perf(v5e): SX1262 SPI 125 kHz -> 4 MHz + TCXO wakeup 10 -> 5 ms (1.72x downlink)

[Mikefly123]

Summary

USP LoRa downlink at P4_GFSK_75K was software-limited to ~2.5 kB/s (~27% of PHY). Per-frame RAM-trace instrumentation on the HIL bench (probes at ComQueue hand-off, UspRadio deferred TX, RAC submit, onPreTx, set_tx, TX_DONE, comStatus) produced this breakdown of the ~97 ms/frame cadence:

stage	125 kHz baseline	4 MHz	4 MHz + TCXO 5 ms
comStatus -> next frame at radio	~8.6 ms	2.4 ms	2.3 ms
dataIn -> deferred handler	0.7 ms	6.2 ms*	6.1 ms*
stop/submit/RAC/planner	~0.2 ms	~0.3 ms	~0.3 ms
onPreTx (SPI config + 249 B FIFO + set_tx)	30.7 ms	12.2 ms	7.2 ms
airtime (set_tx -> TX_DONE)	27.8 ms	27.8 ms	27.8 ms
TX_DONE -> comStatus	3.7 ms	2.7 ms	2.8 ms

* includes the deliberate bounded re-arm-skip wait; overlaps the pipeline hop.

Two board-DTS costs dominate the overhead:

1. spi-max-frequency = 125000 (bring-up value) — ~18 ms/frame of pure SPI time.
2. tcxo-wakeup-time = 10 — the radio planner sleeps the radio after every task, so the TCXO restart is paid per frame.

Measurements (178,704 B //dl200k.bin SD-file downlink over RF, 248 B frames)

• baseline: 69.8-71.5 s (2500-2562 B/s), 181-214 RateGroupCycleSlip/run
• SPI 4 MHz: 45.79 s (3903 B/s), 1 cycle slip
• SPI 4 MHz + TCXO 5 ms: 41.44 s (4312 B/s, 1.72x), 5 cycle slips
• All downlinked files byte-identical (md5 911c2ce7...) to baseline copies
• Bidirectional link, authenticated RF commanding, and P0<->P4 profile switching re-verified after each change

Caveats / follow-ups

• TCXO 5 ms is HWIL-validated but should be confirmed against the E22-400M30S TCXO startup spec before flight; keep 10 ms if the spec disagrees (SPI-only change alone is 1.53x).
• The structural fix — lazy radio sleep between back-to-back TX tasks in the radio planner (saves the remaining ~5-7 ms wake cost) — is a usp fork change, tracked separately.
• Remaining software floor after this PR is ~15 ms/frame (bounded re-arm window + residual SPI/handshake); airtime is now >60% of the frame period.

🤖 Generated with Claude Code

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