Digital IO Inverted

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Joined: Thu Oct 22, 2015 2:17 pm

Digital IO Inverted

Post by » Fri Jun 26, 2020 7:14 pm

Hi all

I've had a RedPitaya v1.1 in my drawer for a couple years, and recently dusted it off, upgraded to the current FW (details as reported in the bottom right of the main screen are "Red Pitaya OS 0.98-694 / STEM 14") to do some SPI debugging using the GPIO interface. For some reason, the logic signals are all inverted in the Logic Analyser app. I feel like I'm going insane, but I've checked it from all angles, and cannot see what I'm doing wrong.

As a test, I've set up a simple flashing signal from an STM32 Discovery board 100ms on, 200ms off. When measure with the oscilloscope, the signal is as I expect to see it.
graphs - Scope.jpg
However, when I have connected the GND to E1:pin 25 and the flashing signal to E1:pin 3, the logic analyser shows the signal as high when at 0V, and low when the signal is 3.3V.
graphs - Logic Analyser.jpg
Am I going mad?

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Posts: 4
Joined: Thu Oct 22, 2015 2:17 pm

Re: Digital IO Inverted

Post by » Thu Jul 02, 2020 12:56 pm

I figured it out eventually... The original v1.1 is now the Stem125-14, which doesn't support this mode as it's configured with differential inputs, and the logic is inverted. Unfortunately, the allowed voltage bands for a logic 1 and 0 don't allow me to simply connect the one side to 3V and use the other side for the probes.

However, there's nothing here that an AM26LV31EIDR can't fix ;)

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Re: Digital IO Inverted

Post by redpitaya » Tue Jul 07, 2020 11:58 am

Hi and thank you for posting.
You were not using the add-on Logic analyzer PRO hardware module, right?? ... ro?cat=c97

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