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https://github.com/YosysHQ/yosys
synced 2025-04-07 09:55:20 +00:00
a855d62b42
The previously generated logic assumed an unconstrained past value in the initial state and did not handle 'x values. While the current formal verification flow uses 2-valued logic, SVA value change expressions require a past value of 'x during the initial state to behave in the expected way (i.e. to consider both an initial 0 and an initial 1 as $changed and an initial 1 as $rose and an initial 0 as $fell). This patch now generates logic that at the same time a) provides the expected behavior in a 2-valued logic setting, not depending on any dont-care optimizations, and b) properly handles 'x values in yosys simulation
52 lines
1.6 KiB
Systemverilog
52 lines
1.6 KiB
Systemverilog
module top (
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input clk
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);
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reg [7:0] counter = 0;
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reg a = 0;
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reg b = 1;
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reg c;
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wire a_fell; assign a_fell = $fell(a, @(posedge clk));
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wire a_rose; assign a_rose = $rose(a, @(posedge clk));
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wire a_stable; assign a_stable = $stable(a, @(posedge clk));
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wire b_fell; assign b_fell = $fell(b, @(posedge clk));
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wire b_rose; assign b_rose = $rose(b, @(posedge clk));
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wire b_stable; assign b_stable = $stable(b, @(posedge clk));
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wire c_fell; assign c_fell = $fell(c, @(posedge clk));
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wire c_rose; assign c_rose = $rose(c, @(posedge clk));
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wire c_stable; assign c_stable = $stable(c, @(posedge clk));
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always @(posedge clk) begin
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counter <= counter + 1;
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case (counter)
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0: begin
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assert property ( $fell(a) && !$rose(a) && !$stable(a));
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assert property (!$fell(b) && $rose(b) && !$stable(b));
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assert property (!$fell(c) && !$rose(c) && $stable(c));
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a <= 1; b <= 1; c <= 1;
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end
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1: begin a <= 0; b <= 1; c <= 'x; end
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2: begin
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assert property ( $fell(a) && !$rose(a) && !$stable(a));
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assert property (!$fell(b) && !$rose(b) && $stable(b));
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assert property (!$fell(c) && !$rose(c) && !$stable(c));
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a <= 0; b <= 0; c <= 0;
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end
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3: begin a <= 0; b <= 1; c <= 'x; end
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4: begin
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assert property (!$fell(a) && !$rose(a) && $stable(a));
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assert property (!$fell(b) && $rose(b) && !$stable(b));
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assert property (!$fell(c) && !$rose(c) && !$stable(c));
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a <= 'x; b <= 'x; c <= 'x;
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end
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5: begin a <= 0; b <= 1; c <= 'x; counter <= 0; end
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endcase;
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end
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endmodule
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