CS641 Class 20 Handout: Intro to Verilog

Note: hw6 has new last problem re Verilog

HDLs: Hardware Description Language: Verilog

In serious use. With today’s complexity of circuits, logic diagrams are almost useless.

Verilog is one of two contenders, other is VHDL.

Verilog uses C syntax, so best for us.

Important feature: can build up little blocks into bigger blocks.

With this, we can “build” little CPUs. Today: just combinational, plus R-S FF

Why not just use C? or Java?

We could write a function for a combinatorial circuit, but in C, we need to call it to get the result, whereas in a circuit, it is calculated continuously.

Verilog has a “continuous assignment” capability: set up a little machine that continuously reads from inputs, expresses outputs.

Of course this is implemented in software via event handling. But the abstraction is there for us to use.

Also, work directly with time, unlike in C/Java—make this happen, then that happen,...

Intro to Verilog: look at handout

Circuit Modules needed for parts of CPUs

Last time: register files

See picture on pg. 304, Fig. 4.2: several Mux’s. Inputs on one side, control signals coming in at top or bottom.

Also in Fig. C.3.6, pg C-19

Data Multiplexor (here 2-to-1, n-bit-wide)

N instances of 1-bit-wide mux

How do we build a 1-bit-wide mux?

Look at diagrams on pg. C-19.

Flip thru Chap 4, see many MUX’s, all with 2 n-bit inputs, one n-bit output, one bit control

Is possible to have more than 2 inputs, and more bits of control

Modules for CPUs, continued

Arithmetic and Logic Unit

° Most processors contain a special logic block called “Arithmetic and Logic Unit” (ALU)

° We’ll show you an easy one that does ADD, SUB, bitwise AND, bitwise OR

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° Our simple ALU

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° Adder/Subtracter Design -- how?

° Truth-table, then determine canonical form, then minimize and implement as we’ve seen before

° Or, look at breaking the problem down into smaller pieces that we can cascade or hierarchically layer

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° Adder/Subtracter – One-bit adder, for ith bit:

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Next time: put together N 1-bit adders to make 1 N-bit adder