Verilog Basics Nattha Jindapetch November 2008

Agenda Logic design review Verilog HDL basics LABs

Logic Design Review

Basic Logic gates

Additional Logic Gates

Two Types Combinational circuit No "memory" Its output depends only upon the current state of its inputs. Sequential circuit Contains memory elements Its output depends not only upon the current state of its inputs, but also on the current state of the circuit itself. Two Styles: Synchronous and Asynchronous

Combinational Circuits: examples Multiplexers 2 n x 1 multiplexer receives 2 n input bits and n selector bits, and outputs exactly one of the input bits, determined by the pattern of the selector bits.

Combinational Circuits: examples Demultiplexers 1 x 2 n DMUX is the inverse of MUX It takes 1 input and transmits that input on exactly one of its outputs, determined by the pattern of its n selector bits.

Combinational Circuits: examples Encoders 2 n x n encoder takes 2 n inputs and sets each of its n outputs, based upon the pattern of its inputs. an encoder is the inverse of a decoder.

Combinational Circuits: examples Decoders An n x 2 n decoder takes n inputs and sets exactly one of its 2 n outputs, based upon the pattern of its inputs.

Sequential Circuits Circuits with feedback Cross-coupled NOR gates R SQ Q’ 0 No change Reset Set 1 Not allowed

Sequential Elements Latches vs Flip-Flops Latches work on Level-sensitive Flip-Flops work on Edge-triggered RS, JK, D, T S RQ Q’ 0 No change Reset Set 1 Not allowed J KQ Q’ 0 No change Toggle DQ TQ 01 10

Counters Synchronous counter Clock of synchronous counter are same clock. Asynchronous counter Clock of second flip- flop is Q of first flip- flop. The clock of asynchronous counter are different source.

Registers A register is used for storing several bits of digital data. It basically consists of a set of flip-flops.flip-flops each flip-flop representing one bit of the register. Thus, an n-bit register has n flip-flops. A Simple Shift Register Consisting of D-type Flip-flops

Verilog HDL basics

HDL (Hardware Description Language) Big picture: Two main HDLs out there VHDL Designed by committee on request of the Department of Defense Based on Ada Verilog HDL Designed by a company for their own use Based on C Both now have IEEE standards Both are in wide use

Verilog Description Styles Verilog supports a variety of description styles Structural explicit structure of the circuit e.g., each logic gate instantiated and connected to others Behavioral program describes input/output behavior of circuits Mixed

Verilog Module module module-name (list-of-port); input/output declarations local net declarations parallel statements endmodule

Structural Module: example1 module half_adder (s, a, b, co); inputa, b; outputs, co; wire w0, w1, w2; assign w0 = a & b, w1 = ~w0, w2 = a | b, s = w1 & w2, co = w0; endmodule a bw0 w1 w2 co s a b s co

Structural Module: example1 module full_adder (s, a, b, co, ci); inputa, b, ci; outputs, co; wire w0, w1, w2; assign co = w1 | w2; half_adder i0 (.co(w1),.s(w0),.a(a),.b(b)); half_adder i1 (.co(w2),.s(s),.a(w0),.b(ci)); endmodule

Structural Module: example1 module adder4 (s, a, b, co, ci); input [3:0] a, b; input ci; output [3:0]s, outputco; wire w0, w1, w2; full_adder i0 (.co(w0),.s(s[0]),.a(a[0]),.b(b[0]),.ci(ci)); full_adder i1 (.co(w1),.s(s[1]),.a(a[1]),.b(b[1]),.ci(w0)); full_adder i2 (.co(w2),.s(s[2]),.a(a[2]),.b(b[2]),.ci(w1)); full_adder i3 (.co(co),.s(s[3]),.a(a[3]),.b(b[3]),.ci(w2)); endmodule

Behavioral Module: example1 module adder4 (s, a, b, co, ci); input [3:0] a, b; inputci; output [3:0]s, outputco; reg [3:0]s; regco; or b or ci) {co, s} = a + b + ci; endmodule

Verilog Data Types  Possible Values:  0: logic 0, false  1: logic 1, true  X: unknown logic value  Z: High impedance state  Registers and Nets (wires) are the main data types  Integer, time, and real are used in behavioral modeling, and in simulation  Note that they are not synthesized !

Verilog Registers Abstract model of a data storage element A reg holds its value from one assignment to the next The value “ sticks ” Register type declarations reg a; // a scalar register reg [3:0] b; // a 4-bit vector register

Verilog Nets Nets (wires) model physical connections They don’t hold their value They must be driven by a “driver” (i.e. a gate output or a continuous assignment) Their value is Z if not driven Wire declarations wire d; // a scalar wire wire [3:0] e; // a 4-bit vector wire

Verilog Parameters Used to define constants parameter size = 16, value = 8; wire [size-1:0] bus; // defines a 15:0 bus

Verilog Operators Arithmetic operators:+, -, *, /, % Logical operators:&&, ||, ! Bitwise operators:&, |, ~, ^, ^~ Equality operators:==, !=, Relational operators:>, =, <= Reduction operators:&, ~&, |, ~|, ^ Shift operators:>>, << Conditional:?:

Example2: 4:1 multiplexer module mux4 (s, d, z); //bitwise operators input [1:0] s; input [3:0] d; output z; assign z =(~s[1] & ~s[0] & d[0]) | (~s[1] & s[0] & d[1]) | ( s[1] & ~s[0] & d[2]) | ( s[1] & s[0] & d[3]) ; endmodule

Example2: 4:1 multiplexer module mux4 (s, d, z); //using conditional operators input [1:0] s; input [3:0] d; output z; assign z = s[1] ? (s[0] ? d[3] : d[2]) : (s[0] ? d[1] : d[0]); endmodule

Verilog Assignments Two types: Continuous Assignments assign values to nets This means combinational logic Procedural Assignments assign values to registers Only allowed inside procedural blocks (initial and always)

Continuous Assignments Models combinational logic using a logical expression instead of gates Assignment is evaluated whenever any signal changes wire a, b, out; assign out = ~(a & b); wire [15:0] sum, a, b; wire cin, cout; assign {cout,sum} = a + b + cin;

Procedural Assignments Assigns values to register types They do not have a duration The register holds the value until the next procedural assignment to that variable They occur only within procedural blocks initial and always They are triggered when the flow of execution reaches them

always Blocks When is an always block executed? always Starts at time 0 or b or c) Whenever there is a change on a, b, or c Used to describe combinational logic foo) Whenever foo goes from low to high Used to describe sequential logic bar) Whenever bar goes from high to low

Inside always blocks Procedural assignments if statements case statements for, while, forever statements wait statements

Blocking vs Non-blocking

begin q1 = x1; q2 = q1; z1 = q2; end Blocking begin q1 <= x1; q2 <= q1; z1 <= q2; end Non-Blocking

Quick Review Continuous assignments to wires assign variable = exp; Result in combinational logic Procedural assignment to regs Always inside procedural blocks (always blocks in particular for synthesis) blocking variable = exp; non-blocking variable <= exp; Can result in combinational or sequential logic

Quick Review module name (args…); input …; // define inputs output …; // define outputs wire… ; // internal wires reg …; // internal regs, possibly output // the parts of the module body are // executed concurrently endmodule

comment in Verilog_HDL // Single-line comment /*…………. …………..*/ multiple-line comment module mux2_1(out,a,b,sel); // port declare. input a,b,sel; output out; wire sel_,a1,b1 /* structural design using logic operator mux2_1 */ not (sel_,sel); and (a1,a,sel),(b1,b,sel); or (out,a1,b1); endmodule;

Naming in Verilog_HDL ข้อกำหนด ในการตั้งชื่อไฟล์ การตั้งชื่ออุปกรณ์ และ การเชื่อมต่อ สายสัญญาณ จะต้องไม่เป็นคำสงวนของภาษา ระหว่างข้อความ หรือตัวเลข สามารถใช้ เครื่องหมาย _ หรือ $ ได้ ตัวแรกที่ตั้งชื่อต้องเป็น ตัวอักษร a-z หรือ A-Z หรือ _ ตัวอักษรตัวใหญ่ และ ตัวเล็ก จะมีความหมายแตกต่างกัน

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