Now that the mode is set, the Si1120 can start being used to take IR proximity measurements. The STX pin is now used as a source to start the measurement and can be controlled by the MCU hardware (by using timers) or by firmware. When the STX line is driven high the measurement cycle begins. At the time the STX goes high, the Si1120 takes the ambient IR reading and stores this for later. After the ambient measurement, the TXO signal is driven low which then starts the constant current flow through the LED. During this time when TXO is asserted, the Si1120 actually measures a combination of ambient IR plus the reflected IR. To get the reflectance, the Si1120 subtracts the ambient IR measured at the beginning of the measurement cycle from the overall measurement when the LED is emitting the IR energy. If the accuracy of the measurement is not required, for example when doing a simple threshold detection, then a measurement cycle can be aborted by bringing the STX line low again, thus raising PRX and TXO. This can reduce overall power consumption by minimizing the pulse width outputs.