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DE III problem witch Voltage of the I/O pins

Honored Contributor II


I’m developing a project in the DE3 and I’m having a problem with the voltage of the GPIO Pins.  

To describe my application, It consists basically of use the switches and the dipswitches to control the value of the GPIO Pins. Above, I give an example: 

GPIO_D11<= DIP_SW_4 



I have all the compilation ok and so the download to the board. But, as I was having some trouble with my other application that uses those signals, I start to measure the GPIO pins with an multimeter. Then, I realize that when I put most of my signals in ON, I have an decrease of the value of the Voltage of some of the pins. 

For example: when I have the SW_1 and the SW_2 in OFF, and the DIP_SW_4 in ON, the voltage of the GPIO_D11 is 3,19 Vdc; 

But when I have the SW_1 and the SW_2 in ON, and the DIP_SW_4 in ON, the voltage of the GPIO_D11 is only 2,42 Vdc; 

Does anyone know what could be happening? 

I’m using the Quartus II 8.0. 

Thanks for the help!!!!!
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Honored Contributor II

I guess the lower voltage on GPIO probably means you have some design issue in the connections outside fpga. For example another signal is driving gpio or excessive current is being drawn from fpga output. 

Are gpios connected to anything?  

Can you also check gpio with an oscilloscope? Maybe the signal is not stable, because some switches configurations introduce an abnormal modulation; since the multimeter is slow, it measures the mean value and you simply see a lower voltage.
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