Why it matters

In a latch-based pipeline, alternate stages use opposite clock phases, and borrowing flows through them. A stage that borrows hands a later launch to the next, which may borrow in turn. The chain is checked as a whole, because the end must meet its final capture edge.

A three-stage example (ns, illustrative)

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# Alternating phases fed by a rising-edge flop, period 10: L1 negative-level opens 5 closes 10; L2 positive-level opens 10 closes 15; L3 negative-level opens 15 closes 20.
windows = [(5.0, 10.0), (10.0, 15.0), (15.0, 20.0)]
dq, setup = 0.30, 0.10
logic = [7.0, 5.6, 5.0]          # logic delay entering each latch (illustrative)
t = 0.40                          # data leaves the launching rising-edge flop (clock-to-Q)
borrowed, slacks = [], []
for (op, cl), lg in zip(windows, logic):
    arrive = t + lg
    slacks.append(round((cl - setup) - arrive, 2))
    assert slacks[-1] >= 0, "latch setup violated"
    borrowed.append(round(max(0.0, arrive - op), 2))
    t = max(arrive, op) + dq      # leaves at the arrival if transparent, else at the opening edge
assert borrowed == [2.4, 3.3, 3.6]
assert slacks == [2.5, 1.6, 1.3]
assert abs(t - 18.9) < 1e-9
print("PASS: borrowed", borrowed, "slacks", slacks, "chain leaves L3 at", round(t, 2), "ns")

L1 and L3 are negative-level latches and L2 is positive-level, so the phases alternate, and the first latch is fed by a rising-edge flop. Each stage's arrival time feeds the next: when a stage arrives before its latch opens (no borrowing) the chain restarts from the opening edge, otherwise the data keeps moving and borrowing accumulates through the chain. The check is end-to-end: the final stage must meet its closing edge less setup, and every intermediate latch must too.

What the review looks for

• Chains are listed with total borrowed time and the end-to-end slack; a single stage's slack is not enough.

• Loops containing latches are analysed with the loop's total delay against the available phases.

• Maximum borrow constraints, where the tool supports them, are set to prevent unbounded borrowing from hiding problems.

Related topics

• Time Borrowing Worked

• Next: Latch Signoff Rules