title: Atmega64 pins question. SCL/SDA same as TXD/RDX?
body: <p>I'm not an Atmega pro by any means whatsoever. In fact, never worked with them.  I'm trying to add an external eeprom using i2c interface to a device that uses an Atmega64 controller.  From the datasheet, I see that pins 25 and 26 are labeled PD0/PD1 and are the SCL/SDA for i2c.  That's fine and dandy.  So then I go to the board and start tracing.  I see that those go to a rocker switch with some simple circuitry that I haven't had a good look at yet, but I assumed it was something i2c compatible.  But then what dawned on me was that this device has two such rockers so I expected to trace the other rocker back to the same two pins (since i2c is a bus that can have multiple slaves).  Instead, they traced back to the two adjacent pins 27 and 28 labeled PD2/PD3 and RXD1/TXD1.  Well, now that confused me.</p>
  
  <p>I expected that I could add the memory in parallel on the SCL/SDA lines, but does what I now figure that what I discovered either means:</p>
  
  <ol>
  <li>SCL/SDA is interchangeable with RXD1/RXD2 or</li>
  <li>I'm confused on how SCL/SDA works and its not a bus or it can be used as a simple input?</li>
  </ol>
  
  <p>Sadly, I'm not really sure what I'm asking.  Can someone shed some light on this?  Should I still be able to add the memory to SCL/SDA or probably not?</p>
  
  <p>Thanks.</p>
  
  <p>Steve</p>