An open tool for reading, configuring and eventually updating Spektrum Smart ESCs, the Avian family, without the manufacturer's USB dongle.
These controllers speak SRXL2 on their throttle wire: a documented protocol on an ordinary serial port. Everything the proprietary programmer does over that wire can be done over any serial port, including the one already inside an aeroplane.
Configuration works, and has been exercised against a real ESC on a bench through the ESP32 adapter. What the tool itself has done, on hardware:
- the handshake, and the link that follows it
- telemetry decoded and read back: rpm, current, voltage, throttle
- the stick-programming menu walked, settings read, changed and saved
The tool holds the throttle at idle and commands nothing else. It configures an ESC; it does not fly one, and there is no throttle command to look for.
Through a flight controller, on a TBS Lucid H7 Wing running INAV with an Avian on one of its ports: the handshake, telemetry and the whole settings menu read back, and a setting changed, saved, and still there after the ESC was power cycled. A bus at 400000 has not been tried, because that Avian offers only 115200 in its handshake.
What has not been done: updating firmware, which is described below and is still a goal rather than a feature.
An Avian's settings, thrust reverse, brake mode, timing, cutoff, BEC voltage, can be changed from a Spektrum transmitter, or from a programming box you buy, or from a USB dongle you buy. If you own a model that came with one of these ESCs and no Spektrum radio, the only route to its own settings is through hardware sold for the purpose.
The wire does not require any of that.
Both present the same interface, so everything above them is written once.
Through an ESP32. This is the route the bench work used. esp32-bridge/ holds
a sketch that turns a dev board into the electrical half of a flight controller's
SRXL2 port: one pad carrying both directions through the GPIO matrix, released
except while transmitting, with microsecond timestamps on everything received.
That last part is why it exists, because a USB serial adapter can tell you what
arrived but not when.
Through a flight controller. A board running INAV with a port assigned to
Spektrum Smart ESC can hand that port to a PC: MSP_SET_PASSTHROUGH with the
serial function id bridges the USB connection straight to the ESC's wire. The
flight controller's own scheduler stops while it does this, so nothing transmits
over the session, and the port stays in single-wire half duplex.
Two more things the passthrough is meant to do need a fix in INAV to happen over
MSP, iNavFlight/inav#12003: ending the session with +++, and mirroring the
host's baud rate onto the wire every 15 ms, which is what lets the SRXL2
handshake negotiate 115200 up to 400000 straight through it. INAV applies both to
the first port it is given, and the MSP passthrough gave it the ESC's. Without
the fix everything above still works, but the board stays in the passthrough
until it is power cycled, and the wire stays at the rate the port was set to.
Through a flight controller the ESC can stay powered. An Avian that stops hearing its master announces itself about twenty times a second, and opening the passthrough is exactly that, so the tool finds it at once.
Through the ESP32, start the tool first, then power the ESC. An Avian with no master to lose introduces itself in the moment after it gains power and is silent from then on, so anything that begins listening later finds a wire with nothing on it. If a session will not link, cycle the ESC's power with the tool already running.
Leaving the menu restarts the ESC, and it then announces itself for less than
a second. The tool shakes hands with it inside that window, but ending INAV's
passthrough takes two seconds of silence on the wire, and the flight controller
does not always catch the ESC afterwards. So after config, menu, save or
reset through a flight controller the tool asks INAV whether it has the ESC
again, and says so. If it has not, switch the ESC off and on before flying; INAV
will not arm until it sees the ESC.
set leaves the ESC in its menu, where it ignores the throttle while INAV still
counts it as connected. Follow it with save, reset or a power cycle before
flying.
A window:
python -m smart_esc_tool.gui
The interface is PySide6 with Fluent components. The ESC will not hold a link for longer than a quarter of a second, so a worker thread owns the transport and the menu while a person reads the screen, and every value shown is one the ESC reported after the change rather than the value that was asked for.
The same window runs in a browser, at https://mrscothh.github.io/smart-esc-tool/, as a web app that installs like an application and works offline afterwards. It needs Web Serial, so Chrome or Edge on a computer. To run it from the sources:
cd web
npm install
npm run dev # http://localhost:5173, or npm run build for web/dist
The browser only lets a page open ports a person has picked: press Add a port
once per board, and it is remembered. Behind the window a worker owns the port,
as in the desktop application, and keeps feeding the ESC's link between requests
as well; the page shows only what the ESC reported. npm test checks the port
byte for byte against this Python code and drives whole sessions against a
simulated Avian, which ?demo also offers in the page. INAV's SITL is not
reachable from a browser, which has no TCP sockets.
Or a command line, where <port> is the serial port and the route defaults to the
ESP32 adapter:
python -m smart_esc_tool COM5 find # who is on the wire
python -m smart_esc_tool COM5 link # handshake and hold the link
python -m smart_esc_tool COM5 telem # decode what it reports
python -m smart_esc_tool COM5 menu # walk the settings menu
python -m smart_esc_tool COM5 set "Thrust Rev.=REVERSE"
python -m smart_esc_tool COM5 save "Thrust Rev.=REVERSE"
python -m smart_esc_tool COM5 reset # back to factory settings
python -m smart_esc_tool COM5 selftest # checks the ESP32 adapter
python -m smart_esc_tool COM5 find --via inav # through a flight controller
menu and config only read, and leave through the menu's exit entry without
writing anything. save writes and leaves through the save entry. set changes a
setting for the session and stays in the menu, where the ESC ignores the throttle
until it is saved, reset or powered off.
Releases carry a Windows .exe with nothing to install, and the web app as a zip
for anyone who hosts it themselves (over HTTPS or on localhost, as Web Serial
requires).
This is the part the dongle is really sold for, and the design is deliberate: this project will never contain a firmware image.
The manufacturer publishes them. HobbyWing, whose design the Avian is and whose
software the Spektrum USB Link is a rebadge of, serves a manifest at
/Upload/software/app_version_control.xml naming the current database, and that
database is a plain SQLite file whose software table holds one image per
hardware revision and version, keyed to the controller table. It is a public
URL, no credentials of any kind.
So the tool would fetch from the vendor, on demand, the same file their own application fetches, and push the image the connected ESC asks for. Nothing is redistributed and nothing is decrypted: the images are opaque and stay opaque, because delivering one does not require understanding it.
What is missing is the last step. How an image is transferred to the ESC is not published, and cannot be inferred from the database. It is the one piece that needs observing rather than reasoning about.
If you own an SPMXCA200 programmer or a Spektrum USB Link, a capture of its USB traffic during an update would unblock this entirely. That is the single most useful contribution anyone could make to this project.
Until then, firmware updating is a stated goal and not a feature. A failed write bricks an ESC, and there is no recovery path we know of, so the order of work is: read parameters, then write parameters, which a factory reset undoes, then, only against a protocol that is understood rather than guessed, firmware.
- No propeller. Not for any step, including the ones that look harmless.
- The throttle connector's middle pin is a BEC output, 5 V to 8.4 V depending on the model and up to 12 V on a 130 Pro. It powers a receiver; it does not accept power, and it will destroy a 3.3 V input. When wiring an ESP32, use the signal and the ground and leave the middle one alone.
- The ESC runs from its own battery. Ground is common, the supplies are not.
GPL-3.0. The SRXL2 implementation is an independent reading of Spektrum's published specification; no manufacturer code or data is included or distributed.
Spektrum, Avian and HobbyWing are trademarks of their respective owners. This project is not affiliated with, endorsed by, or supported by any of them.