user7572: I have a system that uses two Arduino boards.  One is used to control several different functions, and the other is dedicated to driving a 45 second countdown display (comprised of two large 7 segment displays.)  All this second Arduino needs is a single signal to tell it when to start.  I would like to simply connect an output pin (on #1) to an input pin (on #2) and and send a single pulse to get the clock started.  Fo something this simple, I wanted to avoid using I2C.  Can I do this?  If so, is there anything else I need to be aware of (like connecting ground to ground?)

user2881: Yes, you can. That is indeed the most easy way of communication.
And like you said, you have to connect the grounds of both arduino's. 
Optionally you could add a pull-up of pull-down resistor, so the countdown doesn't accidentally start if the first arduino is still starting up, or is disconnected.

7572: <p>I have a system that uses two Arduino boards.  One is used to control several different functions, and the other is dedicated to driving a 45 second countdown display (comprised of two large 7 segment displays.)  All this second Arduino needs is a single signal to tell it when to start.  I would like to simply connect an output pin (on #1) to an input pin (on #2) and and send a single pulse to get the clock started.  Fo something this simple, I wanted to avoid using I2C.  Can I do this?  If so, is there anything else I need to be aware of (like connecting ground to ground?)</p>
 2881: <p>Yes, you can. That is indeed the most easy way of communication.</p>

<p>And like you said, you have to connect the grounds of both arduino's. </p>

<p>Optionally you could add a pull-up of pull-down resistor, so the countdown doesn't accidentally start if the first arduino is still starting up, or is disconnected.</p>
