There are only two valid states for digital input/output peripherals, which are commonly represented by On and Off, High and Low, Open and Off, and On and Off. For example, the on and off states of a relay contact, the on and off states of an indicator light, etc. Storing the status information of a digital input/output peripheral only requires one binary bit. The vast majority of MCU programmable I/O pins can output high and low levels, and the level status of these I/O pins and corresponding interface circuits can control the status of digital output peripherals. Therefore, binary bits “1” and “0” are used inside the MCU to represent the status of digital output peripherals, respectively. At the same time, the state of digital input peripherals can be obtained by reading the level of the MCU’s I/O pins, and this state can be saved using Boolean variables.
