Setting the mode to the Si1120 requires a two stage process that involves controlling three output pins of the MCU: MD, STX and SC. The description of how to set the mode was discussed earlier in this module, so the firmware implementation is shown here. Within the Silicon Labs MCUs there is a crossbar. Each peripheral that passes through the crossbar has to be enabled which will provide the connection to the I/O pins. When setting up the Si1120 mode, the designer will reuse the STX pin as it is used to set the mode as well as initiate measurements. For this reason, step one, in this illustration, disables the PCA through the crossbar allowing the firmware to control the GPIO pin directly. The remaining steps follow the mode setting procedure outlined previously by setting the Si1120 to the low power mode by raising SC and changing the I/O. The numbered lines of code correspond to the steps given and follow the timing diagram for setting the modes.