I2C is a popular communication protocol in embedded systems.  When interfacing with the slave device a pull-up resistor is needed on each bi-directional line.  One common question that arises is "what size pull-up resistor should I use?".  Instead of going through a bunch of theory and calculations I thought it would be easier to show what happens to the signals when different resistor values are used.

Since the Arduino is a popular micro-controller among hobbyists we'll use it for the following examples.  We'll interface the Arduino to a DS3231 Real Time Clock, which is a 5 volt device.  We'll also look at the effects on the signal with varying speeds of I2C (100 and 400kHz clock signals).