Digital System Clocking: High-Performance and Low-Power Aspects

CHAPTER 2

THEORY OF CLOCKED STORAGE ELEMENTS

The function of a clocked storage element is to capture the information at a particular moment in time and preserve it for as long as it is needed by the digital system. Having said this, it is not possible to define a storage element without defining its relationship to a clocking mechanism in a digital system, which is used to determine discrete time events. This definition is general and should include various ways of implementing a digital system. More particularly, the element that determines time in a synchronous system is the clock.

2.1. LATCH-BASED CLOCKED STORAGE ELEMENTS

The simplest storage element consists of an inverter followed by another inverter, which provides positive feedback, as shown in Fig. 2.1a. The information bit at the input is thus locked due to the positive feedback loop, and it can be only changed “by force” (i.e., by forcing the output of the feedback inverter to take another logic value). This configuration is used very frequently, and is also known as the keeper, a circuit that keeps (preserves) the information on a particular node.

If we were to avoid the power dissipation associated with overpowering (forcing) the keeper to change its value, we must introduce nodes that will help us in changing the logic value stored in the feedback loop. For that purpose we are free to use logic NAND or NOR gates, as shown in Fig. 2.1. Of particular interest is a simple modification of the diagram that emphasizes the ...