The term a digital circuit refers to a device that works in a binary world. In the binary
world, the only values are zeros and ones. Hence, the inputs of a digital circuit are zeros
and ones, and the outputs of a digital circuit are zeros and ones. Digital circuits are usually
implemented by electronic devices and operate in the real world. In the real world, there
are no zeros and ones; instead, what matters is the voltages of inputs and outputs. Since
voltages refer to energy, they are continuous1. So we have a gap between the continuous
real world and the two-valued binary world. One should not regard this gap as an absurd.
Digital circuits are only an abstraction of electronic devices. In this chapter we explain this
abstraction, called the digital abstraction.