Digital Output is Floating

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jcrp
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Digital Output is Floating

Postby jcrp » Mon May 13, 2013 4:04 pm

Hello,
I am feeding the digital outputs of the SBC 1072 to another microntroller IO that only takes 3.3 volt input. So I added diodes for the voltage drop from 5V to about ~3.3V (positive of diode toward phidget). All of the outputs were working fine and my code was working well, but all of a sudden, one of the outputs on the SBC is now floating between 2V and 3V. I've disconnected all of the connecting components from the SBC and restarted it, but the one output is still floating. The checkbox in the interface control panel is also flickering as the output floats. I hope I haven't inadvertently destroyed some hardware. Can someone suggest some troubleshooting steps with a multimeter? Maybe it's a bad diode or FET that I can replace?

Thanks

erik
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Re: Digital Output is Floating

Postby erik » Tue May 14, 2013 1:22 pm

The digital outputs on the SBC2 are fed through a resistor right into an IO expander chip. It sounds like the corresponding pin on the chip itself is internally fried.

There isn't really that many tests you can do since there is only a single resistor between the chip and the terminal block. Since the control panel is registering the flickering (and presumably the other outputs are still functional), the expander chip is still able to communicate with the processor. The resistor is located between the battery holder and the analog input connector, as an array of four resistors. Output 0 corresponds to the couple of resistor pads closest to the battery holder. They are connected to the chip immediately beside it, in the same order.

So you could check for continuity between all of those and measure the resistance of the resistor (should be approx 220 Ohm). But if those all look good, then it is most likely the expander chip, which in this case you would have to replace the chip for that output pin to start working again.
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jcrp
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Re: Digital Output is Floating

Postby jcrp » Wed May 15, 2013 8:34 pm

I feel like quite the fool.

We have a few phidget SBC's in our lab here and I think I was using the wrong one while testing the other day. I compiled some simple code to turn the digital output on and off to test the maximum speed of the output. I think the maximum pulse width fluctuates between ~1 and ~6 milliseconds depending on how loaded down the SBC is. The duty cycle is approximately 50%, So of course, measuring this output with a voltmeter reads about half of 5 volts with the digital output acting as an oscillator.

Sorry, and thanks for your help.


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