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Topic: LEDS Light without power supply? (Read 741 times) previous topic - next topic

catatung

Hi, I have an Arduino Nano v3.0.  While connecting a device (in this case the CD-Changer connector interface on a car radio) I noticed something strange.  I connected GND to GND so the devices share the same GND.  While hooking up some of the data pins, one had 5V at the moment and when I attached it to a data pin (D9 in this case) the TX and RX light come on for 2 seconds and then go out.  While these two are on the PWR led is on also, but dimly.  Once the TX and RX lights go out then PWR led gets a little bit brighter.  I also notice the "L" led blinking very dimly after RX and TX go out.  If I plug in my USB connector the PWR led gets brighter than it is in this case.

Is there a problem with my board or is this normal behavior?  I can't imagine it should try and power the board from a digital I/O pin just because voltage is present at the moment. 

Can anyone clarify this is or is not normal behavior?

graynomad

#1
Mar 07, 2012, 03:16 pm Last Edit: Mar 07, 2012, 03:19 pm by Graynomad Reason: 1
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I can't imagine it should try and power the board from a digital I/O pin just because voltage is present at the moment. 

That's exactly what can happen.

The board is probably being powered through the ESD protection diode on D9, so that pin was powering the whole board.

This is not recommended.
______
Rob
Rob Gray aka the GRAYnomad www.robgray.com

catatung

Hmm, I tried it on a bunch of pins and every pin had the same result.  What is the best way to avoid this?  While the board is powered via USB is the board configured to power itself from the "correct" voltage source (USB)?  I have no way to really know because the LEDs are expected to be on when the USB connector is plugged in.

Thanks for the quick reply.

graynomad

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every pin had the same result

They will because they all have ESD protection diodes.

How many pins will be connected to external 5v?

Are you saying that the Arduino will often be powered down even though the other circuitry is not? If so why so?

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What is the best way to avoid this?

I actually can't think of a simple way. You have to isolate the pins somehow and how that's done depends on the nature of the signals, are they digital or analog? Always inputs/outputs? Sometimes inputs sometimes outputs?

If digital the TXB series of chips (TXB0102/4/6/8) can be used to isolate signals.

______
Rob
Rob Gray aka the GRAYnomad www.robgray.com

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