docs: time-distribution — transport submit-to-air floors + the network-PTP bridge#244
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Fold two measured results into docs/time-distribution.md: - The ~94 us per-slave absolute lock of the software downlink is the transport's submit-to-air floor, not a protocol property: USB ~93 us RMS vs PCIe ~12 us RMS (robust-fit, tests/pcie_txegress_tx.cpp + tests/txegress_witness.cpp) — a PCIe master lifts the absolute software-path lock ~8x. - New "Bridging to network PTP" section: the 8821CE TSF as a Linux PHC (tests/pcie_phc/) disciplined by phc2sys to the I226 holds ~290 ns RMS (the TSF's own 1 us-quantization floor) with the ~+42 ppm crystal offset servoed out — combined with the TBTT steering actuator this is the wired-PTP -> Wi-Fi-beacon time bridge, end to end. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Doc-only pass on
docs/time-distribution.md, folding in two bench results that were living only in session notes:Transport floors for the software downlink: the ~94 µs per-slave absolute lock is the transport's submit-to-air floor, not a protocol property — USB ~93 µs RMS vs PCIe ~12 µs RMS (robust outlier-rejecting fit;
tests/pcie_txegress_tx.cpp+tests/txegress_witness.cpp). A PCIe master lifts the absolute software-path lock ~8×; the inter-slave difference and the hardware-beacon path are unaffected (already transport-free).New "Bridging to network PTP (IEEE 1588)" section: the 8821CE TSF exposed as a Linux PHC (
tests/pcie_phc/) and disciplined byphc2systo the Radxa X4's Intel I226 holds ~290 ns RMS (the TSF's own 1 µs-quantization floor: uniform ±0.5 µs → 289 ns) with the ~+42 ppm raw crystal offset servoed out. Combined with the TBTT steering actuator (Jaguar2 fine beacon-TBTT steering via reserved-page re-download — unblock the 8821CE PCIe AP↔PTP loop #242/Jaguar1 hardware beacon + TBTT steering — StartBeacon on every generation #243), that's the complete wired-PTP → Wi-Fi-beacon time bridge: network PTP disciplines the master's TSF, the TSF-locked hardware beacon distributes it, slaves inherit the wired timebase at the sub-µs beacon floor.Numbers cross-checked against
docs/timing-accuracy.md; all referenced tools exist in-tree. (The "Beacon-TBTT steering is Jaguar3-only" wording was already removed by #242 — repo-wide grep is clean.)🤖 Generated with Claude Code