Digital pin interfaced with analog pin

Hi everyone. I’ve a question about interfacing two boards- one a photon, the other an arduino.
Firstly, I think it should be possible, but just to check:
Is there any reason the analog pins on an arduino couldn’t measure a voltage on the photon’s breadboard, as long as they share a ground?

Secondly, since I believe that to be possible, I’ve been trying to implement it by having the arduino analog pins read the voltage from a photon digital pin. however, when I do this, the analog pin reads ~2.5V all the time, regardless of whether the photon’s pin is set to HIGH or LOW.

Any insight on this would be appreciated; it seems to me that the digital pin isn’t actually writing. Sorry if there’s documentation I’ve overlooked, but I’ve been searching without any luck.

You may want to show the code of both sides?

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Haha, thank you! Simple things. In the process of copying the code for you, I discovered I forgot to declare the pin as OUTPUT.
So, that was the issue; I suppose my lesson from this is never assume a problem is more complicated than it is.

Thanks for the help, minimal though it was!