a temporary 1K resistor from the BAT pin to ground and measure across that resistor with your multimeter and then the measured voltage should be neglectable. Schottky diodes are also susceptible to static discharge, so handle the board accordingly.

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