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I am connecting a switch to an input pin on a Nexys A7-100T FPGA development board, like shown in the below image:

enter image description here

The circuit has a 10K pull down resistor (R1 on drawing above) to ground and the FPGA has an internal 200 ohm resistor in series with the input pin (See below image from the FPGA datasheet). I am using pin 1 on the JA PMod connector with the 3.3V and GND pins 5 and 6.

enter image description here

Do I need a larger resistor in series with the FPGA input pin like a 10K? Or is this ok as the FPGA is configured as an input and is high impedance?

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    \$\begingroup\$ Those two resistors have nothing to do with each other. What reasoning leads you to think there is even a possibility you might need a higher series resistor? This might be a good time to clarify what you think each one is actually supposed to do because I feel like you are asking the question because you have no idea what either resistor is supposed to do. I could just tell you what each one does, but without knowing your thinking I won't be able to decouple the two ideas from each other. \$\endgroup\$
    – DKNguyen
    Mar 15, 2021 at 7:58
  • \$\begingroup\$ @DKNguyen The 10k pull down resistor does exactly what it says, pulls the FPGA pin to ground when the switch terminal is floating. The 200 ohm resistor is to prevent damage to the FPGA pin if a voltage is connected when configured as output etc. What I am asking is there any reason to make the resistance in series with the FPGA input pin larger? \$\endgroup\$
    – David777
    Mar 15, 2021 at 8:05
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    \$\begingroup\$ There is absolutely no reason. What I am asking you is why you think there might even be a possibility that there is a reason though. That has to be coming from somewhere rather than nowhere. It's kind of like looking at a boat and asking why it doesn't have wheels to get around because cars do. It's just not a thought that crosses your mind unless you have no idea how a boat or car works and I'm trying to figure out what that thought is to correct it. \$\endgroup\$
    – DKNguyen
    Mar 15, 2021 at 8:09
  • \$\begingroup\$ @DKNguyen That's all I wanted to know, if you leave an answer I will accept it. \$\endgroup\$
    – David777
    Mar 15, 2021 at 8:12
  • \$\begingroup\$ Or is your question about a higher series resistance completely unrelated to the presence of the pulldown and you were just trying to give us context? \$\endgroup\$
    – DKNguyen
    Mar 15, 2021 at 8:12

1 Answer 1

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The 200 Ohm series resistor is just to protect the input from user error and limits fault current so the clamp diodes can handle higher power overvoltages.

Increasing them to 10K provides more protection at the expense of slower transitions due to the higher RC time constant formed by the series resistor and input capacitance. You could choose to do this for a manual switch that is exposed to ESD and isn't high speed.

The high impedance of the FPGA inputs is so high that the series resistance otherwise doesn't matter as far as DC voltage levels go.

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