qtalker---/firmware/esp32p4-sensor-node/README.md

# Quantumancy ESP32-P4 Sensor Node

Firmware for the paired hardware sensor node described in
[`docs/superpowers/specs/2026-07-23-esp32-sensor-node-design.md`](../../docs/superpowers/specs/2026-07-23-esp32-sensor-node-design.md)
("Workstream I — firmware, ESP-IDF C — core sensor node"). Real ESP-IDF C
(FreeRTOS-based), not Arduino, not pseudocode. Connects to the seeker's home
Wi-Fi, samples a small set of sensors, and POSTs readings to the Quantumancy
backend's `POST /api/device/telemetry` endpoint, which feeds them into the
séance's live anomaly-detection pipeline as a sixth signal source alongside
`wire`/`evp`/`radio`/`emf`.

## Honesty policy — READ THIS FIRST

> This app's whole ethos is "real signal processing on real data, and it
> says so when something is unverified."

**Nobody working on this had physical ESP32-P4 hardware, a BME280, or an
LD2410 module to flash and test against.** Everything in this directory is
real, structurally-correct ESP-IDF C, written against ESP-IDF's documented
APIs and each sensor's public datasheet/protocol documentation, and reasoned
about carefully — but it has **never been compiled with a real ESP-IDF
toolchain, never been flashed, and never talked to real hardware.** Treat
every claim below as "should work, per the docs" rather than "confirmed
working." See [What's verified vs. not](#whats-verified-vs-not) for the
specific, itemized breakdown — this mirrors the same convention
`frontend/src/lib/sdr.ts`'s `HARDWARE PASS REQUIRED` header comment uses
elsewhere in this repo.

## Directory layout

```
firmware/esp32p4-sensor-node/
├── CMakeLists.txt              top-level ESP-IDF project file
├── sdkconfig.defaults          seed config (idf.py generates the real sdkconfig)
├── README.md                   this file
├── components/                 reserved for Workstream J (RTL-SDR), empty here
└── main/
    ├── CMakeLists.txt          component registration
    ├── app_main.c              entry point / boot sequence
    ├── device_config.h.example template you copy to device_config.h
    ├── wifi_manager.{h,c}      Wi-Fi station mode connect/reconnect
    ├── telemetry_client.{h,c}  HTTP POST task -> /api/device/telemetry
    ├── sensor_driver.h         the sensor_driver_t registry interface
    ├── sensor_registry.{h,c}   the concrete list of compiled-in drivers
    ├── bme280.{h,c}            temperature/humidity/pressure over I2C
    └── ld2410.{h,c}            presence/distance over UART
```

## Build instructions

Requires an ESP-IDF install (v5.3 or newer — ESP32-P4 target support landed
around v5.2/5.3; this was written without a toolchain available to pin an
exact tested version, see honesty section). With `idf.py` on your `PATH`
(e.g. after sourcing ESP-IDF's `export.sh`):

```sh
cd firmware/esp32p4-sensor-node

# 1. Fill in your Wi-Fi + pairing details (see next section) — the build
#    will fail on a missing #include until you do this.
cp main/device_config.h.example main/device_config.h
$EDITOR main/device_config.h

# 2. Target and build.
idf.py set-target esp32p4
idf.py build

# 3. Flash + monitor (adjust the port for your machine).
idf.py -p /dev/ttyUSB0 flash monitor
```

## Manual configuration (no provisioning UI — by design)

A full BLE/Wi-Fi-AP provisioning flow is explicitly out of scope for this
spec (see the spec's "Explicitly out of scope" section). Instead, you
hand-edit one header before building:

1. In the Quantumancy web app, sign in and create a device from your
   account (name + optional sensor-type hint). The backend shows you a
   **raw pairing token exactly once** — copy it immediately, it cannot be
   retrieved again (same one-time-secret convention as the site's session
   tokens).
2. `cp main/device_config.h.example main/device_config.h`
3. Edit `main/device_config.h` and fill in:
   - `DEVICE_WIFI_SSID` / `DEVICE_WIFI_PASSWORD` — your home Wi-Fi.
   - `DEVICE_BACKEND_BASE_URL` — the backend's base URL, no trailing slash.
   - `DEVICE_PAIRING_TOKEN` — the raw token from step 1.
   - `DEVICE_REPORT_INTERVAL_SEC` — optional, defaults to 15s.
4. `main/device_config.h` is listed in `.gitignore` — it will never be
   committed. Never put real credentials in `device_config.h.example`
   itself; that file is the template everyone else copies.

There is deliberately no other config path (no NVS-based captive portal, no
BLE provisioning) in this build — see the spec's scope boundary.

## What's verified vs. not

**Structurally verified** (reasoned through carefully against ESP-IDF's
documented API surface and each sensor's public protocol docs; internally
consistent; no known syntax errors or obviously-wrong API usage):

- Project skeleton (`CMakeLists.txt` × 2, `sdkconfig.defaults`,
  `idf_component_register` call) follows ESP-IDF's standard project layout.
- Wi-Fi station-mode connect/reconnect (`wifi_manager.c`) follows ESP-IDF's
  documented event-driven pattern (`WIFI_EVENT`/`IP_EVENT` handlers +
  `EventGroupHandle_t`), extended with an exponential-backoff reconnect
  timer instead of giving up after N tries.
- HTTP client (`telemetry_client.c`) builds the exact JSON shape the spec's
  contract defines and POSTs it via `esp_http_client` with
  `Authorization: Bearer <token>` and `Content-Type: application/json`.
- BME280 driver (`bme280.c`): register map and the double-precision
  compensation formulas are transcribed from Bosch's public BME280
  datasheet (rev 1.23, §4.2.2–4.2.3) — this is well-trodden, publicly
  documented territory, and the formulas are checkable line-by-line against
  the datasheet. Uses ESP-IDF's newer `driver/i2c_master.h` API (the
  current idiomatic choice; the older `driver/i2c.h` is being phased out).
- LD2410 driver (`ld2410.c`): UART frame envelope (header/footer magic
  bytes, length-prefixed payload) follows the shape consistently reported
  across public LD2410 protocol write-ups. **The exact payload byte offsets
  for target state / distances / energies are the single least-certain
  piece of code in this entire firmware** — see the detailed note in
  `ld2410_parse_payload()`. The driver defends itself with a head/tail
  marker sanity check (`0xAA`/`0x55`) and silently skips anything that
  doesn't match rather than reporting garbage, but that check catches
  gross corruption, not subtle off-by-one offset errors.
- Sensor driver registry (`sensor_driver.h`, `sensor_registry.c`): a
  `sensor_driver_t { name, init, read }` struct, a compile-time array of
  them, and generic init/collect functions that `app_main.c` and
  `telemetry_client.c` call without knowing which concrete sensors exist.

**NOT verified — requires real hardware bring-up:**

- `idf.py build` has never actually been run in this environment (no
  ESP-IDF toolchain installed here) — there could be a typo, a missing
  include, or an API signature mismatch against whatever exact ESP-IDF
  version you build with that only a real compile will surface.
- I2C timing/electricals: pull-up resistor values, bus speed headroom,
  cable length — none of this has been bench-tested.
- BME280 compensation formula correctness in practice: the math is
  transcribed carefully, but "matches the datasheet" and "produces a
  plausible number when this exact C runs on this exact silicon" are
  different claims until someone compares a real reading to a reference
  thermometer/barometer.
- LD2410 frame parsing, as above — verify against a logic analyzer capture
  or a known-good reference implementation (e.g. the `ncmreynolds/ld2410`
  or `iavorvel/MyLD2410` Arduino libraries, cross-checked) before trusting
  field values.
- Wi-Fi reconnect behavior under real-world conditions (router reboot,
  weak signal, captive portals) — the backoff logic is reasoned about, not
  soak-tested.
- HTTP client behavior against the real backend: TLS handshake against
  its actual certificate, real latency, real error responses. The
  `sdkconfig.defaults` enables mbedTLS's full certificate bundle for this,
  but that's untested against the live deploy.
- Timing/power: task stack sizes (`telemetry_task`'s 8192 words, etc.) are
  reasonable guesses, not measured high-water-marks from a real run.
- **The ESP32-P4-has-no-integrated-Wi-Fi caveat below** — this is a real
  hardware architecture question, not just an untested detail.

## Important hardware caveat: ESP32-P4 has no integrated Wi-Fi radio

The ESP32-P4 SoC (per Espressif's own published specs) has **no built-in
2.4GHz radio**. A real deployment needs one of:

- **A companion Wi-Fi chip** (e.g. ESP32-C6) wired to the P4 via SDIO or
  SPI, running Espressif's "esp-hosted" firmware/driver stack. Critically,
  esp-hosted presents the *same* `esp_wifi`/`esp_netif` API this firmware
  already uses — so `wifi_manager.c` should not need to change, only board
  wiring and `sdkconfig` (host-side esp-hosted config) would.
- **Building this same code against a Wi-Fi-native target instead**, e.g.
  `idf.py set-target esp32s3` or `esp32c6`. The application code
  (`wifi_manager.c`, `telemetry_client.c`, the sensor drivers) is written
  against the standard API surface and doesn't reference P4-specific
  peripherals for anything except I2C/UART GPIO numbers, so it should be
  largely target-portable.

This wasn't in the original spec's framing but matters enough for a real
build that it's called out here explicitly, in the honesty-policy spirit —
better to flag a real hardware-architecture gap than let someone discover
it after ordering a bare P4 dev board expecting it to just join Wi-Fi.

## Wiring / pinout

### BME280 (I2C) — temperature, humidity, pressure

Chosen as the concrete default sensor per the spec ("a common,
well-documented sensor... pick this as the concrete default since no
specific part number was given").

| BME280 pin | Connects to                          |
|------------|---------------------------------------|
| VCC        | 3V3                                    |
| GND        | GND                                    |
| SDA        | GPIO8 (`BME280_I2C_SDA_GPIO`)          |
| SCL        | GPIO9 (`BME280_I2C_SCL_GPIO`)          |
| CSB        | VCC (selects I2C mode, not SPI)        |
| SDO        | GND → I2C address `0x76` (default assumed; tie to VCC + change `BME280_I2C_ADDR` for `0x77`) |

GPIO numbers are `#define`s at the top of `bme280.h` — override them there
(or via a future `idf.py menuconfig` entry) to match your actual wiring.
100kHz I2C clock by default (`BME280_I2C_CLK_HZ`); the part supports faster
modes if your wiring/pull-ups support it.

### LD2410 (UART) — presence, distance, motion

**Chosen over a plain PIR** — see the rationale in `ld2410.h`'s header
comment: the LD2410 reports moving-target and stationary-target distance
and energy separately, not just a boolean, which is richer signal for the
anomaly pipeline and better matches this app's "believable" ethos (it can
distinguish "someone crossed the room" from "the sitter shifted in their
chair" in a way a boolean PIR cannot). The tradeoff is a materially more
complex protocol than a PIR's single GPIO pin — see the honesty note in
[What's verified vs. not](#whats-verified-vs-not) about the LD2410 frame
parser being the least-certain code in this firmware. If you'd rather start
with a boolean PIR for a faster, more certain first bring-up, it fits the
same `sensor_driver_t` interface — see
[Adding a new sensor](#adding-a-new-sensor) below.

| LD2410 pin | Connects to                          |
|------------|----------------------------------------|
| VCC        | 5V (sensor front-end runs at 5V; confirm your board revision's UART logic level before wiring directly to a 3.3V-only UART pin) |
| GND        | GND                                     |
| TX         | GPIO17 (`LD2410_UART_RX_GPIO`, ESP32 RX) |
| RX         | GPIO18 (`LD2410_UART_TX_GPIO`, ESP32 TX) |

Default UART settings: 256000 baud, 8N1 (module factory default), reporting
in "basic" (non-engineering) mode. GPIO numbers and baud rate are
`#define`s at the top of `ld2410.h`.

## Sensor driver registry — the extensibility pattern

`sensor_driver.h` defines:

```c
typedef struct {
    char sensor_type[SENSOR_READING_TYPE_MAXLEN];
    double value;
    char unit[SENSOR_READING_UNIT_MAXLEN];
    cJSON *metadata; // nullable; NULL serializes as {}
} sensor_reading_t;

typedef struct sensor_driver {
    const char *name;
    esp_err_t (*init)(void);
    esp_err_t (*read)(sensor_reading_t *out, size_t max_out, size_t *out_count);
} sensor_driver_t;
```

`sensor_registry.c` holds a compile-time array of these (currently BME280
and LD2410) and two generic functions, `sensor_registry_init_all()` and
`sensor_registry_collect()`, that `app_main.c` and `telemetry_client.c`
call without ever referencing `bme280.c`/`ld2410.c` directly. One driver
failing `init()` or `read()` is logged and skipped — it doesn't take the
whole node offline.

### Adding a new sensor

1. Write `main/my_sensor.h` / `main/my_sensor.c` implementing `init()` and
   `read()` matching `sensor_driver_t`'s function pointer signatures.
2. Add `"my_sensor.c"` to the `SRCS` list in `main/CMakeLists.txt`.
3. `#include "my_sensor.h"` in `sensor_registry.c` and add one line to the
   `s_drivers[]` array:
   ```c
   { .name = "my_sensor", .init = my_sensor_init, .read = my_sensor_read },
   ```

Nothing in `app_main.c`, `telemetry_client.c`, or the main reporting loop's
control flow needs to change — that's the whole point of this structure per
the spec.

## Backend contract this firmware targets

From the spec (binding, see the spec file for the authoritative version):

```json
POST /api/device/telemetry
Authorization: Bearer <raw pairing token>
Content-Type: application/json

{
  "readings": [
    {"sensor_type": "presence", "value": 1, "unit": "bool", "metadata": {}},
    {"sensor_type": "temperature", "value": 21.4, "unit": "c", "metadata": {}},
    {"sensor_type": "humidity", "value": 47.2, "unit": "pct", "metadata": {}},
    {"sensor_type": "pressure", "value": 1013.2, "unit": "hpa", "metadata": {}}
  ]
}
```

This firmware's BME280 driver emits `temperature`/`humidity`/`pressure`
exactly as shown; its LD2410 driver emits `presence` as a `0`/`1` boolean
in `value` with the richer distance/energy data folded into `metadata`
(`moving_distance_cm`, `moving_energy`, `stationary_distance_cm`,
`stationary_energy`, `detection_distance_cm`, `target_state`).

## Out of scope here

Per the spec: thermal camera support, a full BLE/Wi-Fi-AP provisioning UX,
on-device spectrum analysis/FFT (the RTL-SDR module below forwards raw IQ
upstream rather than analyzing on-device), and anything on the
backend/frontend side (Workstreams G, H, K).

---

## Workstream J — RTL-SDR experimental module

**⚠️ This is the least certain part of the entire firmware build. Read
this whole section before touching it.**

### What this is

A clearly-separated, opt-in, experimental module exploring USB-host
communication with an RTL2832U-based SDR dongle over the ESP32-P4's
native USB-OTG host controller (USB Host Library, `usb_host.h` — a real,
documented capability of this specific chip, unlike most ESP32 variants).
The idea, per the design spec: attach a cheap RTL-SDR dongle to the
sensor node, treat "spirit radio scanning" as a hardware-backed mode
instead of only a browser WebUSB feature.

**Be honest about the real constraint (this is the spec's framing, and
it's correct):** wideband IQ sample rates and FFT processing are
demanding relative to an MCU's compute, even one with ESP32-P4's AI
accelerator. **On-device spectrum analysis is not what this module
attempts.** The realistic architecture — and the one implemented here —
is: pull raw IQ samples off the dongle via USB host, and forward them
upstream for the *backend* to FFT/analyze (the same job the browser
already does client-side today via `frontend/src/lib/fft.ts` +
`frontend/src/lib/sdr.ts`'s `SpectrumAnomalyDetector`). Even that
"just pass the bytes through" architecture needs real USB throughput
numbers to know if it's viable — see "What's unverified" below.

### Primary reference

`frontend/src/lib/sdr.ts` — this repo's existing browser-based (WebUSB)
RTL2832U + R820T driver. It's real, already-researched protocol detail
against the public librtlsdr register documentation (vendor commands,
I2C-repeater tuner access, the demod/tuner init sequence), itself marked
`HARDWARE PASS REQUIRED` since it's never been run against a real dongle
either. This firmware module is a direct **port** of that file's control-
transfer sequence from WebUSB JS calls to ESP-IDF USB Host Library C
calls — line-by-line correspondences are called out in code comments
(e.g. `rtlsdr_run_init_sequence()` mirrors `open()` in sdr.ts almost
register-for-register). It intentionally does not re-derive any register
math from scratch; wherever sdr.ts says "simplified" or "HARDWARE PASS
REQUIRED" (e.g. the R820T PLL frequency math, the demod resample-ratio
math), this module carries the exact same simplification forward with the
exact same caveat, rather than inventing new unverified math on top of
already-unverified math.

### What's implemented (structurally — see caveats below)

1. **USB Host Library lifecycle** (`rtlsdr_exp_start()` /
   `rtlsdr_usb_lib_daemon_task()` / `rtlsdr_exp_client_task()`):
   `usb_host_install()`, `usb_host_client_register()` with an async event
   callback, and the two-task pump pattern the USB Host Library's async
   model requires (one task for `usb_host_lib_handle_events()`, one for
   `usb_host_client_handle_events()` — the latter also being how this
   module's own control- and bulk-transfer completion callbacks get
   dispatched, since the Library calls them synchronously from whichever
   task is pumping events, not from a hidden thread or ISR).
2. **RTL2832U/Terratec device enumeration and vendor-ID matching**
   (`rtlsdr_try_bring_up()`, `rtlsdr_vendor_id_matches()`): on a
   `USB_HOST_CLIENT_EVENT_NEW_DEV` event, opens the device, reads its
   device descriptor, and matches `idVendor` against `0x0BDA`
   (RTL2832U) / `0x0CCD` (Terratec-rebadged) — ported directly from
   `sdr.ts`'s `requestDevice()` filter list (`RTL2832U_VENDOR`,
   `TERRATEC_VENDOR`). Faithfully carries over that file's specific
   choice to filter by **vendor ID only**, not product ID (its comment:
   "many dongles report product ids outside the handful we know, so
   filtering by productId hides them from the picker") — the known
   product-id list (`RTLSDR_EXP_KNOWN_PRODUCT_IDS`) is kept as an
   informational log line only, never a hard filter, exactly mirroring
   how `RTL2832U_PRODUCTS` is exported-but-unused-as-a-filter in
   `sdr.ts`.
3. **RTL2832U init vendor-command sequence** over USB control transfers
   (`rtlsdr_run_init_sequence()`, `rtlsdr_demod_write()`,
   `rtlsdr_reg_write()`, `rtlsdr_i2c_write()`, `rtlsdr_exp_set_frequency()`,
   `rtlsdr_exp_set_sample_rate()`): the same demod soft-reset → demod_ctl/
   suspend-off block → standby-off → AGC-mode → R820T tuner power-up
   (through the I2C repeater) → sample-rate program → initial tune →
   streaming-endpoint reset → test-mode-off sequence as `sdr.ts`'s
   `open()`, with the same register addresses/values and the same
   `wValue`/`wIndex` encoding (`(block<<8)|0x10` / `(page<<8)|address`),
   translated from `USBDevice.controlTransferOut()` to
   `usb_host_transfer_submit_control()` with a manually-built
   `usb_setup_packet_t`.
4. **A basic bulk-transfer read loop structure** for pulling IQ sample
   data off the device (`rtlsdr_start_bulk_streaming()`,
   `rtlsdr_bulk_xfer_cb()`): unlike `sdr.ts` (which does one-shot
   `await dev.transferIn(...)` calls from inside its own async `sweep()`
   loop), the USB Host Library is callback-driven, so continuous
   streaming here is a small pipeline — N transfers (`bulk_read_queue_
   depth`, default 4) of a fixed chunk size (`bulk_read_chunk_bytes`,
   default 16384B, enforced as a multiple of 512 the same way `sdr.ts`'s
   `readSamples()` comment requires) are kept perpetually in flight; each
   completion callback copies the received bytes into a heap block, hands
   it to a FreeRTOS queue for the forward task, and immediately resubmits
   itself to keep the pipe full. **This is the part most likely to need
   real-hardware tuning** — see below.
5. **A stub/structure for forwarding raw IQ data upstream**
   (`rtlsdr_exp_forward_iq_block()`, `rtlsdr_forward_task()`) — see the
   architecture decision writeup immediately below.

### IQ-forwarding architecture decision

The spec explicitly leaves this as a product-judgment call ("your call on
whether this warrants a separate endpoint/stream vs. reusing the sensor
telemetry shape with a binary/base64 payload"). Decision made here:
**a separate binary stream/endpoint, not a reuse of the JSON telemetry
shape.** Reasoning:

- **Raw throughput is the wrong order of magnitude for the telemetry
  contract.** Even a modest 2.048 Msps capture at 8 bits/sample/channel
  (RTL2832U's native ADC format) is interleaved I/Q bytes at roughly
  4.1 MB/s. The design spec's `POST /api/device/telemetry` contract caps
  the `readings` array at ~64 entries, caps the request body at ~16KB,
  and rate-limits ingestion to roughly 1 request/second sustained per
  device — sized for a handful of scalar sensor readings (temperature,
  humidity, presence booleans), not a continuous multi-megabyte/second
  binary stream. Forcing IQ data through that shape would mean either
  violating those caps (defeating their whole purpose — bounding what a
  misbehaving/malicious device can push at the backend) or chopping IQ
  into thousands of tiny requests per second, which is worse for both
  sides than one continuous stream.
- **Base64-in-JSON adds ~33% size overhead** on top of a payload that's
  already too large for the telemetry shape, for no benefit — there's no
  reason to pay text-encoding tax on a payload nothing needs to
  eyeball as text.
- **It's a fundamentally different kind of data with different backend
  handling needs.** Telemetry readings feed the anomaly-detection
  baseline/threshold pipeline directly and cheaply (a few floats per
  sensor). IQ data needs FFT processing before it's useful for anything
  — an entirely different backend code path (closer to how the browser's
  own `powerSpectrumDb()` + `SpectrumAnomalyDetector` work today,
  server-side instead of client-side). Conflating the two request shapes
  would couple two things that should scale, rate-limit, and fail
  independently.

Given that, this module's stub (`rtlsdr_exp_forward_iq_block()`) targets
a **separate, currently-hypothetical** endpoint (`iq_upload_url` in
`rtlsdr_exp_config_t`, suggested path `/api/device/iq-stream` in code
comments) carrying a small fixed binary header (magic, sequence number,
center frequency, sample rate, payload length — see
`rtlsdr_iq_chunk_header_t`) followed by the raw IQ bytes, POSTed as
`application/octet-stream` with the same `Authorization: Bearer <device
token>` auth as the regular telemetry loop. **This endpoint does not
exist anywhere in this repo.** Building it is explicitly out of scope for
this workstream (it's backend work, not firmware, and the spec doesn't
assign a backend workstream to receive IQ data at all — only to receive
scalar telemetry). The stub is inert by default (`rtlsdr_exp_forward_
iq_block()` returns `ESP_OK` immediately unless `iq_upload_url` is
configured) specifically so this module can be compiled/enabled for its
USB-host/bring-up behavior without requiring a backend that doesn't
exist.

A real product decision here — genuinely open, not resolved by this
workstream — is whether a POST-per-chunk model is even right versus a
persistent WebSocket stream (the spec's `/ws/device-feed` design already
establishes a WS pub/sub pattern on the backend for telemetry; a
`/ws/device-iq` sibling might fit that architecture better than repeated
HTTP POSTs, especially if backpressure/flow-control matters, which for a
continuous stream it very much does). That's flagged here rather than
silently decided, because it depends on backend design judgment as much
as firmware judgment.

### What's unverified — an honest, specific list for real hardware bring-up

Nothing below has run against real hardware. In rough order of "most
likely to break first":

1. **USB Host Library API surface.** Function names, struct field names,
   and callback signatures (`usb_host_client_config_t`'s `.async.
   client_event_callback` shape in particular — ESP-IDF has changed this
   API's shape across versions) are written from documented/remembered
   API shape, not checked against a real ESP-IDF checkout (none available
   in this environment). **First thing a real bring-up needs: does this
   even compile against the ESP-IDF version Workstream I's project
   targets?**
2. **RTL2832U enumeration over a real ESP32-P4 USB-OTG host port** — does
   `usb_host_device_open()` / `usb_host_get_device_descriptor()` actually
   see the dongle at all on this specific chip's USB-OTG controller (power
   delivery to the dongle over USB-host mode is itself a hardware
   question — does the ESP32-P4 dev board supply VBUS in host mode, does
   the dongle draw more current than it can supply).
3. **The init vendor-command sequence itself** — carried over unchanged
   from `sdr.ts`, which is *itself* unverified. Two layers of "reasoned,
   not tested." A logic analyzer / USB protocol analyzer trace against a
   real dongle (or cross-checking against real librtlsdr `-T` verbose
   output) is needed to confirm register values, not just transfer
   plumbing.
4. **R820T PLL frequency math and demod resample-ratio math** — both are
   the same simplified integer-N approximation `sdr.ts` uses (its own
   comment: "real librtlsdr computes the exact sdm/vco from a 28.8MHz
   crystal reference. Simplified... Marked for hardware."). Likely wrong
   or imprecise until replaced with the real librtlsdr formula and
   checked against an actual received signal.
5. **Bulk transfer chunk size, pipeline depth, and timeout=0 choice**
   (`RTLSDR_EXP_BULK_CHUNK_BYTES`/`RTLSDR_EXP_BULK_QUEUE_DEPTH` Kconfig,
   defaults 16384B / depth 4) — these are guesses. Real ESP32-P4 USB
   Host Library heap/DMA limits, achievable sustained throughput at
   2.048 Msps (~4.1 MB/s), and whether `timeout_ms = 0` (no timeout, i.e.
   "block until data or disconnect") is even the right transfer mode for
   this endpoint all need real measurement.
6. **Drop-on-full backpressure policy** (`iq_blocks_dropped` stat) — is
   silently dropping IQ blocks when the forward task falls behind
   acceptable, or does it need real flow control (e.g. throttling the
   bulk read rate itself)? Depends on (5) and on real WiFi uplink
   bandwidth from a real device on a real seeker's home network.
7. **The proposed IQ-forwarding wire format and endpoint** — entirely
   hypothetical (see architecture section above); no backend exists to
   validate the framing against, and the POST-vs-WebSocket question above
   is unresolved.
8. **Concurrent USB Host Library ownership with the rest of the firmware
   project.** `rtlsdr_exp_start()`/`rtlsdr_exp_stop()` call `usb_host_
   install()`/`usb_host_uninstall()` directly, assuming this module is the
   *only* USB Host client in the project. If Workstream I's skeleton (or
   anything else) also needs USB host for something, this needs to change
   to a shared-ownership model (install once, both modules register as
   clients) — impossible to resolve without seeing that code, flagged
   here for whoever does the merge.
9. **Memory footprint.** Multiple in-flight 16KB bulk transfer buffers
   plus a forward queue plus `esp_http_client` buffers, alongside whatever
   Workstream I's WiFi/HTTP/BME280/presence stack already needs, on a
   single ESP32-P4's RAM — not sized or measured against a real linker
   map.

### Build / integration notes

This component is **off by default** (`RTLSDR_EXP_ENABLE` Kconfig option,
default `n`) so it cannot affect Workstream I's core build unless
explicitly turned on via `idf.py menuconfig` → "RTL-SDR Experimental
Module." To include it in a real project once Workstream I's skeleton
exists:

1. Add this directory's `components/` to the consuming project's
   `EXTRA_COMPONENT_DIRS` in the top-level `CMakeLists.txt` (or copy
   `components/rtlsdr_experimental/` into that project's own
   `components/`).
2. From wherever `app_main()` sets up its other sensor drivers, e.g.:
   ```c
   #include "rtlsdr_experimental.h"

   rtlsdr_exp_config_t sdr_cfg;
   rtlsdr_exp_config_default(&sdr_cfg);
   sdr_cfg.iq_upload_url = NULL; /* leave unset until a real backend
                                     endpoint exists — see README */
   sdr_cfg.device_bearer_token = DEVICE_BEARER_TOKEN; /* from
                                     main/device_config.h, same token
                                     used for the telemetry POST loop */
   rtlsdr_exp_handle_t sdr_handle;
   if (rtlsdr_exp_init(&sdr_cfg, &sdr_handle) == ESP_OK) {
       rtlsdr_exp_start(sdr_handle); /* non-blocking; watches for a dongle */
   }
   ```
3. `idf.py build` — **not run in this environment** (no ESP-IDF
   toolchain available); this module has only been reasoned about, not
   compiled. Treat "compiles" as an open question for the next person
   with a real toolchain, not a claim made here.

Verified in this environment: none of it, against hardware or a real
compiler. What *has* been done: careful structural translation of a
real, already-partially-researched protocol (`sdr.ts`) to the documented
shape of a real, chip-specific API (ESP32-P4's USB Host Library), with
every simplification and open question called out rather than hidden.