The previous slides covered how the MCU controls the hardware interface to the Si1120. This section will illustrate the structure of the firmware used to control the Si1120 (using a C8051F930). The firmware implementation uses the hardware outlined for the three LED system discussed prior. Refer to the three LED schematic on the previous slides to correlate the special function register settings with the pin designations and Si1120 input/output functions. The specific device data sheet should also be consulted for definitions of the specific registers used in these examples. The I/O of the MCU is connected directly to the Si1120 which means they have to be configured to be either input or output depending on the function. For example, the Si1120 MD pin is input so the MCU GPIO pin must be set as a push-pull output. The example shown here provides the I/O settings for all of the control inputs as well as the PCA pin settings for the application. There is a priority encoded crossbar in Silicon Labs MCUs that enable all of the digital peripherals in the device to be placed on the I/O instead of the traditional multiplexed I/O. As a result, there are skip registers and an enable for the crossbar that needs to be set in order to make the PCA signals available to specific pins after they have been configured for the input or output direction. For more information, check out any Silicon Labs MCU data sheet for port input/output more information on the crossbar and its operation.