DigiRadio/Software/docs/si4684-rf-investigation-report.md

Si4684 RF no-lock investigation — status report

Date: 2026-08-13 Board: PCBWay order W96157ASH49, U6 = Si4684-A10 (confirmed genuine, top marking 4684A10-2112AD254YZ-2112AD-E3)

Symptom

Si4684 (U6) boots, loads the ROM patch and FM/DAB application images, and answers every SPI command correctly (CTS, boot sequence, property reads/writes all succeed). FM and DAB tuning never completes: the STCINT status bit never sets, RSQ/DIGRAD metrics stay at zero (RSSI=0, SNR=0, VALID=0, FIC quality=0, empty service list), on both bands, at every frequency tried.

What has been verified correct (do not re-litigate)

All cross-checked byte-by-byte against the official Skyworks documents (Hardware/DATASHEET/AN649.pdf, AN851_Schematics_Layout.pdf):

• Boot sequence: RSTB# pulse, ROM patch stream, image stream, BOOT — matches the AN649 flowchart exactly, on every boot, both bands.
• Crystal / POWER_UP: XTAL_FREQ = 19,200,000 exact (bytes 00 F8 24 01, little-endian 0x0124F800), CLK_MODE = crystal mode (0x17 → bits 5:4 = 01). Matches the physical ABM8-19.200MHZ-10-1-U-T crystal (U7) and BOM/schematic.
• I2S: Si4684 configured as I2S slave (DIGITAL_IO_OUTPUT_SELECT = 0x0000), 48 kHz; ADAU1701 is bus master. Confirmed via GPIO clock probe (LRCLK ≈ 48000 Hz, BCLK ≈ 3.07 MHz) at every boot.
• Front-end matching properties: FM/DAB_TUNE_FE_VARM (0x1710), FM/DAB_TUNE_FE_VARB (0x1711), and FM/DAB_TUNE_FE_CFG/VHFSW switch (0x1712, value 0x0001 = closed) all match AN851's "Silicon Labs Recommended Front End Network" table exactly (FM: 0xEDB5/0x01E3; DAB: 0xF8A9/0x01C6; switch closed on both).
• FM_TUNE_FREQ command: all six ARG bytes decoded bit-by-bit against the AN649 command table (DIR_TUNE, TUNE_MODE, INJECTION, FREQ, ANTCAP, PROG_ID) — correct.
• INT_CTL_ENABLE / INT_CTL_REPEAT (STCIEN/STCREP, properties 0x0000/0x0001): correct bit positions; confirmed these only gate the physical INTB pin, not the STATUS0 STCINT bit our driver polls directly over SPI.
• STC polling mechanism: the same raw SPI byte (pollRx[1]) that reliably reports CTS=1 (bit 7) across hundreds of successful commands also reports STCINT=0 (bit 0) — the read path itself is proven reliable by the CTS side, so the "never sets" result is a real hardware/firmware-image observation, not a polling bug.

Front-end network component mismatch (found this session, not the cause by itself)

The board's actual front-end network (RF1 → C13(33pF) → L1(18nH) → C14(2.7pF shunt) → L3(120nH shunt) → VHFI, L2(22nH) bridging VHFI↔VHFSW) differs from Silicon Labs' AN851 reference network the VARM/VARB constants were derived from (C1=33pF, L1=56nH, L2=120nH‖L3=120nH). This was flagged as a plausible contributor, then tested directly and ruled out as the sole cause (see below).

Empirical sweeps (all negative — zero variation)

• IBIAS/CTUN (crystal startup calibration, POWER_UP ARG3/ARG8): 8 candidates across the practical range, full reboot between each. No change.
• ANTCAP (FM_TUNE_FREQ ARG4/5, bypasses FE_VARM/VARB auto-tune entirely and forces the on-chip antenna varactor directly, per AN851 Appendix A): ~100 of 128 possible values swept, across three antenna conditions (disconnected, loose contact, directly soldered 70 cm — correct quarter-wave for FM). Every single attempt returned byte-identical RSQ raw data (00 80 00 00 c0 00 00 00 00 00 00 00, RSSI=0/SNR=0/VALID=0). If the RF path were electrically functional, at least one of ~100 forced varactor values across the full physical range should have produced resonance. None did.
• Reset type: software RSTB# vs. full USB power-cycle (15 s cold) — no change.
• PCB continuity: RF1 (antenna connector) → C13 → L1 → U6 pin 10 (VHFI) confirmed intact with a multimeter (tested each leg separately to work around C13's DC block). No broken trace/via.

Leading hypothesis: QFN-48 exposed pad (EP) solder defect

U6 is a 7×7 mm QFN-48 with an exposed thermal/ground pad (pin 49, tied to GND). Insufficient solder or voiding under this pad during reflow is a well-documented QFN assembly failure mode that produces exactly this symptom: digital I/O (peripheral pins, less ground-sensitive) works perfectly, while the RF/analog front end (which references the exposed pad for a clean ground) fails entirely. ESD was considered and set aside — the antenna input has ESD clamp protection (D3, BAV99) ahead of the RF path, and no digital-side symptom consistent with ESD damage (SPI glitches, crystal instability) has ever appeared.

Action taken: a technical report was sent to PCBWay (order W96157ASH49) requesting an assembly quality review of U6's solder joints, specifically the exposed pad, laying out the same evidence above (firmware ruled out by the ANTCAP-bypasses-firmware argument).

Action pending: a manual hot-air reflow of U6 (re-melt only, no added solder/paste) was planned as a lower-risk first attempt before considering full chip removal and re-paste. Outcome not yet recorded in this document as of this report's writing — update this section once attempted.

Separate finding this session: audio profile save was broken, now partially fixed

Independent of the Si4684 investigation, while testing the internet radio streaming feature (components/services/webradio), audio profile changes via PUT /api/audio/profile and POST /api/audio/reset were found to fail (store_failed) — which is why the ESP32 mixer channel (needed to hear the web radio stream through the DSP mixer, muted at -96 dB by the default "radio-first" mix) could not be un-muted via the API.

Root-caused and fixed: sigma_i2c_write() (components/drivers/adau1701/src/SigmaStudioFW.c) had no retry on I2C transaction failure. A full EQ apply chains ~55 sequential I2C transactions (5 bands × safeload block); a single transient NACK anywhere in that burst aborted the whole sequence, while short bursts (e.g. the 2-write beep toggle) reliably succeeded. Added a 3-attempt retry with a 2 ms backoff.

After the fix, DSP-side writes (mixer, EQ) succeed. A second, separate failure remains: NvsAudioProfileStore::saveProfile() still fails, now isolated to the NVS write step itself (not the DSP). Diagnostic logging was added (nvs_open/nvs_set_str/nvs_commit error codes) to pin down the exact esp_err_t; the leading suspicion is NVS partition space/fragmentation (the nvs partition is only 24 KB, and this session alone did many repeated writes across streaming config, beep toggles, Wi-Fi, and station data). Not yet confirmed with the actual error code — re-run the diagnostic build and capture the log line to close this out.

Blob/firmware-image integrity check (completed, negative — blobs are genuine)

getPartInfo() and getSysState() (components/drivers/si4684/src/Si4684Driver.cpp) already existed to decode GET_PART_INFO/GET_FUNC_INFO/GET_SYS_STATE, but were never called anywhere in the codebase — dead code, so their byte-offset bugs had never surfaced. Found and fixed two rounds of off-by-one bugs while wiring them into a boot-time diagnostic log:

1. First pass: every field (chip ID, firmware major/minor/build, image type) read one byte too far right — e.g. firmwareBuild was actually reading the NOSVN/LOCATION flag byte, not a version number.
2. That "fix" was itself wrong in the other direction. readRaw() responses carry a one-byte SPI lead-in before STATUS0 — the same convention already confirmed and commented in pollStc() elsewhere in this file — which the first pass didn't account for. Caught empirically: the "fixed" GET_SYS_STATE reported image=192 (0xC0), the exact byte pattern of STATUS3 with PUP_STATE=3 seen dozens of times elsewhere in this investigation — proof the read was still one byte off, in the other direction. All three existing response buffer sizes (7/13/24 bytes) already matched "N response bytes + 1 lead-in", confirming the lead-in-byte offset (not the no-lead-in offset) is correct.

Result after the fix, captured live from the device (DAB boots first at startup):

Si4684: blob streamed: 5796/5796 bytes      (rom_patch_016.bin, matches file size exactly)
Si4684: blob streamed: 517524/517524 bytes  (dab_firmware.bin, matches file size exactly)
Si4684: GET_SYS_STATE: image=2              (2 = DAB active, correct per AN649)
Si4684: GET_PART_INFO/GET_FUNC_INFO: part=4684 rev=4.0.5 svnid=0x00001754

part=4684 matches the expected Si4684 part number exactly; rev=4.0.5 and the SVN ID are plausible, sane values, not garbage. Blob byte counts streamed over SPI match the local file sizes exactly — no truncation in transit. Verdict: the DAB blob loaded on the chip is genuine and intact. (FM boot's GET_FUNC_INFO was not yet captured — a BT1035 AT-init failure, see below, has been blocking the device from reaching the point in the boot sequence where FM is exercised. Not expected to change this verdict; DAB alone already answers the blob-integrity question this check was for.)

This closes the last plausible firmware-side explanation for the no-lock symptom as far as DAB is concerned only — see the 2026-08-14 update below for a correction to how far this actually generalizes to FM.

Open items

1. Confirm the exact NVS error code for the audio-profile save failure and fix accordingly (likely: erase/compact the audio_profile_json key, or address partition fragmentation — do not perform a full NVS erase without explicit confirmation, it would wipe Wi-Fi credentials, stations, and the saved BT speaker pairing).
2. PCBWay dispute closed (2026-08-14) without resolution. PCBWay's response: AOI passed, no X-ray performed (not requested by their process), and they consider the assembly sound unless given photographic evidence — which, per the QFN voiding research below, cannot exist for this failure mode by construction. The user closed the dispute rather than continue arguing a claim neither side can prove without an X-ray neither side is willing/able to obtain (single unique board, international shipping not worth the cost or risk). PCBWay is no longer an active avenue. Remaining options if revisited: local X-ray access (university SMT lab, phone-repair/BGA rework shop), or the hot-air reflow (still requires the user's explicit go-ahead — not to be proposed proactively).
3. Record the hot-air rework outcome (RSSI response test) if/when the user decides to attempt it. Not proposed proactively — the user has explicitly declined to touch/rework the board without a materially stronger reason than what non-invasive diagnostics have produced so far.
4. Visual tilt/float check (2026-08-13/14, two rounds, 8 photos: top-down and genuine side-profile/raking-light): negative — U6 sits flush, solder fillets look even on every edge photographed, no visible gap or lifted corner. Rules out the "gross float from paste over-print" QFN failure mode specifically (a real, documented failure mode found via web research this session). Does not rule out sub-visible exposed-pad voiding, which is undetectable by any optical method — confirmed via research into QFN thermal-pad voiding literature (needs X-ray, see item 2).
5. VA/VCORE analog+core supply rail measured directly at U3 (1.8 V regulator) pin 5: 1.791 V, within the Si4684 datasheet spec (1.71–2.0 V, typ. 1.8 V). Rail confirmed healthy; this was expected going in, since VA and VCORE share the same physical net and VCORE was already known-good (the chip boots and answers SPI). Rules out a gross power-rail fault as the cause.
6. LO-leakage test (second FM radio near the board while attempting a tune, to detect whether the Si4684's local oscillator radiates near the tuned frequency + IF) — proposed, not yet performed/reported by the user. Still the only remaining test that can distinguish "RF synthesizer alive, fails downstream" from "RF block itself never starts."

2026-08-14 update: FM blob verified, BT1035 root-caused (software, not hardware)

FM blob integrity — closed. Live capture, POST /api/tuner/tune {"band":"fm","frequency_khz":95000}:

Si4684: blob streamed: 531300/531300 bytes   (byte-perfect vs local file)
Si4684: FM firmware booted
Si4684: GET_SYS_STATE: image=1
Si4684: GET_PART_INFO/GET_FUNC_INFO: part=4684 rev=5.1.3 svnid=0x000023b3

Genuine, byte-perfect, sane values — same verdict as DAB. Tuning at 101.5 MHz and 95.0 MHz both reproduce the identical no-lock signature already seen on DAB (STC timeout, FM RSQ raw: 00 80 00 00 c0 00 00 00 00 00 00 00, all metrics zero).

New finding: DAB (rev 4.0.5) and FM (rev 5.1.3) are from two different Skyworks release generations roughly two years apart (DAB blob sourced from the PE5PVB community project, FM from a Skyworks eval CD) — confirmed via the official AN649 Table 1 revision history. Both are individually within ROM0.016's documented compatibility window, and each band load is an independent, exclusive HOST_LOAD (never concurrent), so this mismatch is very unlikely to be functionally relevant. Recorded because it was a real, previously-unverified gap, not because it changes the verdict.

This strengthens the hardware hypothesis: two independently-sourced firmware images, different vintage, different origin, fail identically. A shared firmware bug across both is far less plausible than a shared hardware cause (front-end/EP) that doesn't care which application image is loaded.

BT1035 AT init failed — root-caused and fixed, was software, not hardware. Contrary to the working hypothesis from earlier tonight (intermittent physical contact, correlated with the antenna soldering session), the actual cause was two regressions introduced by this session's own earlier commit (6f7b6dd), confirmed by diffing against the last commit explicitly logged as "all companion chips ready" (6ca40f1, 2026-08-05):

1. A redundant software AT+RESET sent over UART immediately after the hardware RESET# pulse — absent from the known-good baseline, which goes straight from the hardware pulse into the init handshake. Landing this command while the module is still processing the hardware reset risks restarting its bring-up mid-sequence.
2. A UART boot-banner diagnostic probe (added earlier this session to distinguish "module silent" from "module garbled") with too short a listen window (1500 ms) — live capture showed the module's real unsolicited boot banner (+VER=FSC-BT1035,V6.1.1,20240521) arriving closer to 5 s, well outside that window.

Both fixed in components/drivers/bt1035/src/Bt1035Driver.cpp: removed the redundant AT+RESET, widened the listen window to 3500 ms. Result: 5/5 clean boots after the fix, versus roughly 1/13 before. No physical intervention, cleaning, or component was involved — a visual inspection of the BT1035 module's castellated pads (zero risk, no rework) found nothing abnormal beyond minor flux residue, consistent with this being a pure software regression, not a solder defect.

Net effect on the Si4684 hypothesis: none directly — BT1035 and Si4684 are separate chips/subsystems — but it's a useful calibration: a fault that looked exactly like a classic "physical handling damage" symptom (persistent after a soldering session, deterministic-then-intermittent) turned out to be 100% software. Worth remembering as a caution against over-attributing intermittent symptoms to hardware without exhausting the code-path diff against a known-good commit first.

2026-08-15 update: front-end network mismatch quantified — does not explain the total blackout

Follow-up on the "Front-end network component mismatch" section above (board network C13 33pF, L1 18nH, C14 2.7pF shunt, L3 120nH shunt, L2 22nH vs AN851's reference C1 33pF, L1 56nH, L2‖L3 120nH‖120nH). The mismatch was flagged as a plausible contributor but never quantified. Real component coordinates were pulled directly from DigiRadio.kicad_pcb (RF1 at 104.064,92.281; C13 111.811,92.281; L1 112.319,89.868; C14 114.097,91.519; L3 115.621,89.868; L2 115.621,91.9; U6 121.717,90.122 — confirming the network's physical path and component identity), then modeled as a two-port ABCD chain (series C13 → series L1 → shunt bank C14‖L3‖L2 at the VHFI node), 50 Ω reference on both ports. This is a lumped-element approximation: it ignores PCB trace parasitics, the chip's real complex input impedance at VHFI, and antenna radiation — good for an order-of-magnitude comparison against the AN851 reference network, not an absolute number.

(A pure EM/gerber-based simulation via gerber2ems/openEMS, initially considered, was ruled out for this specific question: per its own documentation, gerber2ems does not model discrete capacitors/inductors — "capacitors are not simulated... they can be approximated by shorting them using a trace" — which would misrepresent a network that is almost entirely discrete L/C components.)

Result (S21 = insertion loss, S11 = return loss, board network vs AN851 reference):

Band	Board S21	Reference S21	Board S11	Reference S11
FM 87.5–108 MHz	−7.1 to −9.8 dB	−1.2 to −1.5 dB	−0.5 to −0.9 dB	−5.4 to −6.3 dB
DAB 174–240 MHz	−2.4 to −3.4 dB	−2.0 to −3.0 dB	−2.7 to −3.7 dB	−3.1 to −4.4 dB

FM: the board network carries a real 6–9 dB insertion-loss penalty over the reference network — worth correcting, but not by itself the kind of loss that silences a strong local FM station on a working receiver (10 dB of front-end loss is routinely tolerated).

DAB: the board network is within ~0.3–1.4 dB of the reference network — essentially the same insertion loss. The mismatch is not a meaningful factor at DAB frequencies at all.

Conclusion: since DAB shows the identical total-blackout signature as FM (RSSI/SNR/VALID all zero, unmoved by ~100 ANTCAP sweep values) despite the front-end mismatch being nearly irrelevant in that band, the network mismatch cannot be the primary cause of the observed failure on its own. This is a quantitative point in favor of the existing QFN exposed-pad hypothesis (§ "Leading hypothesis" above), not a competing explanation — it narrows, rather than replaces, the open items in that section.

2026-08-16 update: hot-air reflow attempted — no change to RF symptom

The manual hot-air rework of U6 (re-melt only, no added solder/paste) flagged as "action pending" in the Leading hypothesis section was carried out: 100°C for 1 minute, then 220°C for 1.5 minutes, low airflow.

Post-rework, on a fresh build/flash of the current firmware, FM tuning was retested at three frequencies (100.9, 95.0, 87.9 MHz) via POST /api/tuner/tune. Result: byte-for-byte identical to every pre-rework capture in this report.

Si4684: STC timeout: last poll spi_err=0 status=12 c0 00 00 c0 INTB=1
Si4684: FM tune STC timeout at 95000 kHz — settling 150 ms
Si4684: FM RSQ raw: 00 80 00 00 c0 00 00 00 00 00 00 00
Si4684: FM tuned 95000 kHz antcap=0 rssi=0 dBuV snr=0 dB valid=0 readfreq=0

Same at 100.9 and 87.9 MHz. RSSI/SNR/VALID all zero, locked=false, no variation from the reflow.

Item 2/3 (rework outcome) in "Open items" above is now closed: attempted, no effect. This does not rule out the QFN exposed-pad hypothesis — a re-melt without added paste/flux does not reliably resolve a voiding defect under an exposed pad (only adds heat to already-present solder, doesn't add volume where a void is) — but it does mean the easy, low-risk fix attempt is exhausted. Remaining paths are the non-destructive diagnostics proposed this session (mechanical flex test with live RSSI monitoring, controlled thermal stress test with live RSSI monitoring, NanoVNA S11 sweep at RF1 chip-on vs chip-off) or escalating to X-ray/full chip removal, neither attempted yet.

2026-08-16 update: root cause found — FM_TUNE_FREQ/DAB_TUNE_FREQ argument-offset bug, not hardware

This overturns the QFN exposed-pad hypothesis above. The actual cause of the months-long "total RF blackout" was a software bug in Si4684Driver::tuneFm()/tuneDab(), found by diffing our command construction against the official AN649 Command 0x30 (FM_TUNE_FREQ) and Command 0xB0 (DAB_TUNE_FREQ) argument tables directly (page-level read of Hardware/DATASHEET/AN649.pdf, not driver comments), prompted by cross- referencing against the independent hitech95/si468x_dab_receiver Linux driver.

Si4684Driver::writeCommand() always prepends a fixed ARG1 = 0x00 byte before whatever payload array is passed to it:

buffer[0] = static_cast<std::uint8_t>(cmd);
buffer[1] = 0x00U;                 // ARG1, always
std::memcpy(buffer.data() + 2U, payload, length);   // ARG2 onward

POWER_UP and HOST_LOAD callers already accounted for this correctly (their arrays are written starting at ARG2). tuneFm() and tuneDab() did not — both built their argument arrays starting at what the author believed was ARG1, so every byte actually landed one slot to the right of where it belongs, with an extra unused byte tacked on the end:

• FM_TUNE_FREQ (AN649 Command 0x30): real layout is ARG2=FREQ[7:0], ARG3=FREQ[15:8], ARG4=ANTCAP[7:0], ARG5=ANTCAP[15:8], ARG6=PROG_ID. Our code sent FREQ's low byte into ARG3 (should be the high byte), the actual frequency low byte was always sent as a fixed 0x00, and the ANTCAP value landed in ARG5 (the high byte of a 0–128-range field) instead of ARG4. The chip was never told the requested frequency — it received a garbage FREQ value derived from shifted bytes, and the ANTCAP sweep documented earlier in this report (~100 values, byte-identical results) was sweeping the wrong byte entirely, which is exactly why it never produced any variation.
• DAB_TUNE_FREQ (AN649 Command 0xB0): same shift. FREQ_INDEX (real ARG2) was always sent as 0x00; the actual requested index landed in ARG3, which the spec requires to be a fixed 0x00.

Fixed in components/drivers/si4684/src/Si4684Driver.cpp, tuneFm() and tuneDab(): removed the extra leading byte and the extra trailing byte so the arrays start at the real ARG2.

Result, live on hardware immediately after the fix (POST /api/tuner/tune, no other change — same antenna, same board, no rework involved in this result):

Si4684: FM RSQ raw: 00 81 80 00 c0 00 02 2e 22 8d fb fd
Si4684: FM tuned 87500 kHz antcap=0 rssi=-5 dBuV snr=-3 dB valid=0 readfreq=87500

RSSI/SNR now read real, varying, frequency-dependent values (e.g. −13 to +4 dBuV across a 10-point FM sweep, peaking near a plausible local station at 98.5 MHz) instead of the fixed 00 80 00 00 c0 00 00 00 00 00 00 00 / all-zero pattern seen in every capture in this report until now. No STC timeout occurred in any tune or seek attempt after the fix — every prior capture in this document logged one on every single attempt.

locked/valid is still false in this test — expected with the board's improvised antenna and not yet investigated further; that is now an ordinary sensitivity/antenna question, not a "chip never responds to RF" question. DAB was retested at freq_index=10 with no station found (fic_quality=0, cnr_db=0) but also with no STC timeout — most likely no active multiplex at that index/location, to be swept properly with a real antenna as a follow-up, not evidence against the fix (which addresses the identical byte-shift bug in both commands).

What this means for the rest of the investigation: the QFN exposed-pad hypothesis, the front-end network mismatch analysis, the hot-air reflow, and the mechanical flex test were all investigating a symptom that had a software cause. None of that work was wasted — the empirical rigor (ANTCAP sweep producing zero variation, DAB and FM failing identically) is exactly what made this bug's fingerprint recognizable once the actual command bytes were checked against the primary spec instead of trusted from driver comments. The lesson: writeCommand()'s implicit ARG1 prepend is an easy trap for future commands — any new caller must remember its array starts at ARG2, not ARG1.

Follow-up, completed same session: audited every remaining writeCommand() call site in Si4684Driver.cpp against the AN649 page text (not driver comments) and found the identical bug pattern repeated in several more places — writeCommand()'s implicit ARG1=0x00 prepend was either swallowing a real ARG1 value the caller needed, or shifting a whole multi-byte struct one slot right:

• seekFm() (FM_SEEK_START, 0x31): SEEKUP/WRAP (real ARG2) were never sent — the chip always saw ARG2=0x00, so hardware seek always searched down with no wrap regardless of what was requested. This is why every seek in this report's earlier captures fell through to the hitech95-inspired 100 kHz software-step fallback in Si4684Tuner instead of using the chip's real seek.
• startDabService()/stopDabService() (0x81/0x82): SERVICE_ID and COMPONENT_ID (8 bytes, real ARG4-11) were shifted one byte right into ARG5-12, with SERTYPE landing in ARG2 (spec: fixed 0x00) instead of ARG1. Playing a specific DAB service would have started the wrong service/component or failed outright — not yet observed in practice only because tuning itself never worked before this session.
• readDabServiceData() (GET_DIGITAL_SERVICE_DATA, 0x84): same ARG1-only shift as below, plus the STATUS_ONLY bit was coded as 0x08 (bit 3) instead of the correct 0x10 (bit 4) per the AN649 bit table.
• clearFmStc(), readFmRsq(), readFmRds(), fetchDabServiceList(), readDabDigRadStatus(), readDabEventStatus(): all six commands (FM_RSQ_STATUS 0x32, FM_RDS_STATUS 0x34, GET_DIGITAL_SERVICE_LIST 0x80, DAB_DIGRAD_STATUS 0xB2, DAB_GET_EVENT_STATUS 0xB3) have only ARG1 in the AN649 spec — no ARG2 exists at all. Passing anything through the old writeCommand(cmd, payload, length) two-argument form for these could only ever send a spurious extra byte while the intended ARG1 flag (STCACK, INTACK, SERTYPE, DIGRAD ack, EVENT_ACK) silently landed nowhere, since writeCommand() had no way to set ARG1 to anything but a hardcoded 0x00. clearFmStc()'s STCACK never fired in this driver's entire history — masked because FM_TUNE_FREQ/FM_SEEK_START already auto-clear STC per their own AN649 documentation.

Fixed by giving writeCommand() a fourth parameter, std::uint8_t arg1 = 0x00U (default preserves every already-correct call site), and updating each caller above to either pass its flag byte through arg1 with no payload (for the ARG1-only commands) or drop the erroneous leading array element (for the multi-arg commands whose ARG1 is legitimately always 0x00, e.g. seekFm's default tune mode).

Confirmed live after this round of fixes — first locked: true and first hardware (non-software-fallback) seek in this entire investigation:

POST /api/tuner/tune  {"band":"fm","frequency_khz":87500}
POST /api/tuner/seek  {"direction":"up"}
-> {"frequency_khz":98300}
GET /api/tuner/status
-> {"locked":true,"fm":{"frequency_khz":98300,"rssi_dbuv":12,"snr_db":14,"stereo":false}}

The seek jumped directly from 87.5 to 98.3 MHz in one hardware search (not 100 kHz software steps), landing on a real, locked station at a plausible RSSI/SNR. bt1035 also came back to true in /api/health during this same session (cause not yet diagnosed — see the BT1035 section below; unrelated to this fix, separate chip).

DAB was swept across 7 frequency-table indices (5, 10, 15, 20, 25, 30, 35) after the fix — fic_quality/cnr_db stayed at 0 on all of them, no lock yet. The DAB_TUNE_FREQ/DAB service-list/service-start fixes are verified correct against the AN649 spec text the same way the FM fix was, but do not yet have an empirical lock to point to, unlike FM. Not treated as a red flag — no DAB antenna tuning has been attempted yet, and the default European frequency table may not match active local multiplexes at these particular indices. Next step: sweep the full DAB frequency table (not just 7 samples) with a real antenna and confirm a lock the same way FM was confirmed.

2026-08-16 update: first real audio from Si4684 — ADAU1701 SerialInputRegister IBP polarity

After the FM_TUNE_FREQ/DAB_TUNE_FREQ fix above produced a real lock (locked:true, RSSI +12 dBuV, SNR +13 dB on 98.3 MHz), the speaker was still silent. This section covers debugging that separate problem — not a Si4684 RF issue, but the digital audio link from Si4684 into the ADAU1701.

Audit trail, each step verified against a primary source, not assumed:

1. Re-verified every Si4684 audio property against the AN649 page text: DIGITAL_IO_OUTPUT_SELECT, _SAMPLE_RATE, _FORMAT (24-bit sample in 32-bit I2S slots), AUDIO_MUTE (unmuted), AUDIO_OUTPUT_CONFIG (was incorrectly written with a stray bit — 0x0302's only real field is bit0 MONO, not an I2S enable; fixed to 0x0000). All correct.
2. PIN_CONFIG_ENABLE (0x0800): bit1 I2SOUTEN, bit0 DACOUTEN — AN649 states "only I2SOUTEN or DACOUTEN can be enabled at a time; if both enabled, only analog output is enabled." The driver was writing 0x0003 (both bits) — the chip was falling back to its unused analog DAC output on every boot. Fixed to I2SOUTEN-only. Cross-checked against hitech95/si468x_dab_receiver's ALSA codec driver (sound/soc/codecs/si468x.c): their working value is SI468X_PROP_I2S_ENABLED = 0x8002 (I2SOUTEN + INTBOUTEN, bit15) — not just 0x0002. Adopted 0x8002.
3. Traced the full ADAU1701 SigmaStudio netlist (Firmware/ADAU1701-Firmware/DigiRadio_NetList.xml, generated export, not guessed): Si4674 gain cell → St Mixer1 → PEQ1 → Master gain → Limiter → Output, confirmed reachable and correctly addressed (ADDR_SI4674/ ADDR_SI4674_1 come from the generated DigiRadio_IC_1_PARAM.h, not hand-typed). Confirmed this whole downstream chain works independently — both the Beep1 test tone and the web radio (ESP32) path were audible through it before any Si4684 fix.
4. Checked MpCfg0/MpCfg1 (ADAU1701 pin-mux registers, DigiRadio_IC_1_REG.h) against the ADAU1701 datasheet (Hardware/DATASHEET/adau1701.pdf): MP0, MP1, MP4, MP5 (SDATA_IN0/1, INPUT_LRCLK, INPUT_BCLK) are correctly configured as "Serial data port" function, not left as GPIO.
5. Found a PCB net-name vs. silicon pin-name mismatch while tracing the Si4684→ADAU1701 connection in DigiRadio.kicad_pcb: Si4684 (U6 pin 33) lands on ADAU1701 (U9) physical pin 11, which the datasheet identifies as MP0 (silicon channel SDATA_IN1) — the PCB net is labeled "SDATA_IN0", which does not match the silicon function at that pin. Tested by unmuting both the "Si4684" and "ESP32" mixer gain legs simultaneously (Si4674/ESP32 cells in the netlist) — this did not by itself fix the silence, so the SigmaStudio channel assignment for Input1 was not actually the blocking issue (kept both legs unmuted as a harmless no-op change of practice, not reverted).
6. Root cause: SerialInputRegister (ADAU1701 register 0x081F, Table 49 in the datasheet), which controls the serial input port's clock polarities — ILP (bit4, LRCLK polarity) and IBP (bit3, BCLK edge the input data changes/is clocked on). This register is baked into the compiled SigmaStudio DSP program export and is not something the ESP32 firmware wrote at runtime before now; its compiled value is the default 0x00 (ILP=0, IBP=0). Added a diagnostic runtime override in Adau1701Driver::boot() (via SIGMA_WRITE_REGISTER_BLOCK, the same primitive the DSP program loader itself uses) to test alternate polarities live, without touching the SigmaStudio project:
   • IBP=1 alone (0x08): real, recognizable music instead of pure static on a locked, strong FM signal — first time ever.
   • ILP=1 added on top (0x18): made it worse (pure white noise again).
   • Back to IBP=1 alone (0x08): music confirmed again, though inconsistently — RSSI/SNR fluctuated significantly between otherwise identical retunes (SNR seen anywhere from 2 to 14 dB on the same station), consistent with a marginal/improvised antenna connection rather than a firmware regression. Kept IBP=1 as the fix.

Fixed in components/drivers/adau1701/src/Adau1701Driver.cpp (SerialInputRegister override after DSP program load) and components/drivers/si4684/src/Si4684Driver.cpp (PIN_CONFIG_ENABLE = 0x8002, AUDIO_OUTPUT_CONFIG = 0x0000).

Status: first confirmed end-to-end audio path (Si4684 → ADAU1701 → BT1035 → Bluetooth speaker) in this project's history. Remaining noise on top of the music is attributed to antenna quality, not yet independently confirmed with a proper antenna — flagged as follow-up, not closed.

2026-08-16 update: DAB lock confirmed on 3 ensembles + response-offset bug

With the tune fix in place, a full sweep of freq_index 0-35 found three real ensemble locks (fic_quality=100 on all three): index 5 (CNR 7 dB), index 22 (CNR 15 dB), index 23 (CNR 20 dB, strongest). First confirmed DAB lock in this project's history.

Chasing why /api/tuner/services returned service_list_empty even after 30+ seconds on a solid lock found a second bug class, this time in response parsing, not command construction: readFmRds(), readDabDigRadStatus()'s acquired field, and readDabEventStatus() all read raw[4] expecting AN649's "RESP4" field, but this driver's own established convention elsewhere (getPartInfo(), and the already-correct ficQuality/cnrDb fields in readDabDigRadStatus()) is raw[5]=RESP4 (raw[0]=SPI lead-in, raw[1..4]=STATUS0-3). Fixed all four call sites to the correct offset; readFmRds()'s fifoUsed/blockA-D fields were consequently also all off by one and fixed together with it.

After the fix, serviceListReady now correctly gates open and /api/tuner/services returns real data instead of service_list_empty — but the entries themselves are still garbled (implausible service_id values, component_id fields that decode as ASCII spaces, e.g. 538976288 = 0x20202020, mostly-empty labels). This points to a third, separate bug in fetchDabServiceList()'s service-list body parsing (the entry structure walked in the loop over serviceCount), not yet investigated — the DAB service list binary format is documented in AN649 §7 "Digital Services User's Guide" (starts around page 418), not the command tables checked so far. Confirmed live: POST /api/tuner/play with one of these garbled IDs accepted ({"status":"playing"}) but produced no audio, consistent with a wrong service/component ID rather than a new audio-path regression.

Follow-up, not done this session: fix fetchDabServiceList() entry parsing against AN649 §7; then confirm actual DAB audio playback end to end the same way FM was confirmed.

Unrelated finding from the same session, logged for completeness: BT1035 began failing boot deterministically (no spontaneous UART bytes after hardware reset, then AT init failed) starting from this session, on both the firmware build that predates and the one that includes the boot-sequence fix from fd9d4ae — ruling out that fix's absence as the cause. Extending the diagnostic listen window from 3.5 s to 12 s (temporary, reverted) produced zero bytes either way, confirming this is not the previously-fixed "banner arrives late" timing issue but a harder, total UART silence. The BT1035 module was not physically touched during the U6 rework. Cause not yet identified; unrelated to the Si4684 investigation (separate chip), but HardwareBootstrap::boot() was changed (main/hardware_bootstrap.cpp) to treat BT1035 boot failure as non-fatal rather than halting the whole device, so the rest of the system (Si4684 tuning, web UI, Wi-Fi) remains usable while this is investigated separately.

2026-08-19 update: fetchDabServiceList() entry parsing fixed; DAB audio

confirmed, quality traced to signal strength

Live retest on real hardware found fetchDabServiceList()'s body parsing (the third bug flagged as "not yet investigated" above) double-counted the already-consumed SIZE field: it treated the payload as starting 2 bytes later than it actually does (serviceCount read from body[11] instead of body[9], service entries starting at body[15] instead of body[13]). AN649 doesn't actually document the DAB service-list entry layout itself — it defers to a supplemental "Digital Services User's Guide" this project doesn't have a copy of — so the exact field layout was re-derived by cross-checking hitech95/si468x_dab_receiver's si468x_core_cmd_dab_get_service_list() (a working Linux driver for the same command over the same command set), which also confirmed the payload carried after SIZE is SIZE-2 bytes, not SIZE bytes (fixed the read sizing to match).

Confirmed live immediately after reflashing: GET /api/tuner/services on a locked DAB ensemble (freq_index 5) now returns 22 real, correctly-decoded Italian DAB station labels (R.M.T., Radio Cuore, GR News, Radio Sportiva, Lifegate, ...) instead of an empty list. POST /api/tuner/play against one of these real service/component IDs was confirmed audible — crackly/broken but present, not silence — on a second try after the first selected service (R.M.T., cnr_db=7) produced no audible sound at all. Switching to GR News (cnr_db=8) did produce audible (if degraded) audio. This matches DAB's two-tier robustness by design: the FIC channel (fic_quality 94-98 throughout) is far more error-protected than the actual audio sub-channel, so a receiver can report ensemble lock and a clean, complete service list while individual programme audio is too weak (CNR ~7-8 dB here) to decode cleanly or at all — the chip's own soft-mute is the most likely explanation for the first service's total silence, not a firmware defect. This is consistent with what FM already showed this session ("works, but badly") and with the still-open antenna/front-end TODO below.

Also found and fixed, unrelated to the Si4684: SetupWebServer was registering 41 HTTP routes against httpd_config_t::max_uri_handlers = 40 — httpd_register_uri_handler() fails past the limit with only a generic "no slots left" warning, no indication of which handler was dropped. The 41st and therefore last-registered route, POST /api/stations/tune, was silently unroutable (404) on every boot since whichever commit first pushed the route count past 40. Bumped to 56 for headroom.

2026-08-19 update (2): DAB_EVENT_INTERRUPT_SOURCE never configured;

intermittent multi-second HTTP unresponsiveness noted, still open

Retesting DAB service-list retrieval later the same night found it far less reliable than the earlier confirmation: locked:true, fic_quality:97-100 sometimes took anywhere from ~15s to ~60s to appear after a fresh POST /api/tuner/tune (full Si4684 reboot for the FM->DAB band switch), and even once locked with excellent FIC quality, GET /api/tuner/services kept returning service_list_empty for a further 30-65s.

Root cause candidate found by re-reading AN649's DAB_GET_EVENT_STATUS section (command 0xB3) carefully: the SVRLISTINT bit this driver polls via readDabEventStatus() is explicitly documented as gated by Property 0xB300 DAB_EVENT_INTERRUPT_SOURCE, bit 0 = SRVLIST_INTEN, default 0x0000 (disabled) at power-on — and this driver never wrote that property anywhere. configureAfterBoot()'s DAB branch already wrote a similarly-named DIGITAL_SERVICE_INT_SOURCE (property 0x8100), but AN649's own text for 0x8100 is internally inconsistent between its prose ("configures digital service interrupt sources") and its bit table (VHFCAPS/VHFSW, a front-end switch config field) — almost certainly a pdftotext -raw extraction artifact merging two adjacent property tables, the same failure mode noted earlier this session for AN649/adau1701.pdf text extraction. 0x8100 and 0xB300 are two different properties; only 0xB300's own section (page ~236) reads internally consistent, so it — not 0x8100 — is the one that gates SVRLISTINT. Added setProperty(kPropDabEventIntSource=0xB300, 0x0001) right after the existing 0x8100 write.

Verified live after reflashing: the service list did come back complete and correct (all 22 real station labels) on the next test. Not proven conclusively faster than before — DAB acquisition/list-assembly timing is inherently variable and this was only tested once post-fix — but the property write is unambiguously correct per its own AN649 section regardless, so it stays.

Separately, and NOT explained by the above: the HTTP server went fully unresponsive (connection timeouts on /api/health, the simplest possible route) for 5-10 second stretches, more than once, both before and after this fix. The heartbeat log line kept appearing on schedule throughout (digiradio: heartbeat every 5s, confirmed via serial), proving the whole system did not crash or panic — only the HTTP server (or whatever it was waiting on, most likely a blocking SPI/CTS wait inside the Si4684 driver triggered from a DAB status/event read) stalled and then recovered on its own. This was reproducible independent of the 0xB300 change (first observed hours earlier, unrelated, during the ANTCAP sweep in the antenna calibration work). Not investigated further tonight — candidate causes to check next: whether any Si4684Driver SPI wait loop lacks a bound tight enough for interactive HTTP use, and whether httpd_config_t:: max_open_sockets = 3 (components/net/src/SetupWebServer.cpp) is simply too small once anything blocks even briefly.

TODO (next session)

• Antenna/front-end calibration — now the real blocker for DAB/FM audio quality, not firmware. Both bands are confirmed working end to end (real lock, real service list, real audio) but both are signal-limited: FM "works, but badly" per live listening test, and DAB audio ranges from crackly to fully soft-muted depending on the service's CNR (~7-8 dB observed, on the low side). Redo the ANTCAP sweep — done this session for FM (see the ANTCAP antenna calibration feature commit); antcap=102 saved as the board's default, +6 to +11 dB RSSI/SNR across the band. DAB doesn't have an equivalent calibrated-default mechanism yet — added and swept 2026-08-20, see below; no default saved (auto-tune already best on the ensembles tested).
• Try a proper FM/DAB antenna to see how much of the crackle/noise clears up versus how much is inherent to the current antenna's gain/placement.
• Investigate the intermittent multi-second HTTP unresponsiveness noted above — reproducible, not yet root-caused, not obviously related to any single change this session.
• BT1035 boot-failure root cause still open (see section above) — non-fatal now, so it's no longer blocking, but still unexplained. Recurred 2026-08-20, see below — still open, confirmed not caused by physical handling.

2026-08-20 update: DAB ANTCAP override added and swept live; BT1035 "total UART silence" recurred

DAB ANTCAP — implemented, built, flashed, swept live via the HTTP API. Extended the ANTCAP override (AN649 Command 0x30 ARG4/5 for FM, Command 0xB0 ARG4/5 for DAB) from FM-only to DAB, mirroring the existing FM mechanism end to end: ITuner::tuneDab/Si4684Driver::tuneDab gained an antCap parameter (was hardcoded 0x00/auto); TunerService gained defaultDabAntCap_/setDefaultDabAntCap(); Eeprom24aa gained readDabAntCap()/writeDabAntCap() at word address 0x01 (FM stays at 0x00); HardwareBootstrap gained dabAntCapCalibration()/ saveDabAntCapCalibration(), loaded at boot alongside the FM one; the net::AntennaCalibration bridge gained saveDab; both POST /api/tuner/tune (one-shot override, {"band":"dab","freq_index":N, "antcap":V}) and POST /api/tuner/calibrate-antenna (persists to EEPROM, {"band":"dab","antcap":V}, band defaults to "fm" so old clients are unaffected) now accept DAB. Host build + 20/20 ctest + doxygen + check-manual-sync all green before flashing.

Swept live via the API (freq_index 0-128 step 8) against three real ensembles:

• freq_index 5 (weakest known ensemble, 7 dB CNR baseline from the 2026-08-16 sweep): did not lock at all this session, at any ANTCAP including auto — signal currently below threshold, not a code issue (indices 22/23 locked normally in the same session).
• freq_index 23 (strongest, 21-26 dB): CNR jittered ±3 dB across the whole ANTCAP range with no discernible trend — already saturated, sweep can't discriminate on a signal this strong.
• freq_index 22 (medium, 16-20 dB): auto (0) and antcap=32 tied for best (20 dB CNR); antcap=72 and 80 caused total loss of lock (a dead zone to avoid); the rest of the range gave no systematic gain over auto, unlike FM's clean +6 to +11 dB improvement.

Decision: left DAB on auto-tune, nothing saved to EEPROM. Unlike FM, no ANTCAP value tested beat the chip's own auto-tune by a margin worth trusting. If DAB audio quality is still the limiting factor later, retest specifically on a weak ensemble (index 5 or similar) once it's receivable again — ANTCAP calibration matters most on weak signals, which is exactly the case that wasn't testable this session.

BT1035 "total UART silence" recurred — same still-open issue as before, confirmed (again) not physical. During the DAB sweep, the board was reset several times via opening a pyserial connection for log capture — each open triggers a hardware EN/reset pulse on this ESP32-S3 (confirmed: happens even with dsrdtr=False, rtscts=False and explicit setDTR(False)/setRTS(False) — this is the USB-native auto-reset circuit firing on port open, not a pyserial default that can be disabled from the Mac side). One of these resets left BT1035 silent: no spontaneous UART bytes after hardware reset on both boot attempts (2/2), then silent across all 8 probed baud rates (9600-921600). This is not the "banner arrives late" issue fixed 2026-08-20 earlier this same session (kBootBannerWaitMs = 25000 was already in effect and made no difference) — it's the harder, total-silence failure mode already logged above (the "Unrelated finding from the same session" note before the 2026-08-19 entry), recurring. Confirmed again this time that it is not caused by physical handling: a full physical power-off for 60 s did not recover it (Si4684/ADAU1701 both came back up fine on the same power cycle, ruling out a board-wide power issue). Root cause still not identified. /api/bluetooth/status and /api/bluetooth/paired correctly report {"status":"error","reason":"at_timeout"} while in this state; the rest of the device (tuner, web UI) stays usable per the existing non-fatal-BT1035-boot design.

2026-08-21 update: BT1035 total silence confirmed intermittent (not

hardware); background retry mitigation added

Follow-up session dedicated entirely to the "total UART silence" BT1035 failure mode above. Summary: confirmed intermittent on genuinely identical hardware, root cause narrowed to the module's internal crystal (not our PCB, not fixable by us), and mitigated (not fixed) with an indefinite background boot retry.

Diagnostic instrumentation (temporary, added then reverted this session): added BT1035 AT TX: <line> / BT1035 UART RX RAW: <hex or <empty>> / BT1035 AT RESULT: OK|ERROR|TIMEOUT logging around Bt1035Driver::transmitAndCollect(), and temporarily dropped kBootAttempts to 1 for single-attempt clarity. This confirmed the failure signature precisely: AT is transmitted, zero bytes ever come back (<empty>), timeout. Reverted via git checkout once the manual diagnosis was done — not kept in the codebase.

Multimeter checks, all normal (scope-level checks — crystal oscillation, power-on transient — remain out of reach without an oscilloscope):

• VBAT_IN: 3.3V (datasheet range 3.0-4.2V) ✓
• 1.8V_OUT (module's internal regulator): 1.8V ✓ — proves the module's own power management is running, it isn't simply unpowered
• SYS_CTRL / RESET (post-boot): ~3.27V, matching the firmware's own GPIO readback log (post-reset: SYS_CTRL=1 RESET=1) ✓
• BT1035 TX pin (module side) to GND: 3.29V, idle-HIGH, no short/float/ reversed polarity ✓ (though idle-HIGH alone doesn't prove the module's firmware is executing — some pads default HIGH from reset state alone)
• A 10kΩ pull-down the user had added on SYS_CTRL (matching the datasheet's own recommendation for an undriven pin) was checked and is not the cause — the ESP32 GPIO drives push-pull and its own readback confirms it reaches a valid HIGH regardless.

Crystal location determined: the BT1035 datasheet's own block diagram shows "32MHz Crystal" as an internal block of the QCC3056 die, and the DigiRadio schematic netlist (Netlist_Schematic1_2026-08-07.asc) has no XTAL_IN/XTAL_OUT pins wired to any external crystal for U11 — confirming the oscillator is sealed inside the Feasycom module, not on our PCB. This is why nothing on our side (layout, load caps, our firmware) can affect it; if the failure really is a marginal oscillator-startup margin, it's a property of that specific physical module unit (or the part's design tolerance in general).

Decisive evidence of intermittency, not a dead unit: across repeated reboots in the same session (physical power-cycles and serial-port-open resets, which also hard-reset this ESP32-S3's native USB-CDC), the identical physical module was observed to boot completely successfully at least once — spontaneous banner +VER=FSC-BT1035,V6.1.1,20240521 + +DEVSTAT=1, then AT and AT+AUXCFG=3 both answered OK — and to fail completely silently on other attempts, with no physical change in between. This rules out "defective/dead module" as an explanation; ordering a replacement module is therefore not a guaranteed fix, since the same physical unit demonstrably works when it works.

UART loopback test attempt — inconclusive, logged for future reference. Tried to isolate ESP32 vs. module by bridging the ESP32-S3's own GPIO40 (BT1035 UART TX)/GPIO41 (BT1035 UART RX) pins with a jumper held by hand on the ESP32 module's castellated pads (no series resistor/test point exists on this net per the schematic netlist — U8.33 ↔ U11.P$14 and U8.34 ↔ U11.P$13 directly, nothing else). Twice reproducibly, bridging those pins from cold boot caused the ESP32 itself to hang very early in boot (right after the bootloader's "Disabling RNG early entropy source" line, before app_main() even starts) — harmless (board recovers fully once the jumper is removed) but unexplained, and it sidesteps the actual test rather than answering it. Not pursued further this session given the practical difficulty of hand-holding a wire onto castellated pads without a proper SMD test hook. If retried: attach the jumper after the ESP32 has already booted past that early stage (there's a ~25s window before the BT1035 AT command is actually sent) rather than from a cold boot.

Mitigation implemented: indefinite background boot retry. Since the module's own internal fault (if that's what it is) isn't something we can fix, and since it demonstrably self-clears on a later attempt rather than needing repair, main/hardware_bootstrap.cpp now spawns a bt1035RetryTask FreeRTOS task whenever the initial HardwareBootstrap:: boot()'s call to Bt1035Driver::boot() fails. The task loops calling boot() again with no artificial delay between attempts — each attempt already blocks for ~25-60s on its own (the banner wait times kBootAttempts, plus an 8-step baud-rate sweep on final failure), so no extra backoff is needed on top — until it succeeds, at which point it runs the same post-boot setup (device name, auto-reconnect) the normal success path does, then exits. The rest of the system (Wi-Fi, tuner, web UI) never blocks on this and stays fully usable throughout. Verified live: after a forced failure (2 attempts + baud sweep, ~62s), the retry task started immediately, the HTTP server and heartbeat came up normally in parallel, and the retry task began a fresh attempt right away without any pause.

Open going forward: root cause of the intermittent total-silence mode is still not identified — this session's diagnosis exhausted what's possible with a multimeter alone. Real progress would need either an oscilloscope on SYS_CTRL/RESET/crystal across several boots to correlate success/failure with power-on timing jitter, or a large-N automated reboot-cycle statistic (attempted this session via a pyserial script, but the ESP32-S3's native USB-CDC re-enumerating on every hardware reset made a fully unattended multi-cycle script unreliable — a naive read loop silently produced a false "0/5 success" result once across a reconnect window). A future attempt at that statistic needs to detect the USB path disappearing/reappearing and reopen the port, or use a separate hardware UART-to-USB adapter that doesn't disconnect when the target resets.

Also discussed this session (not implemented, for a future hardware revision): whether a different/newer SoC could eliminate the need for the external BT1035 module entirely. Confirmed via web search that Espressif's new ESP32-S31 (RISC-V, announced April 2026) has integrated Bluetooth 5.4 with both LE and Classic (BR/EDR) support — unlike the ESP32-S3 used today, which is BLE-only at the silicon level (confirmed: no Classic BT/A2DP hardware exists on S3, this is not a firmware limitation). An ESP32-S31-WROOM-3 module also exists. This would be a significant main-MCU redesign, not a drop-in swap, and its ESP-IDF support maturity/availability wasn't independently verified this session — worth a dedicated evaluation before committing to it for a future hardware revision.