Use of D53 (INIT_B) as an input

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I'm using the 16 bit input wing (LXC16245 based) and, for physical reasons, I would like to plug this into ports DH/DL on the Duo.

When I do this, I'm sometimes experiencing a failure for the design to configure, until the wing is physically removed.

I've tracked this down to pin Bit 0 of the wing being low. This connects to D53 which is then routed to the Xilinx INIT_B pin.

So it seems like D53 cannot safely be used as a user input (both the shields use this as an output so are fine).

Is this a known issue?

I can work around this by simply not using Bit 0 of the input wing, and tying it high. But I'm concerned of the possibility of conflicts during configuration, as INIT_B is an open drain output, and can this be driven low.

I guess I could go one step further and de solder the pin on the wing that connects to the Duo.

Anyone got any thoughts on this?


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Hello Dave,

I was trying to squeeze every pin possible into the DUO design to fill the Arduino Mega footprint. INIT_B has a pullup resistor on it so it will be at 1 during bootup and everything can proceed, but unfortunately it does mean that any input that pulls it down to 0 at bootup will hang up the FPGA...

Sorry that was not documented, I'm putting it on my task list to update the documentation for this gotcha.


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Thanks for confirming this Jack.

In the end I just de-soldered bit 0 of the wing, and reconnected the signal elsewhere.

It's now working fine.



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