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Recap : Always block module and_gate (out, in1, in2); inputin1, in2; outputout; regout; or in2) begin out = in1 & in2; end endmodule zAlways.

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Presentation on theme: "Recap : Always block module and_gate (out, in1, in2); inputin1, in2; outputout; regout; or in2) begin out = in1 & in2; end endmodule zAlways."— Presentation transcript:

1 Recap : Always block module and_gate (out, in1, in2); inputin1, in2; outputout; regout; or in2) begin out = in1 & in2; end endmodule zAlways waiting for a change to a trigger signal zThen executes the body Not a real register!! A Verilog register Needed because of assignment in always block specifies when block is executed ie. triggered by which signals

2 Always block zA procedure that describes the function of a circuit yCan contain many statements including if, case yStatements in the always block are executed sequentially (except a case we will cover soon…) yThe entire block is executed at once yThe final result describes the function of the circuit for current set of inputs xintermediate assignments don’t matter, only the final result ybegin/end used to group statements

3 “Complete” Assignments zIf an always block executes, and a variable is not assigned yvariable keeps its old value yNOT combinational logic  latch is inserted yThis is (most of the times) not what you want zAny variable assigned in an always block should be assigned for any execution of the block

4 module and_gate (out, in1, in2); input in1, in2; output out; reg out; begin out = in1 & in2; end endmodule Incomplete Triggers zLeaving out an input trigger usually results in a sequential circuit - again something we don’t usually want.. zExample:

5 Sequential Verilog zSequential circuits are registers along with combinational logic  Register is synthesized when assignment is triggered by “ posedge clk ” module dreg (clk, d, q); input clk, d; output q; reg q; clk) q = d; endmodule

6 8-bit Register with Synchronous Reset module reg8 (reset, CLK, D, Q); inputreset; inputCLK; input[7:0]D; output[7:0]Q; reg [7:0]Q; CLK) if (reset) Q = 0; else Q = D; endmodule// reg8

7 N-bit Register with Asynch Reset module regN (reset, CLK, D, Q); inputreset; inputCLK; parameter N = 8;// Allow N to be changed input[N-1:0] D; output[N-1:0] Q; reg [N-1:0] Q; CLK or posedge reset) if (reset) Q = 0; else if (CLK == 1) Q = D; endmodule// regN

8  Blocking assignments ( Q = A ) yvariable is assigned immediately before continuing to next statement ynew variable value is used by subsequent statements  Non-blocking (delayed) assignments ( Q <= A ) yvariable is assigned only after all statements already scheduled are executed xvalue to be assigned is computed here but saved for later yusual use: register assignment xregisters simultaneously take their new values after the clock tick Blocking & Nonblocking Assignments

9 Example CLK) begin temp = B; B = A; A = temp; end Swap : C style.. CLK) begin A <= B; B <= A; end But in hardware we can do things in parallel..

10 Example zAll delayed assignments scheduled at the same time (even across always blocks) happen together yAnother way to Swap : CLK) begin A <= B; end

11 What does this do? clk) begin B = A; C = B; D = C; end clk) begin {D, C, B} = {C, B, A}; end clk) begin B <= A; C <= B; D <= C; end

12 // 8-bit counter with clear and count enable controls module count8 (CLK, clr, cntEn, Dout); inputCLK; inputclr;// clear counter inputcntEn;// enable count output[7:0]Dout;// counter value reg[7:0]Dout; CLK) if (clr)Dout <= 0; else if (cntEn)Dout <= Dout + 1; endmodule Counter Example

13 Mealy vs Moore machines zMealy Machines - output depends on state as well as inputs. yNeeds two always blocks - one for the state change (on posedge-clock) and one for the outputs. zMoore Machine - output depends only on state yCan do in one always block, but gets very confusing sometimes. yBest to always separate the “things happening on clock edge” and “things not happening on clk” into two always blocks.

14 Example: Mealy Machine module reduce (clk, reset, in, out); input clk, reset, in; output out; reg out; reg state;// state register reg next_state; parameter zero = 0, one = 1; //state assignment // the stuff that happens on “clock edge” clk) if (reset) state = zero; else state = next_state; // the “non clock” stuff or state) case (state) zero: begin// last input was a zero out = 0; if (in) next_state = one; else next_state = zero; end one:// we've seen one 1 if (in) begin next_state = one; out = 1; end else begin next_state = zero; out = 0; end endcase endmodule 1/0 0/0 1/1 zero [0] one1 [0]

15 Moore Machine example zChange the first 1 to 0 in each string of 1’s yExample Moore machine implementation zero [0] one1 [0] two1s [1]

16 Verilog code for Moore.. module reduce (clk, reset, in, out); input clk, reset, in; output out; reg out; reg [1:0] state; // state register reg [1:0] next_state; // State assignment parameter zero = 0, one1 = 1, two1s = 2; // Implement the state register clk) if (reset) state = zero; else state = next_state;

17 Verilog code for Moore.. // now for the part that “does not depend on the clock” or state) case (state) zero: begin // last input was a zero out = 0; if (in) next_state = one1; else next_state = zero; end one1: begin // we've seen one 1 out = 0; if (in) next_state = two1s; else next_state = zero; end two1s:begin // we've seen at least 2 ones out = 1; if (in) next_state = two1s; else next_state = zero; end default: begin // in case we reach a bad state next_state = zero; out = 0; endcase endmodule


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