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ECM585 Special Topics in Computer Design Lecture 2. Combinational Logic Design 1

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### ECM585Special Topics in Computer DesignLecture 2. Combinational Logic Design 1

Prof. Taeweon Suh

Computer Science Education

Korea University

Introduction

A logic circuit is composed of:

- Inputs
- Outputs
- Functional specification
- Timing specification

Circuits

- Nodes
- Inputs: A, B, C
- Outputs: Y, Z
- Internal: n1
- Circuit elements
- E1, E2, E3

Types of Logic Circuits

- Combinational Logic
- Memoryless
- Outputs determined by current values of inputs
- Sequential Logic
- Has memory
- Outputs determined by previous and current values of inputs

Rules of Combinational Composition

- Every circuit element is itself combinational
- Every node of the circuit is either designated as an input to the circuit or connects to exactly one output terminal of a circuit element
- The circuit contains no cyclic paths: every path through the circuit visits each circuit node at most once
- Example:

Boolean Equations

- Functional specification of outputs in terms of inputs
- Example:

S = F(A, B, Cin)

Cout = F(A, B, Cin)

Terminology

- The Complementof a variable A is A
- A variable or its complement is called literal
- The AND of one or more literals is called a product orimplicant
- Example: AB, ABC, B
- OR of one or more literals is called a sum
- Example: A + B
- Order of operations
- NOT has the highest precedence, followed by AND, then OR
- Example: Y = A + BC

Sum-of-Products (SOP) Form

- All Boolean equations can be written in SOP form
- A mintermis a product (AND) of literals involving all of the inputs to the function
- Each row in a truth table has a minterm that is TRUE for that row (and only that row)
- The function is formed by ORing the minterms for which the output is TRUE
- Thus, a sum (OR) of products (AND terms)

Y = F(A, B, C) = AB + AB

Product-of-Sums (POS) Form

- All Boolean equations can be written in POS form
- A maxterm is a sum (OR) of literals
- Each row in a truth table has a maxterm that is FALSE for that row (and only that row)
- The function is formed by ANDing the maxterms for which the output is FALSE
- Thus, a product (AND) of sums (OR terms)

Y = F(A, B, C) = (A + B)(A + B)

Boolean Equations Example

- You are going to the cafeteria for lunch
- You won’t eat lunch (E: eat)
- If it’s not open (O: open)
- If they only serve corndogs (C: corndogs)
- Write a truth table for determining if you will eat lunch (E)

When to Use SOP and POS?

- SOP produces the shortest equations when the output is true on only a few rows of a truth table
- POS is simpler when the output is false on only a few rows of a truth table

HDL

- Hardware description language (HDL): allows designer to specify logic function only. Then a computer-aided design (CAD) tool produces the optimized gates.
- Most commercial designs built using HDLs
- Two leading HDLs:
- Verilog
- developed in 1984 by Gateway Design Automation
- became an IEEE standard (1364) in 1995
- VHDL
- Developed in 1981 by the Department of Defense
- Became an IEEE standard (1076) in 1987

HDL to Gates

- Simulation
- Input values are applied to the circuit
- Outputs checked for correctness
- Millions of dollars saved by debugging in simulation instead of hardware
- We are going to use ModelSim Xilinx Edition-III (MXE-III) for simulation
- http://www.xilinx.com/ise/optional_prod/mxe.htm
- Synthesis
- Transforms HDL code into a netlist describing the hardware (i.e., a list of gates and the wires connecting them)
- You can see the synthesized result with ISE Webpack

IMPORTANT:

When describing circuits using an HDL, it’s critical to think of the hardware the code should produce.

Verilog Modules

- Two types of Modules:
- Behavioral: describe what a module does
- Structural: describe how a module is built from simpler modules

Behavioral Verilog Example

Verilog:

module example(input a, b, c,

output y);

assign y = ~a & ~b & ~c | a & ~b & ~c | a & ~b & c;

endmodule

Behavioral Verilog Simulation

Verilog:

module example(input a, b, c,

output y);

assign y = ~a & ~b & ~c | a & ~b & ~c | a & ~b & c;

endmodule

Behavioral Verilog Synthesis

Verilog:

module example(input a, b, c,

output y);

assign y = ~a & ~b & ~c | a & ~b & ~c | a & ~b & c;

endmodule

Synthesis:

Verilog Syntax

- Verilog is case sensitive. So, reset and Reset are not the same signal.
- Verilog does not allow you to start signal or module names with numbers. So, for example, 2mux is an invalid name.
- Verilog ignores whitespace.
- Comments come in single-line and multi-line varieties:
- // single line comment
- /* multiline

comment */

Structural Modeling - Hierarchy

module and3(input a, b, c,

output y);

assign y = a & b & c;

endmodule

module inv(input a,

output y);

assign y = ~a;

endmodule

module nand3(input a, b, c

output y);

wire out; // internal signal

and3 andgate(a, b, c, out); // instance of and3

inv inverter(out, y); // instance of inverter

endmodule

Bitwise Operators

module gates(input [3:0] a, b,

output [3:0] y1, y2, y3, y4, y5);

/* Five different two-input logic

gates acting on 4 bit busses */

assign y1 = a & b; // AND

assign y2 = a | b; // OR

assign y3 = a ^ b; // XOR

assign y4 = ~(a & b); // NAND

assign y5 = ~(a | b); // NOR

endmodule

// single line comment

/*…*/ multiline comment

Reduction Operators

module and8(input [7:0] a,

output y);

assign y = &a;

// &a is much easier to write than

// assign y = a[7] & a[6] & a[5] & a[4] &

// a[3] & a[2] & a[1] & a[0];

endmodule

Conditional Assignment

module mux2(input [3:0] d0, d1,

input s,

output [3:0] y);

assign y = s ? d1 : d0;

endmodule

? : is also called a ternary operator because it

operates on 3 inputs: s, d1, and d0.

Internal Variables

module fulladder(input a, b, cin, output s, cout);

wire p, g; // internal nodes

assign p = a ^ b;

assign g = a & b;

assign s = p ^ cin;

assign cout = g | (p & cin);

endmodule

Numbers

Format: N\'Bvalue

N = number of bits, B = base

N\'B is optional but recommended (default is decimal)

Bit Manipulations: Example 1

assign y = {a[2:1], {3{b[0]}}, a[0], 6’b100_010};

// if y is a 12-bit signal, the above statement produces:

y = a[2] a[1] b[0] b[0] b[0] a[0] 1 0 0 0 1 0

// underscores (_) are used for formatting only to make it easier to read. Verilog ignores them.

Bit Manipulations: Example 2

Verilog:

module mux2_8(input [7:0] d0, d1,

input s,

output [7:0] y);

mux2 lsbmux(d0[3:0], d1[3:0], s, y[3:0]); // from slide 12

mux2 msbmux(d0[7:4], d1[7:4], s, y[7:4]); // from slide 12

endmodule

Synthesis:

Z: Floating Output

Verilog:

module tristate(input [3:0] a,

input en,

output [3:0] y);

assign y = en ? a : 4\'bz;

endmodule

Synthesis:

Delays

module example(input a, b, c,

output y);

wire ab, bb, cb, n1, n2, n3;

assign #1 {ab, bb, cb} = ~{a, b, c};

assign #2 n1 = ab & bb & cb;

assign #2 n2 = a & bb & cb;

assign #2 n3 = a & bb & c;

assign #4 y = n1 | n2 | n3;

endmodule

Delays

module example(input a, b, c,

output y);

wire ab, bb, cb, n1, n2, n3;

assign #1 {ab, bb, cb} =

~{a, b, c};

assign #2 n1 = ab & bb & cb;

assign #2 n2 = a & bb & cb;

assign #2 n3 = a & bb & c;

assign #4 y = n1 | n2 | n3;

endmodule

Testbenches

- HDL code written to test another HDL module, the device under test (dut), also called the unit under test (uut)
- Not synthesizeable
- Types of testbenches:
- Simple testbench
- Self-checking testbench
- Self-checking testbench with testvectors (will be explained later)

Example

Write Verilog code to implement the following

function in hardware:

y = bc + ab

Name the module sillyfunction

Example

Write Verilog code to implement the following

function in hardware:

y = bc + ab

Name the module sillyfunction

Verilog:

module sillyfunction(input a, b, c, output y);

assign y = ~b & ~c | a & ~b;

endmodule

Simple Testbench

module testbench1();

reg a, b, c;

wire y;

// instantiate device under test

sillyfunctiondut(a, b, c, y);

// apply inputs one at a time

initial begin

a = 0; b = 0; c = 0; #10;

c = 1; #10;

b = 1; c = 0; #10;

c = 1; #10;

a = 1; b = 0; c = 0; #10;

c = 1; #10;

b = 1; c = 0; #10;

c = 1; #10;

end

endmodule

Self-checking Testbench

module testbench2();

reg a, b, c;

wire y;

// instantiate device under test

sillyfunction dut(a, b, c, y);

// apply inputs one at a time

// checking results

initial begin

a = 0; b = 0; c = 0; #10;

if (y !== 1) $display("000 failed.");

c = 1; #10;

if (y !== 0) $display("001 failed.");

b = 1; c = 0; #10;

if (y !== 0) $display("010 failed.");

c = 1; #10;

if (y !== 0) $display("011 failed.");

Self-checking Testbench (cont.)

a = 1; b = 0; c = 0; #10;

if (y !== 1) $display("100 failed.");

c = 1; #10;

if (y !== 1) $display("101 failed.");

b = 1; c = 0; #10;

if (y !== 0) $display("110 failed.");

c = 1; #10;

if (y !== 0) $display("111 failed.");

end

endmodule

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