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Exploring Basic Sequential Circuits in Verilog: Latches, Flip-Flops, Registers, and More

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timothy-barber
65 diapositives · 813 Views · Nov 17, 2014

This lecture delves into the fundamental concepts of basic sequential circuits using Verilog, covering latches, flip-flops, registers, counters, shift registers, datapaths, and state machines. We explore the R-S Latch operation, detailing both active HIGH and active LOW configurations. The lecture emphasizes Verilog coding techniques for implementing these circuits, including example modules for D flip-flops, counters, and shift registers. By understanding these basic building blocks, students can design and simulate more complex digital systems effectively.

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  1. Logic Design Review – 3Basic Sequential Circuits Lecture L14.3 Verilog

  2. Basic Sequential Circuits • Latches • Flip-Flops • Registers • Counters • Shift Registers • Datapaths • State Machines

  3. R-S Latch R Q S R-S Latch Q is set to 1 when S is asserted, and remains unchanged when S is disasserted. Q is reset to 0 when R is asserted, and remains unchanged when R is disasserted. Assertions can be active HIGH or active LOW

  4. R-S Latch R Q S module RSlatch ( Q ,R ,S ); input R ; wire R ; input S ; wire S ; output Q ; reg Q ; always @(R or S) begin if(S == 1 && R == 0) Q = 1; elseif(S == 0 && R == 1) Q = 0; end endmodule Active HIGH

  5. R-S Latch -- Active High

  6. R-S Latch R Q S module RSlatch ( Q ,R ,S ); input R ; wire R ; input S ; wire S ; output Q ; reg Q ; always @(R or S) begin if(S == 0 && R == 1) Q = 1; elseif(S == 1 && R == 0) Q = 0; end endmodule Active LOW

  7. R-S Latch -- Active Low

  8. Note that this is different from the "textbook" RS latch module RSlatchNOR ( Q ,R ,S ); input R ; wire R ; input S ; wire S ; output Q ; wire Q ; wire F1, F2; nor #10 (F1,F2,R); nor #10 (F2,F1,S); assign Q = F1; endmodule 10ns propagation delay

  9. R S Q 0 0 Q0 store 0 1 1 set 1 0 0 reset 1 1 0 disallowed

  10. R-S Latch R Q S How can you make this R-S latch from gates? Q is set to 1 when S is asserted, and remains unchanged when S is disasserted. Q is reset to 0 when R is asserted, and remains unchanged when R is disasserted. Assertions can be active HIGH or active LOW

  11. SQ R-S Latch 00 01 11 10 R R Q 0 S 1 Q is set to 1 when S is asserted (1), and remains unchanged when S is disasserted (0). Q is reset to 0 when R is asserted (1), and remains unchanged when R is disasserted (0). R S Q Q 0 0 0 0 0 0 1 1 0 1 0 1 0 1 1 1 1 0 0 0 1 0 1 0 1 1 0 0 1 1 1 1 store 1 1 1 set 1 reset store Q = ~R & Q | ~R & S | S & Q

  12. R-S Latch R Q S RS Latch R S Q Q 0 0 0 0 0 0 1 1 0 1 0 1 0 1 1 1 1 0 0 0 1 0 1 0 1 1 0 0 1 1 1 1 store set reset store Q = ~R & Q | ~R & S | S & Q

  13. module RSlatchGates ( Q ,R ,S ); input R ; wire R ; input S ; wire S ; output Q ; wire Q ; assign #10 Q = ~R & Q | ~R & S | S & Q; endmodule

  14. D Latch D Q EN D Latch Q follows D when EN is high, and remains unchanged when EN is low..

  15. D Latch D Q EN module Dlatch ( Q ,EN ,D ); input EN ; wire EN ; input D ; wire D ; output Q ; reg Q ; always @(D or EN) if(EN == 1) Q = D; endmodule

  16. D Latch

  17. Basic Sequential Circuits • Latches • Flip-Flops • Registers • Counters • Shift Registers • Datapaths • State Machines

  18. Q D clk !Q D clk Q !Q 0 0 1 1 1 0 X 0 Q0 !Q0 D Flip-Flop Positive edge triggered D gets latched to Q on the rising edge of the clock. Behavior always @(posedge clk) Q <= D;

  19. Q D clk !Q DFF.v module DFF (D, clk, Q, notQ ); input clk ; wire clk ; input D ; wire D ; output Q ; reg Q ; output notQ ; wire notQ ; always @(posedge clk) Q <= D; assign notQ = ~Q; endmodule Note non-blocking assignment

  20. D Flip-Flop

  21. DFFclr.v module DFFclr (D, clk, clr, Q, notQ ); input clk ; wire clk ; input clr ; wire clr ; input D ; wire D ; output Q ; reg Q ; output notQ ; wire notQ ; always @(posedge clk orposedge clr) if(clr == 1) Q <= 0; else Q <= D; assign notQ = ~Q; endmodule Asynchronous clear

  22. D Flip-Flop with Asynchronous Clear

  23. Basic Sequential Circuits • Latches • Flip-Flops • Registers • Counters • Shift Registers • Datapaths • State Machines

  24. A 1-Bit Register Behavior always @(posedge clk) if(LOAD == 1) Q0 <= INP0;

  25. A 4-Bit Register

  26. A Generic Register // An n-bit register with asynchronous clear and load module register(clk,clr,load,d,q); parameter n = 8; input [n-1:0] d; input clk,clr,load; output [n-1:0] q; reg [n-1:0] q; always @(posedge clk orposedge clr) if(clr == 1) q <= 0; elseif(load) q <= d; endmodule

  27. A Generic Register

  28. Basic Sequential Circuits • Latches • Flip-Flops • Registers • Counters • Shift Registers • Datapaths • State Machines

  29. count3 clr Q(2 downto 0) clk 3-Bit Counter Behavior always @(posedge clk orposedge clr) begin if(clr == 1) Q <= 0; else Q <= Q + 1; end

  30. counter3.v module counter3 (clk, clr, Q ); input clr ; wire clr ; input clk ; wire clk ; output [2:0] Q ; reg [2:0] Q ; // 3-bit counter always @(posedge clk orposedge clr) begin if(clr == 1) Q <= 0; else Q <= Q + 1; end endmodule Asynchronous clear Output count increments on rising edge of clk

  31. counter3 Simulation

  32. Clock 4.0 MHz Q0 2.0 MHz Q1 1.0 MHz Q2 0.5 MHz Q3 0.25 MHz Clock Divider Clk4 = 4 MHz clock

  33. Basic Sequential Circuits • Latches • Flip-Flops • Registers • Counters • Shift Registers • Datapaths • State Machines

  34. 4-Bit Shift Register

  35. shift4.v module ShiftReg(clk,clr,data_in,Q); input clk; input clr; input data_in; output [3:0] Q; reg [3:0] Q; // 4-bit Shift Register always @(posedge clk orposedge clr) begin if(clr == 1) Q <= 0; else begin Q[3] <= data_in; Q[2:0] <= Q[3:1]; end end endmodule Note non-blocking assignment

  36. shift4 simulation

  37. Ring Counter

  38. ring4.v module ring4(clk,clr,Q); input clk; input clr; output [3:0] Q; reg [3:0] Q; // 4-bit Ring Counter always @(posedge clk orposedge clr) begin if(clr == 1) Q <= 1; else begin Q[3] <= Q[0]; Q[2:0] <= Q[3:1]; end end endmodule

  39. ring4 simulation

  40. Johnson Counter

  41. johnson4.v module johnson4(clk,clr,Q); input clk; input clr; output [3:0] Q; reg [3:0] Q; // 4-bit Johnson Counter always @(posedge clk orposedge clr) begin if(clr == 1) Q <= 0; else begin Q[3] <= ~Q[0]; Q[2:0] <= Q[3:1]; end end endmodule

  42. Johnson Counter

  43. A Random Number Generator

  44. Q3 Q2 Q1 Q0 0 0 0 1 1 1 0 0 0 8 1 1 0 0 C 1 1 1 0 E 1 1 1 1 F 0 1 1 1 7 1 0 1 1 B 0 1 0 1 5 Q3 Q2 Q1 Q0 1 0 1 0 A 1 1 0 1 D 0 1 1 0 6 0 0 1 1 3 1 0 0 1 9 0 1 0 0 4 0 0 1 0 2 0 0 0 1 1

  45. rand4.v module rand4(clk,clr,Q); input clk; input clr; output [3:0] Q; reg [3:0] Q; // 4-bit Random number generator always @(posedge clk orposedge clr) begin if(clr == 1) Q <= 1; else begin Q[3] <= Q[3] ^ Q[0]; Q[2:0] <= Q[3:1]; end end endmodule

  46. A Random Number Generator

  47. outp inp Q2 Q1 Q0 clk Clock Pulse

  48. clk_pulse.v module clk_pulse(clk,clr,inp,outp); input clk; input clr; input inp; output outp; wire outp; reg [2:0] Q; // clock pulse generator always @(posedge clk orposedge clr) begin if(clr == 1) Q <= 0; else begin Q[2] <= inp; Q[1:0] <= Q[2:1]; end end assign outp = Q[2] & Q[1] & ~Q[0]; endmodule

  49. outp inp Q2 Q1 Q0 clk