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I am using Cyclone IV FPGA. Does it have the IO_PULLUP pin you mentioned? Also, how I control this IO_PULLUP pin?
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The Cyclone IV does not appear to have an IO_PULLUP control (the Stratix II has that pin).
A search of pull-up in the Cyclone IV handbook yields the following statements:
"The user I/O pins and dual-purpose I/O pins have weak pull-up resistors, which are always enabled (after POR) before and during configuration."
and then a little later on
"In user mode, the user I/O pins function as assigned in your design and no longer have weak pull-up resistors."
So, the FPGA has weak pull-ups enabled when the device is not configured. If you want an external signal held low, then you'll have to use a pull-down.
Cheers,
Dave
Honored Contributor
