In Chapter 6 we saw how to synthesize combinational logic circuits manually from a specification. In this chapter we show how to describe combinational circuits in the VHDL hardware description language, building on our discussion of Boolean expressions in VHDL (Section 3.6) and the initial discussion of VHDL (Section 1.5). Once the function has been described in VHDL, it can be automatically synthesized, eliminating the need for manual synthesis.
Because all optimization is done by the synthesizer, the main goal in writing synthesizable VHDL is to make it easily readable and maintainable. For this reason, descriptions that are close to the function of a design (e.g., a truth table specified with a case statement) are preferable to those that are close to the implementation (e.g., equations using a concurrent assignment statement, or a structural description using gates). Descriptions that specify just the function tend to be easier to read and maintain than those that reflect a manual implementation of the function.
To verify that a VHDL design entity is correct, we write a testbench. A testbench is a piece of VHDL code that is used during simulation to instantiate the design entity to be tested, generate input stimulus, and check the design entity's outputs. While design entities must be coded in a strict synthesizable subset of VHDL, testbenches, which are not synthesized, can use the full VHDL language, including looping constructs. In a typical modern digital design project, at least as much effort goes into design verification (writing testbenches) as goes into doing the design itself.
THE PRIME NUMBER CIRCUIT IN VHDL
In describing combinational logic using VHDL we restrict our use of the language to constructs that can easily be synthesized into logic circuits.
Specifically, we restrict combinational circuits to be described using only concurrent signal assignment statements, case statements, if statements, or by the structural composition of other combinational design entities.
In this section we look at four ways of implementing the prime number (plus 1) circuit we introduced in Chapter 6 as combinational VHDL.
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