Adding components to an Uno with SM130 Evaluation Shield on board


I’m using an Arduino Uno with SparkFun’s RFID Evaluation Shield (link)

I’m using the shield with an attached XBee Pro as a timer.

I’m noticing that the times I’m getting aren’t as precise as I need.

When testing, Google Stopwatch will show 48 seconds and the Arduino will report 42.
Stopwatch: 50 Arduino 44
Stopwatch 1072 Arduino 1040

I’d like to add an RTC to the Arduino.

Which pins should I connect it to?