Verification #15 · 2026-10-08 · Reference models · Medium · about 15 min

The scoreboard that never stalls

add_pipe is a two-stage pipelined adder with a stall input. At a rising edge with en = 1, both stages move: if in_valid is 1 it accepts a and b, and two enabled edges later out_valid is 1 with sum = a + b. With en = 0 nothing moves and the outputs hold. rst_n = 0 empties the pipeline.

scoreboard in scoreboard.sv watches the same signals and predicts each result:

• A result is delivered at every rising edge where rst_n, en and out_valid are all 1. Each one is counted once in checked.
• If the delivered sum isn't the sum of the a and b that produced it, it is also counted in errors.

The DUT is correct, but when a test stalls the pipeline the scoreboard reports mismatches. Fix scoreboard.sv.

Write and run your answer in the browser. The hidden tests check it, and it’s free.