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Altera_Forum
Honored Contributor I
932 Views

GPIO Input Configuration

Hi all, I thought I would post an extension of my GPIO problems in a new thread to address a specific issue. I'm trying to receive a simple 3.3v signal through one of the GPIO pins on the expansion header as the return from a basic range finder. It's going to enable a clock setup eventually that will measure the length of the pulse. All this is being done without VHDL or Verilog as our instructor wants the project built primarily using primitives. I've got to the point where I can get GPIO[0] assigned to an input and my test circuit looks like this: GPIO[0] (input) ==> LEDR[0] (output). My intention was to test the reception of the signal by connecting a jumper between the 3.3v vcc on a nearby pin to cause the LED to light up. That's not how things are working, however.  

 

As soon as the board is programmed, LEDR[0] lights up indicating it's receiving a voltage from somewhere even when nothing is connected to the input pin GPIO[0]. It stays lit until I connect a jumper from that pin to ground to short the signal at which time the LED goes out. It is my suspicion that I have something configured wrong, maybe in the pin planner, and I'm hoping someone can help. I've been playing around with different settings to no avail. If anyone could offer suggestions I'd greatly appreciate it. It seems like such a simple thing - just receive one 3.3v signal.
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2 Replies
Altera_Forum
Honored Contributor I
27 Views

Please take a look at your previous post in this forum. I've addressed your issues.

Altera_Forum
Honored Contributor I
27 Views

Wanted to post a final update in case someone searches for this issue later. The reason the LED was lit constantly was because of some kind of transient voltage around 0.94V that was present in the input pin and was enough to light the LED and barely enough to tigger a logic high in other things I tested like a multiplexer. The issue, however, went away completely once I read something about unifying the grounds between your input and output devices. I went ahead and hooked up the range sensor via a level shifter (since it runs on 5v logic) and once everything was tied together and grounded the problem went away completely. Hope this helps some poor engineering student in the future.