Digilent A7 PMOD Input Voltage

[Altis]

Hello,

I'm fairly new to this so please bear with me. I'm looking to control the A7 trainer board remotely from an Arduino (digital signals to board only), effectively to be used in place of the physical switches on the board. From what I understand, I can read in the inputs on the PMOD connectors.

The PMOD uses 3.3V and the Arduino (nano) uses 5V. Would sending a 5V signal to the PMOD inputs risk causing damage? I'm not sure how sensitive they are.

Alternatively, I would use a pull-up resistor to connect the PMOD input to the PMOD 3.3V, and use the Arduino to pull the PMOD input low only (high-Z for 'High'). I believe you can change the Arduino GPIO from output low to input (which is high-Z). I've attached a sample schematic of how this would look.

Thank you for your input.

Altis

[zygot]

2 hours ago, Altis said:

Alternatively, I would use a pull-up resistor to connect the PMOD input to the PMOD 3.3V, and use the Arduino to pull the PMOD input low only (high-Z for 'High').

What you propose is to emulate an open collector or open drain driver on your Arduino. A true open drain driver might work out ok. I doubt that trying to emulate one in software would be an acceptable alternative. What you should do is create a breadboard or use a PMOD that either uses a logic level translator or has 5V logic open collector gates between your Arduino and the FPGA pins. Can you get away with doing things the wrong way? I don't have an answer to that question because doing things the right way isn't much bother. It does remind me of a quote from one of the old Dirty Harry movies. 

So, if you're feeling lucky, what do you think happens before and after your Arduino application is executing with your boards connected together?

[xc6lx45]

You may be able to make it work with e.g. a 10 kOhms serial resistor that limits the current into the FPGA input through the FPGA's clamping diode to the FPGA's supply rail at 3.3 V.

But please validate this for yourself e.g. with a google search.