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Cadenti
logic family
medium scale integration
metal oxide semiconductor
ac register
accumulator logic
adder circuit
basic computer design
branch unconditionally
flowchart
input and output communication
input output instruction
input register
interrupt cycle
logic adder circuits
logic gates
output register
register and memory
binary code
binary number
clock pulse
data types
decimal numbers
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Excitation Tables
The characteristic tables of flip-flops specify the next state when the inputs and the present state are known. During the design of sequential circuits we usually know the required transition from present state to next state and wish to find the flip-flop input conditions that will cause the required transition. For this reason we need a table that lists the required input combinations for a given change of state. Such a table is called a flip-flop excitation table. Table 1-3 lists the excitation tables for the four types of flip-flops. Each table consists of two columns, Q(t) and Q(t + 1), and a column for each input to show how the required transition is achieved. There are four possible transitions from present state Q(t) to next state Q(t + 1). The required input conditions for each of these transitions are derived from the information available in the characteristic tables. The symbol x in the tables represents a don't-care condition; that is, it does not matter whether the input to the flip-flop is 0 or 1.
The reason for the don't-care conditions in the excitation tables is that there are two ways of achieving the required transition. For example, in a JK flip-flop, a transition from present state of 0 to a next state of 0 can be achieved by having inputs J and K equal to 0 (to obtain no change) or by letting J = 0 and K = 1 to clear the flip-flop (although it is already cleared). In both cases J must be 0, but K is 0 in the first case and 1 in the second. Since the required transition will occur in either case, we mark the K input with a don't-care x and let the designer choose either 0 or 1 for the K input, whichever is more convenient.