This uses one node of a GA144 multi-computer chip to find a crystals resonant frequency and continually pump it to maintain oscillation. This is done to provide an accurate time or frequency reference to the GA144 computers, which are fully asynchronous. In this case it allows us to spend most of our time in a suspended state to conserve power. When running at full speed a node consumes about 4.5 milliwatts, when suspended it only consumes 100 nanowatts.
This was done as part of following these directions from Greenarrays for using the GA144 to control a Sensortag. They go more in depth on this topic in this application note.
These images show the crystal attached the the evaluation board.
The crystal wires are soldered directly to the board, while this is
is not necessary the wires should be kept as short as possible.
The waveform can be monitored with a good oscilloscope.
The probe must be attached before it starts or the disruption
will cause the oscillation to stop.
The vertical lines on the rising edge are the points at which the computer wakes up and pumps a bit of power into the crystal. On the falling edge the pin acts as a sink instead, pulling the wave down.
Here is the code from block 980 that powers the crystal: