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9.4 Choice of Flip-Flops

After state reduction and state assignment, the next step in the design process is to choose flip-flop types for the state registers. The issues are identical to those in the counter case studies of Chapter 7.

Usually, we have to decide whether to use J-K flip-flops or D flip-flops. J-K devices tend to reduce the gate count but increase the number of connections. D flip-flops simplify the implementation process and are well suited for VLSI implementations, where connections are at more of a premium than gates. Because the CAD tools mentioned in the previous section were developed to assist in VLSI implementations, it is not surprising that they implicitly assume D flip-flops as the targets of the assignment. Their best assignment may not lead to the minimum logic for a J-K flip-flop implementation.

The following procedure completes the finite state machine implementation, given a particular choice of flip-flops:

1. Given the state assignments, derive the next-state maps from the state transition table.
   
   
2. Remap the next-state maps given the excitation tables for the flip-flops chosen to implement the state bits.
   
   
3. Minimize the remapped next-state function.

9.4.1 Flip-Flop Choice for the Four-Bit Sequence Detector

Let's illustrate the procedure with the 4-bit sequence detector, using the state assignment of Figure 9.39, the encoded state transition table.

Each state has been replaced by its binary encoding given by the state assignment.

Figure 9.40 is the encoded next-state map, organized according to the standard binary sequence and showing the don't cares.

D Implementation To obtain the direct form for determining the state machine implementation with D flip-flops, represent the encoded next-state functions as K-maps. Figure 9.41 contains the four-variable K-maps for the next-state functions Q2+, Q1+, Q0+, given the current state Q2, Q1, Q0 and the input I. The reduced equations that describe the inputs to the D flip-flops are

There are six unique product terms and 15 literals. In terms of discrete gates, the implementation requires 3 three-input gates, 5 two-input gates, and 4 inverters, a total of 12 gates.

J-K Implementation For the J-K implementation, we begin by remapping the inputs based on the J-K excitation tables. Figure 9.42 gives the remapped next-state table, and Figure 9.43 shows the K-maps. The J-K logic equations become

This implementation requires nine unique terms and 14 literals. The gate count is 1 three-input gate, 6 two-input gates, and 3 inverters, a total of 10 gates. This is slightly fewer than the D flip-flop implementation. However, when you use structured logic such as a PLA to implement the functions, the option with fewer product terms is better. In this case, it would be the D implementation.

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This file last updated on 07/15/96 at 06:40:56.
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