At this point the MCU has controlled the I/O to the Si1120 in order to set the modes and start a measurement and, as a result, the PRX signal is driven low to represent the IR energy detected. The amount of time the PRX signal is low is a representation of the amount of energy that is reflected. The longer the pulse the more reflected energy has been detected, or in other words, the closer an object is to the sensor. The MCU used to control the I/O is also responsible for measuring the PRX pulse width. In the Silicon Labs C8051F microcontrollers, the PCA timing unit can be used. These modules are capable of performing rising and falling edge triggered captures of the PRX signal. When the PRX signal is de-asserted, the PCA module captures the time value of the counter. After the measurement completes, the PCA can then capture the rising edge of the PRX signal. There are now two time values that represent the complete low pulse interval. The subtraction of the falling edge count from the rising edge counts provides the time for the PRX pulse and can be passed on to the proximity algorithms.