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Honored Contributor I
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Cyclone III Starter Kit Input Pins

I have an HSMC-GPIO breakout board (THDB-HSMC) attached to the HSMC port of my Cyclone III Starter Kit. I am attempting to program the FPGA to wait for an input signal and then begin some predetermined routine. However, the input pin is giving me trouble; it seems to be held at a weird voltage (~0.76 V) above the ground that is provided by the breakout board. 

 

This necessitates me providing a pulse of ~3.3 V before it is recognized by the FPGA; I'm worried about damaging it so naturally I'm not too excited about doing this. 

 

If I instead change this pin to an output and don't drive it, it sits nicely at 0V with respect to the same ground. 

 

Any suggestions as to why this might be happening?
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Honored Contributor I
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However, the input pin is giving me trouble; it seems to be held at a weird voltage (~0.76 V) above the ground that is provided by the breakout board. 

 

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Driver conflict? 

 

 

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If I instead change this pin to an output and don't drive it, it sits nicely at 0V with respect to the same ground. 

 

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That comment does not make sense, if its an output, then you must be driving it. If you really mean input, then that indicates your I/O board is driving 0V onto that wire. So back to the original suggestion - driver conflict? 

 

Cheers, 

Dave
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Honored Contributor I
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I'm not sure what that means, precisely. I wrote a simple program that has two pins in use (one input and one output - separate), which is supposed to toggle a light when the input is triggered. I'm not sure what a driver conflict would mean / how I could correct for it.

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Honored Contributor I
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--- Quote Start ---  

I'm not sure what that means, precisely. I wrote a simple program that has two pins in use (one input and one output - separate), which is supposed to toggle a light when the input is triggered. I'm not sure what a driver conflict would mean / how I could correct for it. 

--- Quote End ---  

 

 

If you've somehow managed to get your pin assignments incorrect, and you are driving a signal from your FPGA to your external board and you have attached that FPGA output to an output on your external board, then both of those drivers will fight, i.e., if one is trying to output a high logic level, and the other is trying to output a low logic level, you will get some voltage between the logic high and low levels. The voltage you get depends on the output impedance and current strength of the two fighting drivers. 

 

One way to check the signals, is to separate the boards, and check that each bit on each board does what you expect, eg., for the FPGA, check you can toggle the output pin you are planning on using, and for the external device, look with an oscilloscope to see that the signal acts like an input (and not an output), eg., use a pull-up or down resistor to make the signal go high or low. If you cannot make the input signal on the external board go high or low, then you have a problem. 

 

Cheers, 

Dave
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