Basic Theory of Latches

To be able to develop an abstract concept of a latch, we need something from which to abstract. Section 1 introduces us to a number of devices and a circuit that intuitively have in common a property we call memorisation. Devices with this property are referred to as latches. Section 2 presents a formal concept of memorisation discussing it quite thoroughly. The unconventional definition was first proposed in Vingron [1979] in a slightly different form. This section, with its introduction of setting, resetting, and memorising subsets of input events, and the specification of a memory function in a reduced VK-map, poses the prime part of this chapter. In Section 3 we start the logical description of latches (or memory) by introducing inclusions and exclusions. Section 4 introduces and develops the basic memory-evaluation formulas for latches. A generalisation of these formulas is presented in Section 5. The application of both the basic memory-evaluation formulas and the generalised memory-evaluation formulas is an extensive topic, and therefore covered in its own right in the next chapter. Conversely, the present chapter contains no applications.