Digital VLSI Design and Simulation with Verilog

2 Sequential Circuit Design

This chapter describes flip-flops (F/F), registers, counters, and the finite state machine. F/Fs are part of all sequential circuit designs [1]. A sequential logic circuit can be designed using flip-flop and combinational logic circuits. Registers and counters are commonly used in digital systems. The designs of all flip-flops are described in the next subsection.

2.1 Flip-flops (F/F)

F/Fs are a single-bit binary storage device, that have two states, one is pre-set and the other is clear state. It has two outputs and these are complementary to each other. Each flip-flop also has three control inputs. The first control signal is the clock; it is used to synchronize the circuit. Other control signals are set and reset; these are used to set or reset the value of the corresponding flip- flops. These are also used to design registers, counters and other sequential circuits. There are four types of flip-flop.

2.1.1 S-R F/F

S-R is the basic F/F; it means other F/Fs can be designed using this F/F. It has two inputs and two outputs. It has an indeterminant condition (or undefined) when S = 1, R = 1. F/Fs are designed with the help of logic gates and the clock.

The S-R F/F designed with NAND-gate and with clock input is depicted in Figure 2.1.

Figure 2.1 Clocked S-R F/F.

A truth table of S-R F/F is given in Table 2.1.

Table 2.1 Truth table of an S-R F/F. ...