FAILURE MAP
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FA-88936 / Digital logic simulation / Open access

Clock capture overrides an asserted reset · case 01

A rising edge while reset is held loads d, so q is not 0 during reset.

Verified by executionVariant 1 · 7 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

Capture is evaluated as an independent if after the reset branch, so it overrides reset.

THE FAILURE

Capture is evaluated as an independent if after the reset branch, so it overrides reset.

Unsuccessful approach: Adding a redundant clock-high condition does not restore reset priority.

Case contract

Input: samples [clk, d, en, rst_n] at successive instants. q starts as "x". Active-low rst_n==0 forces q=0 immediately regardless of clock. Otherwise a rising clock edge (previous sample clk 0, this sample clk 1; the first sample is never an edge) with en==1 loads d. The previous clock value is tracked on every sample, including while reset is asserted. Return q after each sample.

Why this case matters

Register models in cycle simulators must distinguish asynchronous from synchronous control and detect true clock edges.

1 / The failure

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(samples):
    out = []
    q = 'x'
    prev = None
    for c, d, en, rst_n in samples:
        edge = prev == 0 and c == 1
        prev = c
        if rst_n == 0:
            q = 0
        if edge and en == 1:
            q = d
        out.append(q)
    return out
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('uninitialized flop holds x until first edge', [[[0, 1, 1, 1], [0, 0, 1, 1], [1, 1, 1, 1]]], ['x', 'x', 1]), ('async reset between edges', [[[0, 0, 1, 1], [1, 1, 1, 1], [1, 1, 1, 0], [1, 1, 1, 1]]], ['x', 1, 0, 0]), ('reset held low while clock rises', [[[0, 0, 1, 0], [1, 1, 1, 0], [0, 1, 1, 0], [1, 1, 1, 0], [0, 1, 1, 1], [1, 1, 1, 1]]], [0, 0, 0, 0, 0, 1]), ('reset released during clock high', [[[0, 1, 1, 1], [0, 1, 1, 0], [1, 1, 1, 0], [1, 1, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1]]], ['x', 0, 0, 0, 0, 0]), ('falling edges do not capture', [[[1, 0, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 0, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1]]], ['x', 'x', 1, 1, 1, 0]), ('clock held high does not recapture', [[[0, 1, 1, 1], [1, 1, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1]]], ['x', 1, 1, 1]), ('enable low holds value', [[[0, 1, 1, 1], [1, 1, 1, 1], [0, 0, 0, 1], [1, 0, 0, 1], [0, 0, 1, 1]]], ['x', 1, 1, 1, 1])], [('uninitialized flop holds x until first edge', [[[0, 1, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1]]], ['x', 'x', 0]), ('async reset between edges', [[[0, 0, 1, 1], [1, 1, 1, 1], [1, 1, 1, 0], [1, 1, 1, 1], [0, 1, 1, 1]]], ['x', 1, 0, 0, 0]), ('reset held low while clock rises', [[[0, 0, 1, 0], [1, 1, 1, 0], [0, 1, 1, 0], [1, 1, 1, 0], [0, 1, 1, 1], [1, 1, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1]]], [0, 0, 0, 0, 0, 1, 1, 1]), ('reset released during clock high', [[[0, 1, 1, 1], [0, 1, 1, 0], [1, 1, 1, 0], [1, 1, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1]]], ['x', 0, 0, 0, 0, 0]), ('falling edges do not capture', [[[1, 0, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 0, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1]]], ['x', 'x', 1, 1, 1, 0, 0]), ('clock held high does not recapture', [[[0, 1, 1, 1], [1, 1, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1]]], ['x', 1, 1, 1]), ('enable low holds value', [[[0, 1, 1, 1], [1, 1, 1, 1], [0, 0, 0, 1], [1, 0, 0, 1], [0, 0, 1, 1], [1, 0, 1, 1]]], ['x', 1, 1, 1, 1, 0])], [('uninitialized flop holds x until first edge', [[[0, 1, 1, 1], [0, 0, 1, 1], [1, 1, 1, 1]]], ['x', 'x', 1]), ('async reset between edges', [[[0, 0, 1, 1], [1, 1, 1, 1], [1, 1, 1, 0]]], ['x', 1, 0]), ('reset held low while clock rises', [[[0, 0, 1, 0], [1, 1, 1, 0], [0, 1, 1, 0], [1, 1, 1, 0], [0, 1, 1, 1], [1, 1, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1]]], [0, 0, 0, 0, 0, 1, 1, 1, 1, 1]), ('reset released during clock high', [[[0, 1, 1, 1], [0, 1, 1, 0], [1, 1, 1, 0], [1, 1, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1]]], ['x', 0, 0, 0, 0, 0]), ('falling edges do not capture', [[[1, 0, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 0, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1]]], ['x', 'x', 1, 1, 1, 0, 0, 0]), ('clock held high does not recapture', [[[0, 1, 1, 1], [1, 1, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1]]], ['x', 1, 1, 1]), ('enable low holds value', [[[0, 1, 1, 1], [1, 1, 1, 1], [0, 0, 0, 1], [1, 0, 0, 1]]], ['x', 1, 1, 1])], [('uninitialized flop holds x until first edge', [[[0, 1, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1]]], ['x', 'x', 0]), ('async reset between edges', [[[0, 0, 1, 1], [1, 1, 1, 1], [1, 1, 1, 0], [1, 1, 1, 1]]], ['x', 1, 0, 0]), ('reset held low while clock rises', [[[0, 0, 1, 0], [1, 1, 1, 0], [0, 1, 1, 0], [1, 1, 1, 0], [0, 1, 1, 1], [1, 1, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1]]], [0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1]), ('reset released during clock high', [[[0, 1, 1, 1], [0, 1, 1, 0], [1, 1, 1, 0], [1, 1, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1]]], ['x', 0, 0, 0, 0, 0]), ('falling edges do not capture', [[[1, 0, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 0, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1]]], ['x', 'x', 1, 1, 1, 0, 0, 0, 0]), ('clock held high does not recapture', [[[0, 1, 1, 1], [1, 1, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1]]], ['x', 1, 1, 1]), ('enable low holds value', [[[0, 1, 1, 1], [1, 1, 1, 1], [0, 0, 0, 1], [1, 0, 0, 1], [0, 0, 1, 1]]], ['x', 1, 1, 1, 1])], [('uninitialized flop holds x until first edge', [[[0, 1, 1, 1], [0, 0, 1, 1], [1, 1, 1, 1]]], ['x', 'x', 1]), ('async reset between edges', [[[0, 0, 1, 1], [1, 1, 1, 1], [1, 1, 1, 0], [1, 1, 1, 1], [0, 1, 1, 1]]], ['x', 1, 0, 0, 0]), ('reset held low while clock rises', [[[0, 0, 1, 0], [1, 1, 1, 0], [0, 1, 1, 0], [1, 1, 1, 0], [0, 1, 1, 1], [1, 1, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1]]], [0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1]), ('reset released during clock high', [[[0, 1, 1, 1], [0, 1, 1, 0], [1, 1, 1, 0], [1, 1, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1]]], ['x', 0, 0, 0, 0, 0]), ('falling edges do not capture', [[[1, 0, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 0, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1]]], ['x', 'x', 1, 1, 1, 0, 0, 0, 0, 0]), ('clock held high does not recapture', [[[0, 1, 1, 1], [1, 1, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1]]], ['x', 1, 1, 1]), ('enable low holds value', [[[0, 1, 1, 1], [1, 1, 1, 1], [0, 0, 0, 1], [1, 0, 0, 1], [0, 0, 1, 1], [1, 0, 1, 1]]], ['x', 1, 1, 1, 1, 0])]]
for label, args, expected in fixtures[N-1]:
    check(label, solve(*args), expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
Boundary fixtureActualExpectedOutcome
uninitialized flop holds x until first edge['x', 'x', 1]['x', 'x', 1]Passed
async reset between edges['x', 1, 0, 0]['x', 1, 0, 0]Passed
reset held low while clock rises[0, 1, 0, 1, 1, 1][0, 0, 0, 0, 0, 1]Failed
reset released during clock high['x', 0, 1, 1, 1, 0]['x', 0, 0, 0, 0, 0]Failed
falling edges do not capture['x', 'x', 1, 1, 1, 0]['x', 'x', 1, 1, 1, 0]Passed
clock held high does not recapture['x', 1, 1, 1]['x', 1, 1, 1]Passed
enable low holds value['x', 1, 1, 1, 1]['x', 1, 1, 1, 1]Passed

SHA-256 / cd6bd730074f4922fa0bf2aacfb225ac6b8b3481b37b94867dc286fb8f42e885

2 / The unsuccessful fix

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(samples):
    out = []
    q = 'x'
    prev = None
    for c, d, en, rst_n in samples:
        edge = prev == 0 and c == 1
        prev = c
        if rst_n == 0:
            q = 0
        if edge and en == 1 and c == 1:
            q = d
        out.append(q)
    return out
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('uninitialized flop holds x until first edge', [[[0, 1, 1, 1], [0, 0, 1, 1], [1, 1, 1, 1]]], ['x', 'x', 1]), ('async reset between edges', [[[0, 0, 1, 1], [1, 1, 1, 1], [1, 1, 1, 0], [1, 1, 1, 1]]], ['x', 1, 0, 0]), ('reset held low while clock rises', [[[0, 0, 1, 0], [1, 1, 1, 0], [0, 1, 1, 0], [1, 1, 1, 0], [0, 1, 1, 1], [1, 1, 1, 1]]], [0, 0, 0, 0, 0, 1]), ('reset released during clock high', [[[0, 1, 1, 1], [0, 1, 1, 0], [1, 1, 1, 0], [1, 1, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1]]], ['x', 0, 0, 0, 0, 0]), ('falling edges do not capture', [[[1, 0, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 0, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1]]], ['x', 'x', 1, 1, 1, 0]), ('clock held high does not recapture', [[[0, 1, 1, 1], [1, 1, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1]]], ['x', 1, 1, 1]), ('enable low holds value', [[[0, 1, 1, 1], [1, 1, 1, 1], [0, 0, 0, 1], [1, 0, 0, 1], [0, 0, 1, 1]]], ['x', 1, 1, 1, 1])], [('uninitialized flop holds x until first edge', [[[0, 1, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1]]], ['x', 'x', 0]), ('async reset between edges', [[[0, 0, 1, 1], [1, 1, 1, 1], [1, 1, 1, 0], [1, 1, 1, 1], [0, 1, 1, 1]]], ['x', 1, 0, 0, 0]), ('reset held low while clock rises', [[[0, 0, 1, 0], [1, 1, 1, 0], [0, 1, 1, 0], [1, 1, 1, 0], [0, 1, 1, 1], [1, 1, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1]]], [0, 0, 0, 0, 0, 1, 1, 1]), ('reset released during clock high', [[[0, 1, 1, 1], [0, 1, 1, 0], [1, 1, 1, 0], [1, 1, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1]]], ['x', 0, 0, 0, 0, 0]), ('falling edges do not capture', [[[1, 0, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 0, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1]]], ['x', 'x', 1, 1, 1, 0, 0]), ('clock held high does not recapture', [[[0, 1, 1, 1], [1, 1, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1]]], ['x', 1, 1, 1]), ('enable low holds value', [[[0, 1, 1, 1], [1, 1, 1, 1], [0, 0, 0, 1], [1, 0, 0, 1], [0, 0, 1, 1], [1, 0, 1, 1]]], ['x', 1, 1, 1, 1, 0])], [('uninitialized flop holds x until first edge', [[[0, 1, 1, 1], [0, 0, 1, 1], [1, 1, 1, 1]]], ['x', 'x', 1]), ('async reset between edges', [[[0, 0, 1, 1], [1, 1, 1, 1], [1, 1, 1, 0]]], ['x', 1, 0]), ('reset held low while clock rises', [[[0, 0, 1, 0], [1, 1, 1, 0], [0, 1, 1, 0], [1, 1, 1, 0], [0, 1, 1, 1], [1, 1, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1]]], [0, 0, 0, 0, 0, 1, 1, 1, 1, 1]), ('reset released during clock high', [[[0, 1, 1, 1], [0, 1, 1, 0], [1, 1, 1, 0], [1, 1, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1]]], ['x', 0, 0, 0, 0, 0]), ('falling edges do not capture', [[[1, 0, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 0, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1]]], ['x', 'x', 1, 1, 1, 0, 0, 0]), ('clock held high does not recapture', [[[0, 1, 1, 1], [1, 1, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1]]], ['x', 1, 1, 1]), ('enable low holds value', [[[0, 1, 1, 1], [1, 1, 1, 1], [0, 0, 0, 1], [1, 0, 0, 1]]], ['x', 1, 1, 1])], [('uninitialized flop holds x until first edge', [[[0, 1, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1]]], ['x', 'x', 0]), ('async reset between edges', [[[0, 0, 1, 1], [1, 1, 1, 1], [1, 1, 1, 0], [1, 1, 1, 1]]], ['x', 1, 0, 0]), ('reset held low while clock rises', [[[0, 0, 1, 0], [1, 1, 1, 0], [0, 1, 1, 0], [1, 1, 1, 0], [0, 1, 1, 1], [1, 1, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1]]], [0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1]), ('reset released during clock high', [[[0, 1, 1, 1], [0, 1, 1, 0], [1, 1, 1, 0], [1, 1, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1]]], ['x', 0, 0, 0, 0, 0]), ('falling edges do not capture', [[[1, 0, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 0, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1]]], ['x', 'x', 1, 1, 1, 0, 0, 0, 0]), ('clock held high does not recapture', [[[0, 1, 1, 1], [1, 1, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1]]], ['x', 1, 1, 1]), ('enable low holds value', [[[0, 1, 1, 1], [1, 1, 1, 1], [0, 0, 0, 1], [1, 0, 0, 1], [0, 0, 1, 1]]], ['x', 1, 1, 1, 1])], [('uninitialized flop holds x until first edge', [[[0, 1, 1, 1], [0, 0, 1, 1], [1, 1, 1, 1]]], ['x', 'x', 1]), ('async reset between edges', [[[0, 0, 1, 1], [1, 1, 1, 1], [1, 1, 1, 0], [1, 1, 1, 1], [0, 1, 1, 1]]], ['x', 1, 0, 0, 0]), ('reset held low while clock rises', [[[0, 0, 1, 0], [1, 1, 1, 0], [0, 1, 1, 0], [1, 1, 1, 0], [0, 1, 1, 1], [1, 1, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1]]], [0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1]), ('reset released during clock high', [[[0, 1, 1, 1], [0, 1, 1, 0], [1, 1, 1, 0], [1, 1, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1]]], ['x', 0, 0, 0, 0, 0]), ('falling edges do not capture', [[[1, 0, 1, 1], [0, 1, 1, 1], [1, 1, 1, 1], [0, 0, 1, 1], [0, 0, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1]]], ['x', 'x', 1, 1, 1, 0, 0, 0, 0, 0]), ('clock held high does not recapture', [[[0, 1, 1, 1], [1, 1, 1, 1], [1, 0, 1, 1], [1, 0, 1, 1]]], ['x', 1, 1, 1]), ('enable low holds value', [[[0, 1, 1, 1], [1, 1, 1, 1], [0, 0, 0, 1], [1, 0, 0, 1], [0, 0, 1, 1], [1, 0, 1, 1]]], ['x', 1, 1, 1, 1, 0])]]
for label, args, expected in fixtures[N-1]:
    check(label, solve(*args), expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
Boundary fixtureActualExpectedOutcome
uninitialized flop holds x until first edge['x', 'x', 1]['x', 'x', 1]Passed
async reset between edges['x', 1, 0, 0]['x', 1, 0, 0]Passed
reset held low while clock rises[0, 1, 0, 1, 1, 1][0, 0, 0, 0, 0, 1]Failed
reset released during clock high['x', 0, 1, 1, 1, 0]['x', 0, 0, 0, 0, 0]Failed
falling edges do not capture['x', 'x', 1, 1, 1, 0]['x', 'x', 1, 1, 1, 0]Passed
clock held high does not recapture['x', 1, 1, 1]['x', 1, 1, 1]Passed
enable low holds value['x', 1, 1, 1, 1]['x', 1, 1, 1, 1]Passed

SHA-256 / e8f1c595552f09df84baa2506c423b67a8f5f3b6e25d3e63de8a14f417f21a5e

HELD IN THE MEMBER ARCHIVE

The verified repair and its recorded checks are member-only.

This mechanism has 7 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

Every case sharing this mechanism uses the same contract and the same repair, so this one record is held back for all of them.

Member access is invitation-based. Sign in with your invited account to inspect the repair.

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Verification & scope

A deterministic bounded teaching model of one simulator rule set; the contract is stipulated and is not a claim of conformance to any HDL standard or commercial simulator. This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.

Observations recorded using Python 3.12.14 at 2026-09-29T14:51:12.720703+00:00.

Case digest / 6b18524049f082b9be525fc8cf3fdeea1ca408eed82f214593ec5924680026ba