I created these notes to help better understand the role of the components we are soldering on the MAKERbuino. They also contribute towards an understanding of the schematic. Please feel free to suggest changes.
The 16MHz Crystal
This component contains a quartz crystal in a basic circuit which produces an oscillating electrical signal when voltage is applied to it. This up and down signal is the clock for the entire circuit. Digital circuits need a clock signal to advance through the sequence of events that characterize the operation of binary circuits.
There is an adequate explanation of clock signals on Wikipedia but a full understanding requires an understanding of how CPUs operate.
In this case, the crystal is connected to pins 9 and 10 on the microcontroller which are designated for this purpose (according to the datasheet). This crystal will deliver 16 million pulses per second to the microcontroller and that will determine the speed the microcontroller operates at. Simple crystals, like this one, don’t have a specific polarity so you can place it any way around.
Note: the microcontroller does have an internal clock generator but it can be tricky to use - it is much simpler to install your own external one.