FSM-based Digital Design using Verilog HDL

Appendix B: Counting and Shifting Circuit Techniques

This appendix contains a number of techniques to help in the development of synchronous binary counters and shift registers. These are used in some of the designs covered in chapters throughout the book.

B.1 BASIC UP AND DOWN SYNCHRONOUS BINARY COUNTER DEVELOPMENT

The development of synchronous pure binary up/down counters can be mechanized to produce a general n-stage pure binary counter. This can then be implemented directly using PLDs/complex PLDs (CPLDs)/FPGA devices. To illustrate how this is achieved, a four-stage down-counter is described below.

Table B.1 shows a down-counter with Q0 the least significant bit. This counter is to be designed as a synchronous counter so all flip-flops will be clocked by the same clock edge. Also, the flip-flops will be T flip-flops. Most CPLDs and FPGAs can support the T flip-flop, either directly or by using D-type flip-flops with an exclusive OR input.

The equation for the T input of each flip flop can be obtained by inspection of Table B.1 and entering a product term for every 0-to-1 and 1-to-0 transition required by each flip flop. For example, from Table B.1 the equation for flip flop q0 · t will be

Each state where the T flip-flop is to change state (0 to 1 or 1 to 0) is entered into the equation.

This can then be written in terms of the Q0Q1Q2Q3 outputs, or simply entered into a Karaugh ...