Once the initialization sequence is completed, the firmware has to be set up to service the operation of the Si1120 using the PCA interrupt service routines (ISR). PCA channel one was configured to be the square wave output which was generated by adding the DIVIDE_RATIO value to the current count to always provide the same overflow rate. Within the channel one ISR, the next count value is determined and loaded into the compare registers to provide the next overflow. That is all the ISR does for this PCA output driving the STX signal.