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EEGN-CSCI 660 Introduction to VLSI Design Lecture 7

EEGN-CSCI 660 Introduction to VLSI Design Lecture 7. Khurram Kazi. Behavioral Modeling with Verilog. Verilog allows design functionality in an algorithmic manner: i.e. describe the behavior of the model.

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EEGN-CSCI 660 Introduction to VLSI Design Lecture 7

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  1. EEGN-CSCI 660Introduction to VLSI DesignLecture 7 Khurram Kazi CSCI 660

  2. Behavioral Modeling with Verilog • Verilog allows design functionality in an algorithmic manner: i.e. describe the behavior of the model. • Design at this level resembles C programming more than it resembles digital circuit design. • Verilog is rich in behavioral construct CSCI 660 2

  3. Topics under Behavioral Modeling • Structured procedures always and initial • Define blocking and nonblocking procedural assignments • Delay based timing control mechanism in behavioral modeling • Event based timing control mechanism in behavioral modeling • Use the level-sensitive timing control mechanism in behavioral modeling • Conditional statements using if and else • Multiway branching, using case, casex and casx statements CSCI 660 3

  4. Topics under Behavioral Modeling • Looping statements such as while, for, repeat and forever • Define sequential and parallel blocks • Some examples CSCI 660 4

  5. Structured procedures always and initial:Review • All statements inside an initial statement constitute an initial block • Initial block starts at time 0 • Executes only once during a simulation • If there are multiple initial blocks, each block starts to execute concurrently at time 0 • Each block finishes execution independently of the other blocks • Multiple behavioral statements must be grouped, typically using begin and end. CSCI 660 5

  6. Structured procedures alwaysand initial:Review • All statements inside an always statement constitute an always block • always block starts at time 0 • Executes the statements continuously in a looping fashion • This statement is typically used to model a block of activity that is repeated continuously in a digital circuit CSCI 660 6

  7. Procedural assignments • Procedural assignments update values of reg, integer, real or time variables • The values placed on a variable will remain unchanged until another procedural assignment updates the variable with another value CSCI 660 7

  8. Blocking assignments • Blocking assignments are executed in the order they are specified in a sequential block • A blocking assignment will not block execution of statements that follow in a parallel block • The = operator is used to specify blocking assignments CSCI 660 8

  9. Blocking assignments reg x, y, z; reg [15:0] reg_a, reg_b; integer count; //All behavioral statements must be inside an initial or always block initial begin x = 0; y = 1; z = 1; //scalar assignments count = 0; //Assignment to integer variables reg_a = 16’b0; reg_b = reg_a; //initialize vectors #15 reg_a[2] = 1’b1; //bit select assignment with delay #10 reg_b[15:13] = {x, y, z} //assign result of concatenation to part of a vector // {variable within these braces are concatenated} count = count + 1; // assignment to an integer (increment) end Executed at time 0 Executed at time = 15 Executed at time = 25 CSCI 660 9

  10. Nonblocking assignments • Nonblocking assignments allow scheduling of assignments without blocking execution of the statements that follow in a sequential block. • A <= operator is used to specify nonblocking assignments CSCI 660 10

  11. Nonblocking assignments affects on the code // describing the differences between blocking and non-blocking operator // "=" blocking and "<=" non-blocking operator initial begin x = 0; y = 1; z = 1; counter = 0; reg_a <= 16'b0; reg_a <= reg_b; reg_a[2] <= #15 1'b1; //this value changes @ time 15 reg_b[15:13] <= #10 {x, y, z}; //this value changes @ time10 // i.e. changes before reg_a counter <= counter + 1; //this value changes @ time 0 end endmodule concatenation operator: values within the curly braces are concatenated module blocking; reg clk, reset, enable; reg x, y, z; reg [15:0] reg_a, reg_b; integer counter; initial begin reset = 1'b0; enable = 1'b0; #25 reset = 1'b1; #40 enable = 1'b1; end initial begin clk = 0; forever #10 clk = !clk; end Simulator schedules a nonblocking assignment statement to execute and continues to the next statement in the block without waiting for the nonblocking statement to complete execution CSCI 660 11

  12. Blocking vs non Blocking • The difference between the two is that one is similar to variable in VHDL and the other acts like a signal • Blocking in synonymous to = assignment (more like a variable) • Where as non blocking is represented by <= assignment (more like a signal) CSCI 660 12

  13. Non blocking assignment: Non blocking infers two flip flop after synthesis (when clock is in the sensitivity list) module SimpleFlipFlop (clk, a, b, c); //Input ports input clk; input a; //Output ports output b, c; reg b, c; //Input ports data type //By rule all the input ports should be wires wire clk, a; always @(posedge clk) begin b <= a; c <= b; end endmodule CSCI 660 13

  14. blocking assignment: blocking statement infers one flip flop in synthesis all the time (code dependent) module SimpleFlipFlop_blocking (clk, a, b, c); //Input ports input clk; input a; //Output ports output b, c; reg b, c; //Input ports data type //By rule all the input ports should be wires wire clk, a; always @(posedge clk) begin b = a; c = b; end endmodule Not a good way of inferring a flip flop CSCI 660 14

  15. Why at times blocking assignment is preferred At times some designers prefer blocking as they can see sharing of resources more readily: reg [15:0] a, b, c, d, e, f, g, h; reg [16:0] x, y, z; always @ (posedge clk) begin x = a + b + c + d + e + f; y = x + g; z = x + h; end reg [15:0] a, b, c, d, e, f, g, h; // In this example resource sharing is synthesizer reg [16:0] y, z; // dependant always @ (posedge clk) begin y <= (a + b + c + d + e + f) + g; z <= (a + b + c + d + e + f) + h; end CSCI 660 15

  16. Why at times blocking assignment is not desired Blocking statement can cause race condition: file first.v module first (a, b, clk) input b, clk; output a; always @ (posedge clk) begin a = b; end file second.v module first ( b, c, clk) input c, clk; output b; always @ (posedge clk) begin b = c; end Some simulators may evaluate module second.v and then module first. This effectively transfers contents of c to a in ONE clock cycle. This is known as simulator race condition. While some other simulators will execute module first followed by second. Hence two different simulation results AVOID USING BLOCKING STATEMENT CSCI 660 16

  17. Inferring Latches always @ (a1 or a2 or a3 or preset1 or preset2 or preset3 or enable) begin if (preset1) y1 <= 1’b1; else if (enable) y1 <= a1; if (preset2) y2 <= 1’b0; else if (enable) y2 <= a2; if (preset3) y3 <= 1’b1; else if (enable) y3 <= a3; end endmodule module latches (y1, y2, y3, enable, a1, preset1, a2, preset2, a3, preset3); output y1, y2, y3; input a1, preset1; input a2, preset2; input a3, preset3; input enable; reg y1, y2, y3; CSCI 660 17

  18. Golden Rule • Golden Rule 1:To synthesize combinational logic using an always block, all inputs to the design must appear in the sensitivity list. • http://www.doulos.com/knowhow/verilog_designers_guide/if_statement/ Good website that gives simple Verilog examples CSCI 660 18

  19. Inferring counters always @ (posedge clk) begin if (clear) // Sync clear counter y3 <= 0; else y3 <= y3 + 1; end always @ (posedge clk) begin if (up_down) // up down counter direction <= 1; else direction <= -1; y4 = y4 + direction; // NOTICE HERE // y4 assignment is outside the if else end endmodule WOULD THIS CODE GENERATE DANGLING/ UNCONNECTED WIRES OR PORTS??? module counters (y1, y2, y3, y4, y5, y6, d, clk, enable, clear, load, up_down); output [7:0] y1, y2, y3, y4, y5, y6; input [7:0] d; input clk, enable, clear, load, up_load; reg [7:0] y1, y2, y3, y4, y5, y6; integer direction; always @ (posedge clk) begin if (enable) // enable counter y1 <= y1 + 1; end always @ (posedge clk) begin if (load) // loadable counter y2 <= d; else y2 <= y2 + 1; end CSCI 660 19

  20. Frequency Divider module div11 (clkdiv11, clk, reset_n); output clkdiv11; input clk, reset_n; reg div11; reg [3:0] counter; always @ (posedge clk or negedge reset_n) begin if (!reset_n) counter <= 0; else if (counter == 10) counter <= 0; else counter <= counter + 1; end always @ (posedge clk or negedge reset_n) begin if (!reset_n) div11 <= 0; else if (counter == 10) div11 <= 1; else div11 <= 0; end CSCI 660 20

  21. Some simple ALU functions module alu (f, a, b, opcode); parameter addab = 4’b0000, inca = 4’b0001, incb = 4’b0010, andab = 4’b0011, orab = 4’b0100, nega = 4’b0101, shal = 4’b0110, shar = 4’b0111, passa = 4’b1000, passb = 4’b1001; output [7:0] f; input [7:0] a, b; input [3:0] opcode; reg [7:0] f; always @ (a or b or opcode) begin case (opcode) addab: f <= a + b; //add inca: f <= a + 1; //increment a incb: f <= b + 1; //increment b andab: f <= a & b; //a and b orab: f <= a | b; // a or b nega: f <= !a; //negation of a shal: f <= a << 1; //shift “a” left shar: f <= a >> 1; //shift “a” right passa: f <= a; //pass a as is passb: f <= b; //pass b as is default: f <= 8’bx; defautl output endcase end endmodule CSCI 660 21

  22. FSM with four states else begin next_state <= st_three; lsb <= 1; msb <= 1; end st_one: if (up_down == 0) begin next_state <= st_two; lsb <= 0; msb <= 1; end else begin next_state <= st_zero; lsb <= 0; msb <= 0; end module state_machine (lsb, msb, up_down, clk, reset_n); output lsb, msb; input up_down, clk, reset_n; parameter [1:0] st_zero = 2’b00, st_one = 2’b01, st_two = 2’b10, st_three = 2’b11; reg lsb, msb; reg present_state, next state; always @ (up_down or present_state) // combinatorial part begin case (present_state) //0 = up, 1 = down st_zero: if (up_down == 0) begin next_state <= st_one; lsb <= 1; msb <= 0; end CSCI 660 22

  23. FSM with four states cont’d else begin next_state <= st_two; lsb <= 0; msb <= 1; end endcase end always @ (posedge clk or negedge reset_n) //sequential part begin if (!reset_n) present_state <= st_zero; else present_state <= next_state; end endmodule st_two: if (up_down == 0) begin next_state <= st_three; lsb <= 1; msb <= 1; end else begin next_state <= st_one; lsb <= 1; msb <= 0; end st_three: if (up_down == 0) begin next_state <= st_zero; lsb <= 0; msb <= 0; end CSCI 660 23

  24. While Loop example module While_example; `define TRU 1'b1; `define FALSE 1'b0; reg [15:0] flag; integer i; //integer to keep the count in "flag" reg continue; integer count; initial begin count = 0; while (count < 128) // Execute loop till count is 27 //Exit loop at count of 128 begin $display ("Count = %d", count); count = count + 1; end end initial begin flag = 16'b 0010_0000_0000_0000; i = 0; continue = `TRU; while ((i < 16) && continue) //Multiple conditions using operators begin if (flag[i]) begin $display ("Encountered a TRUE bit at element number %d", i); continue = `FALSE; end i = i + 1; end end endmodule CSCI 660 24

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