Can pull-up and pull-down resistors be in the same circuit? (FPGA)

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nrhznd

Joined Mar 31, 2022
10
I also said this in post #3. I cannot say why the signal is hanging at 2.7V for 13 milliseconds. If I had to guess, it looks like you have TWO driving sources on that line. We had a somewhat vulgar expression for that when it was two TTL gates, which had the property of weak drive in the HIGH state and strong drive in the LOW state. This should not be happening unless the Xilinx part is trying to drive the INIT_B line simultaneously. There should NEVER be a reason to have both a pullup and a pull-down on the same line.

Since I'm not there and don't have access to the documents you're going to have to dig around a bit. If the docs convince you need one of them, I would go with the pullup.
Got it.
Thank you for explaining.
I will try to remove the pull down and do the test again,
if necessary(if the waveform is weird or anything) I will try mounting pull up.
 

drjohsmith

Joined Dec 13, 2021
852
I also said this in post #3. I cannot say why the signal is hanging at 2.7V for 13 milliseconds. If I had to guess, it looks like you have TWO driving sources on that line. We had a somewhat vulgar expression for that when it was two TTL gates, which had the property of weak drive in the HIGH state and strong drive in the LOW state. This should not be happening unless the Xilinx part is trying to drive the INIT_B line simultaneously. There should NEVER be a reason to have both a pullup and a pull-down on the same line.

Since I'm not there and don't have access to the documents you're going to have to dig around a bit. If the docs convince you need one of them, I would go with the pullup.
The reason for the "odd" voltages has been covered on the xilinx forums

The pin on the FPGA , is bi directional,
Its intended that this pin has a pull u on it,

It is being driven by a totem poll chip, as opposed to open collector ,
so at some point both the FPGA and the inverter are driving the line ,

If I remember, the conclusion was some one has tried to perform a power up reset using the init_b pin instead of the prog_b pin.
 
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