4.1 Digital Design Methodology and Flow

Electronic signals can be categorized as analog and digital. An analog signal is a waveform with continuous values, while a digital signal is a waveform with discrete values represented only by 1s and 0s. The performance of analog circuits can be evaluated by SPICE-like simulations, where SPICE is a simulation program with integrated circuit emphasis. The performance of digital circuits can be assessed using static timing analysis. Unlike the analog IC designs described in other chapters in this book, digital IC designs follow a different methodology and design flow. Digital design normally involves complicated logic functions with relatively low operating frequencies as compared to its analog counterpart. The most distinctive difference between digital and analog IC designs is that a complicated digital system can be designed using a hardware description language (HDL) such as Verilog or VHDL (VHSIC HDL, or very high-speed integrated circuit HDL), and a synthesis tool can automatically convert the HDL code into the gate-level representation of the system based on the timing requirements. Here, gates are taken to be simple combinatorial logic gates, such as inverters, AND, OR, NAND, NOR, XOR, XNOR, as shown in Figure 4.1, and simple storage gates, such as latches and flip-flops, which will be discussed in much further detail in Chapter 5. In contrast, analog circuits cannot be synthesized using HDL codes, although recent research has explored the feasibility of synthesizable analog circuits. For this reason, we still treat a high-speed frequency-divider circuit...