Introduction to Digital Systems: Modeling, Synthesis, and Simulation Using VHDL

8.3 LATCHES

Latches and flip-flops are the basic building blocks of most sequential circuits. A latch is a sequential circuit that watches all the inputs continuously and changes its outputs at any time independently of a clocking signal. On the other hand, a flip-flop changes its outputs only at times determined by a clocking signal. A latch is a bistable (two stable output states) device that can store one bit (a logic 0 or 1) of data. Because of their storing capacity, latches are sometimes referred to as bistable memory devices. Latches may be used in groups of 4, 8, 16, or 32 for temporary storage of a nibble, byte, or word of data. They are also used often in microprocessor-based design. The most commonly used types of latches are described in the following sections.

8.3.1 SR Latch

A set-reset latch, commonly referred to as an SR latch, has two inputs (S and R), one true output (Q), and one complemented output (), as shown in Figure 8.2. The crossing of the outputs is known as cross coupling. This circuit is said to employ cross-coupled feedback. The feedback connects the output of a circuit to its input. When output Q is equal to 1, the latch is said to be in the set state; similarly, when Q is equal to 0, the latch is said to be in the clear (or reset) state. For the basic NOR latch circuit, both inputs normally are at the 0 logic level. If one input (S or R) changed to the ...