Why it matters

An exception is a promise that a path does not need to meet timing, and a wrong promise is silicon that fails at speed after passing signoff. A common integration failure is exactly this: a false path covering a real path, or a multicycle valid for only one clock ratio, both green in the reports and red in the lab. A multicycle written for today's 8-to-1 CPU-to-peripheral ratio silently becomes wrong the day the PLL plan or a divider changes; the exception must name the ratio it holds for and the check that fails when the ratio moves. The review below keeps exceptions honest.

The core idea

Every exception carries four fields besides the path itself: the reason the path cannot matter functionally, the evidence proving the reason, usually a CDC report, a protocol argument or a constrained-random test, the scope of modes and ratios it holds for, and an expiry no further than the next signoff milestone. The review reads each exception against its evidence and refuses on sight: exceptions without evidence, exceptions that are active in a shipped mode the evidence does not cover (that mode would leave the path untimed), exceptions copied from a block whose paths changed at the top, and false paths written over multi-bit data. On NanoSoC the last case is the APB bridge: the single-bit request and acknowledge toggles entering the first synchroniser flop are a protocol-reviewed crossing and can be false paths, but the held address and write-data buses are multi-bit and need a maximum-delay bound, not a blanket false path.

The audit below holds the four fields as a list of dictionaries, applies the rules in the order a reviewer would, and requires exactly one survivor. It then moves the ratio and the expiry to show those rules can refuse too.

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PLAN_RATIO = 8        # clk_cpu : clk_per today (NanoSoC teaching plan)

def verdict(e, plan_ratio=PLAN_RATIO):
    if e.get("origin") == "block" and not e.get("requalified"):
        return "re-qualify at top"
    if not e.get("evidence"):
        return "refuse: no evidence"
    if e["kind"] == "false_path" and e.get("bits", 1) > 1:
        return "refuse: multi-bit path needs a maximum-delay bound"
    if e["active_modes"] - e["proven_modes"]:
        return "refuse: mode without evidence"
    if e.get("ratio") not in (None, plan_ratio):
        return "refuse: ratio changed"
    if e["expiry_days"] <= 0:
        return "refuse: expired"
    return "keep"

EXC = [
    {"name": "req/ack toggle", "kind": "false_path", "bits": 1,
     "evidence": "bridge protocol review", "active_modes": {"mission", "test"},
     "proven_modes": {"mission", "test"}, "expiry_days": 90},
    {"name": "status to INTC", "kind": "multicycle", "evidence": None,
     "active_modes": {"mission"}, "proven_modes": {"mission"}, "expiry_days": 90},
    {"name": "test mux select", "kind": "false_path", "evidence": "mode-matrix case",
     "active_modes": {"mission", "test"}, "proven_modes": {"mission"}, "expiry_days": 90},
    {"name": "DMA status path", "kind": "false_path", "origin": "block",
     "evidence": "block report", "active_modes": {"mission"},
     "proven_modes": {"mission"}, "expiry_days": 90},
    {"name": "apb data bus", "kind": "false_path", "bits": 32,
     "evidence": "bridge protocol review", "active_modes": {"mission", "test"},
     "proven_modes": {"mission", "test"}, "expiry_days": 90},
]
got = [verdict(e) for e in EXC]
assert got == ["keep", "refuse: no evidence", "refuse: mode without evidence",
               "re-qualify at top", "refuse: multi-bit path needs a maximum-delay bound"], got
assert got.count("keep") == 1            # exactly one survivor

mc = {"name": "per-ratio multicycle", "kind": "multicycle", "ratio": 8,
      "evidence": "ratio-8 timing argument", "active_modes": {"mission"},
      "proven_modes": {"mission"}, "expiry_days": 30}
assert verdict(mc) == "keep"
assert verdict(mc, plan_ratio=6) == "refuse: ratio changed"     # the plan moved
assert verdict(dict(mc, expiry_days=0)) == "refuse: expired"
print("PASS: 5 exceptions audited, 1 kept; ratio change and expiry are refused")

What the review looks for

• The four fields filled for every exception, with evidence the reviewer can open, not a reference to a hallway conversation.

• Scope covering every shipped mode and ratio; a multicycle valid for one ratio names that ratio and the check that enforces it.

• Expiry dates in the future with owners; an expired exception fails the gate exactly like an expired waiver.

Verify and debug

Re-run the exception audit before every signoff milestone, not just at tapeout: modes added late silently narrow scopes written early, and a changed clock ratio invalidates every multicycle written for the old one. When silicon fails at speed on a path the reports called green, read the exception list first; the wrong promise is usually one row with a plausible reason and no evidence. For clock-gating checks that interact with exceptions see Clock Gating Checks.

Related topics

• Merging Block Constraints

• Next: DFT Integration

• STA: Clock Gating Checks