Course Topics - Outline

Transcription

1 Course Topics - Outline Lecture 1 - Introduction Lecture 2 - Lexical conventions Lecture 3 - Data types Lecture 4 - Operators Lecture 5 - Behavioral modeling A Lecture 6 Behavioral modeling B Lecture 7 Behavioral modeling C Lecture 8 Data flow modeling Lecture 9 Gate Level modeling Lecture 10 Tasks and Functions Lecture 11 Advanced Modeling Techniques Lecture 12 - Coding Styles and Test Benches Lecture 13 - Switch Level modeling 1

2 Lecture 7 - Behavioral modeling C Loop Statements while, for, repeat, forever Sequential and Parallel Blocks -, fork & join Naming of blocks and disabling of named blocks Exercise 7 2

3 Loop Statements There are four types of loop statements: - while - for - repeat - forever The syntax for those loops is very similar to the syntax of loops in C programming All loop statements can appear only inside an or always blocks Loops may contain delay expressions 3

4 while loop The while loop executes until the while-expression is not true. If the loop is entered when the while-expression is not true, the loop is not executed at all. Syntax: ::= while (<expression>) <statement> The while statement should not be used to wait for a change in a value generated external to its always statement. The while-expression can have multiple conditions using operators 4

5 5 while loop example - counter module count_mod ; integer count ; count = 0; while (count < 128) /*Execute loop till count is 127. Exit at count 128 */ $display ("Count = %d", count) ; count = count + 1 ; module

6 A riddle How many times does the following while loop say hello? a = 5 ; c = 0 ; while (a!= c) $display("hello") ; c = c + 1 ; 6

7 7 for loop The for loop contains three parts: - An condition - A check to see if the termination condition is true - A procedural assignment to change value of the control variable Initialization condition and the incremental procedural assignment are included in the for loop and do not need to be specified separately. Provides more compact loop structure than the while loop Executed at compile time. Used in Test Benches! Example: module counter ; integer count ; for (count = 0 ; count < 128 ; count = count + 1) $display("count = %d", count) ; module

8 8 for loop example Priority Encoder module pri_encode (clk, nrst, in, out) ; input clk, nrst ; // clock and active low reset input [7:0] in ; // 8-bit input vector output reg [2:0] out ; // encoded priority output reg [7:0] temp ; // internal shift register integer i ; clk or negedge nrst) if (!nrst) // Reset state i = 0 ; out = 3'b000 ; temp = 8'b ; else // Out of reset temp = 8'b ; for (i = 0 ; i < 8 ; i = i + 1) // The for loop if(temp[i] == in[i]) out = #2 i ; // Intra-delay temp = temp << 1 ; module

9 9 for loop example memory ization /* Initialize mem: each location contains its own address plus 24 i.e. memory16_256[0] contains , (24 in decimal) */ module initmem ; reg [15:0] mem16_256 [255:0] ; reg [8:0] address ; // Why 9-bits address? integer index ; address = 24 ; for(index = 0 ; index < 256 ; index = index + 1) mem16_256 [index] = address ; address = address + 1 ; $display("%d", mem16_256[index]) ; module

10 10 repeat loop The repeat loop is executed the given number of times. Cannot be used to loop on a general logic expression. The while loop is used for that purpose Must contain a number constant, variable, signal value Syntax: repeat (<expression>) <statement> The value of the loop count is determined once at the ning of the execution of the loop repeat (posedge clock) ; #1 data_in = $random ;

11 11 repeat loop example - counter module counter ; // increment and display count from 0 to 127 integer count ; count = 0 ; repeat(128) $display("count = %d", count) ; count = count + 1 ; module

12 12 forever Loop The forever loop does not contain any expression and executes forever until the $finish task is encountered Syntax: ::= forever <statement> A forever loop can be exited by the use of the disable statement. Example: // clock generation for Test Bench reg clock ; forever clock = 1'b0 ; #5 clock = 1'b1 ; #5; // clock cycle time = 10

13 forever loop example module clock_gen ; //Clock generation reg clock ; clock = 1'b0 ; forever #5 clock = ~clock ; // clock cycle time = 10 #1000 $finish; module 13

14 14 Exiting Loops on Exceptional Conditions Any of the loop statements may be exited through use of the disable statement A disable statement terminates any named - block and execution then s with the statement following the block - blocks may be named by placing the name of the block after a colon following the keyword: : break for (i = 0 ; i < n ; i = i + 1) : continue if (a == 0) disable continue ; // proceed with i = i+1... if (a == b) disable break ;... // exit for loop

15 15 Block types - Sequential and Parallel There are two Block types: Sequential and Parallel Block statements are used to group multiple statements to act together as one If a Sequential Block contains more than one statement, keywords and are used to group the statements If a Parallel Block contains more than one statement, keywords fork and join are used to group the statements Fundamental difference between sequential and parallel blocks: Statements in a Sequential Block are processed in the order they are specified Statements in a Parallel Block are executed concurrently

16 16 Sequential Blocks Statements in a sequential block are processed in the order they are specified A statement is executed only after its preceding statement completes execution (except for non-blocking assignments) If delay or event control is specified it is relative to the simulation time, i.e. execution of the previous statement Example: // x =1, y=2, z=3 x = y * z ; // x = 6 y = x + 2 ;// y = 8 z = y 1 ;// z = 7 x = y * z ; // x completes at simulation time 0 #4 y = x + 2 ; // y completes at simulation time 4 #8 z = y 1 ; // z completes at simulation time 12

17 17 Parallel Blocks All statements (or blocks) between a fork/join pair execution simultaneously upon execution flow hitting the fork Execution continues after the join upon completion of the longest running statement or block between the fork and join Makes the blocking assignments to become unblocking Order of statements is controlled by the delay or event control assigned to each statement If delay or event control is specified, it is relative to the time the block was entered fork - splitting a single flow into indepent flows join - joining indepent flows back into a single flow

18 Example - Parallel Block reg x, y ; reg [1:0] z, w ; fork x = 1'b0 ; // x completes at simulation time 0 #5 y = 1'b1 ; // y completes at simulation time 5 #10 z = {x, y} ; // z completes at simulation time 10 #20 w = {y, x} ; // w completes at simulation time 20 join 18

19 19 Parallel block with deliberate race conditions // All statements start at simulation time 0. // The order in which the statements will execute is not known reg x, y ; reg [1:0] z, w ; fork x = 1'b0 ; y = 1'b1 ; z = {x, y} ; /* will get the value 1 if x = 1'b0 and y = 1'b1 execute first, 2'bxx if execute last */ w = {y, x} ; /* will get the value 2 if x = 1'b0 and y = 1'b1 execute first, 2'bxx if execute last */ join

20 Blocks can be nested. Nested, mixed Blocks Sequential and Parallel blocks can be mixed: Initial x = 1'b0 ; fork #5 y = 1'b1 ; #10 z = {x, y} ; join #20 w = {y, x} ; //Nested mixed blocks 20

21 Naming of blocks and Disabling named blocks Blocks can be given names. Using -, a name can be given to that group. This is called named blocks. Local variables can be declared for the named blocks Variables in named blocks can be accessed by using hierarchical name referencing Named blocks can be disabled their execution can be stopped disable statement terminates execution of named block Disable a block causes execution control to pass to the statement immediately succeeding the block 21

22 22 Named blocks examples Example: module top () ; : block1 // sequential block named block1 integer i ; // can be accessed by hierarchical name top.block1.i. fork : block2 // parallel block named block2 reg i ; /* reg i is static and local to block2. can be accessed by hierarchical name top.block2.i */. join

23 23 Disable block example module find_true_bit ; // Find the first bit with a value 1 in flag (vector variable) reg [15:0] flag ; integer i ; //integer to keep count flag = 16'b 0010_0000_0000_0000; // 16 h2000 i = 0; : block1 //The main block inside while is named block1 while(i < 16) if (flag[i]) $display("encountered a TRUE bit at element number %d", i) ; disable block1; //disable block1 because you found true bit. i = i + 1 ; module

24 Exercise 7 Part 1: Design and test Data Buffer, exercising for and repeat loops and disable loop. Initialize data buffer with for loop. Repeat reading input data for parametric 8 clock cycles. Use disable loop for partial data buffer ization. Part 2: Design a dual elevator control system. The system responds to two elevators request events and operates the elevators concurrently. Use two sequential blocks enclosed in a fork-join block for concurrency. Test system with concurrent requests. 24