A sequential circuit is a circuit with memory. Modern development follows synchronous design methodology and uses a common clock signal to control storage elements. In this chapter, we describe the HDL codes for basic storage elements, introduce the design and coding of “regular sequential circuits,” in which the state transitions in the circuit exhibit a “regular” pattern, as in a counter or shift register, and discuss the use and inference of FPGA's internal memory module.


A sequential circuit is a circuit with memory, which forms the internal state of the circuit. Unlike a combinational circuit, in which the output is a function of input only, the output of a sequential circuit is a function of the input and the internal state. The synchronous design methodology is the most commonly used practice in designing a sequential circuit. In this methodology, all storage elements are controlled (i.e., synchronized) by a global clock signal and the data are sampled and stored at the rising or falling edge of the clock signal. It allows designers to separate the storage components from the circuit and greatly simplifies the development process. This methodology is the most important principle in developing a large, complex digital system and is the foundation of most synthesis, verification, and testing algorithms. All of the designs in the book follow this methodology.

Figure 5.1 Block diagram and functional table of a D FF.

5.1.1 ...

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