Behavioral Modelling - 1

Verilog Behavioral Modelling Behavioral Models represent functionality of the digital hardware. It describes how the circuit will operate without specifying hardware. There are two important keywords for behavioral description. initial Specifies a single-pass behavior. always Specifies cyclic behavior.

Behavioral Modelling initial keyword is followed by a single statement or a begin…end block The statement following initial is executed once at t=0 and it expires. always keyword is followed by a single statement or a begin…end block The statement following always is executed repeatedly until simulation is stopped. Chapter 5 3

Use of initial and always Chapter 5 4

Use of initial and always. Defines a clock with 20 unit clock period. Chapter 5 5

always and event control expression) begin is called event control operator. waits until the event occurs. When event takes place begin.. end block will be executed. Example: wait for positive edge of clock clock) clock or negedge reset) Chapter 5 6

(A or B or C or OUT2) /* Note: OUT2 is output of one statement but input of another statement */ begin OUT1 = (A&B) | (B&C) | (C&A) ; OUT2 = {A,B,C}; OUT3 = OUT2 >>> 2; OUT4 = OUT2 << 1; /* inside the always block all instructions are executed sequentially, like in programming languages (exceptions will be mentioned later) */ end Multiple statements in always block

(A or B or C) OUT1 = (A&B) | (B&C) | (C&A); (A, B, C) OUT2 = {A, B, C}; (*) OUT3 = OUT2 >> 1; /* All the three always blocks will run concurrently i.e. all the LHS of each of the always blocks will be calculated at the same instant of time */ Multiple always blocks

Procedural Blocks module MultiplexerD (input a, b, s, output w); reg w; b, s) begin if (s) w = b; else w = a; end endmodule alwaysstatement if-elsestatement Can be used when the operation of a unit is too complex to be described by Boolean or conditional expressions Sensitivity list

There are two types of assignments in verilog. In blocking assignment the instuctions are executed sequentially one after the other. In non blocking the left hand side is assigned the value of the right hand side simultaneously at the end of the simulation timestep. These statements are executed in parallel. Blocking and non blocking assignments

Blocking and Nonblocking assignment examples  Blocking assignment: Evaluation and assignment are immediate ( a or b or c) begin x = a | b; //Evaluate a|b and assign value to x y = a ^ b ^ c; // Evaluate a^b^c and assign value to y z = b & ~c; //Evaluate b & ~c and assign value to z end  Non Blocking assignment: Assignments deferred until right hand side has been evaluated (assigned at the end of timestep) (a or b or c) begin x <= a | b; //Evaluate a|b but defer assignment of x y <= a ^ b ^ c; // Evaluate a^b^c but defer assignment of y z <= b & ~c; // Evaluate b & ~c but defer assignment of z end

Blocking vs. Nonblocking If A=3 and B=5 B=A ; C=B will set B=3 and C=5 B<=A ; C<=B+2 …… will result B=3 and C=7 Chapter 5 12

Swap Operation  Blocking (*) begin temp = b; b=a; a=temp; end  Non blocking (*) begin a<=b; b<=a; end Blocking and non blocking assignments

module blocking (in, clk, out); input in, clk; output out; reg out, q1, q2; (posedge clk) begin q1 = in; q2 = q1; out = q2; end endmodule Blocking and non blocking assignment examples module nonblocking(in, clk, out); input in, clk, out; output out; reg q1, q2, out; (posedge clk) begin q1 <= in; q2 <= q1; out <= q2; end endmodule

Circuits which the above code is synthesized to Blocking Assignment Non Blocking Assignment

You represent all your combinational logic by blocking assignment. You represent all your sequential logic by non- blocking assignment Rule of thumb

Flip-Flop `timescale 1ns/100ps module Flop (reset, din, clk, qout); input reset, din, clk; output qout; reg qout; clk) begin if (reset) qout <= 1'b0; else qout <= din; end endmodule Synchronous reset input A Signal declared as a reg to be capable of holding its values between clock edges Flip-Flops are used in data part for flags and data storage A Non-blocking Assignment A Software-Like Procedural Coding Style Flip-Flop triggers on the falling edge of clk Input The Body of always statement is executed at the negative edge of the clk signal

D flip-flop with active-low asynchronous reset D clk Q Q_n reset_n Behaviour of the circuit  reset_n is active low signal which clears output when low  Q and Q_n are complimentary outputs of the flip flop  On the positive edge of the clock the input D is passed on to the output Q.

D flip-flop with active-low asynchronous reset (posedge clk or negedge reset_n) begin end if(reset_n == 0) begin Q <= 0; Q_n<=1; end else begin Q <= D; Q_n <= !D; end