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User Defines Explained

With more features being added consistently, ./src/user_defines.txt has gotten complicated 🤷‍♂️. So we will break it down! 🔨


This is the full list of currently supported User Defines and would help you should you intend to compile the firmware using the Toolchain or manual mode on the ExpressLRS Configurator.

Defines 101

  • If these are used in Configurator Manual Mode or user_defines.txt, the value must begin with -D. Example: NO_SYNC_ON_ARM would be -DNO_SYNC_ON_ARM.
  • A user define that begins with # is "commented out", i.e. not active.

Binding Phrase

MY_BINDING_PHRASE="default ExpressLRS binding phrase"
This step is simple but important. Both the TX and RX NEED to have the same binding phrase or ExpressLRS WILL NOT WORK. Anyone using the same binding phrase as you will be able to control your model, so be unique. Set something memorable, and limit to alphanumeric phrases conforming to the Latin alphabet*. Receivers flashed with firmware builds that do not have binding phrase enabled will support and require the traditional binding method. 📜

This feature can, but should not be used as a model match feature (to lock a single specific transmitter to a single specific receiver). For that use, the Model Match option.

* This phrase gets md5 hashed and gets built into the binary you will be flashing.

Regulatory Domain

This is a relatively simple one - enable whatever regulatory domain you are in. EU 868 🇪🇺 is compliant to the frequency but is not LBT compliant 👂 . Every other band is near compliant 👿 but may not be fully compliant for your regulatory domain.

It makes the TX module send the telemetry data to the OpenTX every 320 ms by default. This stops the telemetry lost warnings when running a high telemetry ratio, or low rates like 50hz.

Default value is 320LU. If you want to change that you have to suffix your milliseconds value with LU. For example, in order to specify 100 ms telemetry update rate you have to enter it like this: 100LU.

Typically, you want to keep 320LU value for OpenTX based radios, and 100LU for ErskyTx ones.

Output Power Limit

By default the max power of hardware is limited to what it can safely output without extra cooling. Some hardware supports increasing the power by enabling the following option. Check the supported hardware page to see if this is available and what cooling modifications can be made. By enabling this, you are risking perminant damage to your hardware, sometimes even when you add extra cooling. For example, R9M modules will burn out without cooling.


Performance Options

no sync on arm doesn't transmit sync packets while armed. This is useful for racing as there is less time & packets wasted 🗑️ on sending sync packets (one packet every 5 seconds if connected). HOWEVER if you are doing serious long range ⛰️, keep this disabled because in the case of a sustained failsafe, link can not be regained while armed.

AUX1 is the channel ExpressLRS uses to detect "ARMED", and this feature assumes that a low value of the arm switch is disarmed, and a high value is armed. OpenTX can invert your switch if you prefer it to be mechanically inverted. It is best not to enable no sync on arm when you are first setting up ExpressLRS as it can be a source of confusion.

RF Mode Locking - When the RX is waiting for a connection, it cycles through all available rates waiting for a connection on each one. By default, ExpressLRS will go back to this mode after a disconnect (failsafe). If LOCK_ON_FIRST_CONNECTION is used, ELRS will not cycle after a disconnect, but instead just stay on whatever rate the last connection was. This makes connection re-establishment quick, because the RX is always listening at the proper rate. This is generally what everyone wants, but there is utility in being able to switch the TX to the lowest rate to get more range to re-establish a link with a downed model, which can't happen if the RX is locked at the previous rate.

When cycling through the rates, the RX starts with the fastest packet rate and works down to the slowest, then repeats. It waits PACKET_INTERVAL * PACKS_PER_HOP * HOP_COUNT * 1.1 at each rate. Example: 4ms * 4 * 80 * 1.1 = 1.408s for 250Hz. The duration is extended 10x if a valid packet is received during that time. Even with LOCK_ON_FIRST_CONNECTION, the rate can be changed by changing the TX rate using ELRS.lua while connected, or by power cycling the RX.

Enable antenna-switching diversity for RX that support it. Safe to leave on for hardware that doesn't have diversity except DIY builds which did not populate the RF switch.

Change the RSSI thresholds used by the Dynamic Power algorithm. If the RSSI moving average is below DYNPOWER_THRESH_UP dBm from the sensitivity limit, the algorithm will increase the power output by one step. Similarly, if the RSSI is above DYNPOWER_THRESH_DN from the sensitivity limit, the power will be decreased one step.

For TX devices with fans, FAN_MIN_RUNTIME keeps the fan running even after the power level has dropped below the configured Fan Threshold. This prevents the fan from turning on and off every few seconds if the power level is constantly changing. Default is 30 seconds if not defined, value can be 0-254. There is always a short delay before the fan is activated, which can not be disabled.

Compatability Options

This only works with ESP32 based TXes (will not work with modules without built-in inversion/uninversion), but enables compatibility with radios that output inverted CRSF, such as the FrSky QX7, TBS Tango 2, RadioMaster TX16S. You want to keep this enabled in most of the cases. If your radio is T8SG V2 or you use Deviation firmware turn this setting off.

This only works with ESP8266/ESP8285 based RXes. Invert the TX pin in the receiver code to allow an inverted RX pin on the flight controller to be used (usually labeled SBUS input or RXI). Inverted CRSF output. RX pin (telemetry) is unaffected. Update via_BetaflightPassthrough will not work, only via_Wifi. Note that just because this description includes the word SBUS, it doesn't mean the RX will output SBUS. It is still CRSF protocol, only inverted, so CRSF should still be the receiver protocol selected in the flight controller software.

Use a custom baud rate on the receiver instead of the default 420000 baud. This is useful for a KISS v1 FC (which runs at 400000) or any other oddball baud, like 115200 for interfacing with an Arduino.

This does not turn on SBUS protocol it simply changes the pin used for communication from those two side pins (A9 and A10) to use the pin labeled "SBUS" on the RX, which is inverted. This is useful for F4 FCs which only have an inverted receiver input UART RX. 🔼. This is only one way, so you lose the telemetry downlink to your radio as well as passthrough flashing. Enabling this turns on CRSF protocol output on the S.BUS 🚌 pin on your R9MM/R9Mini. set serialrx_inverted = ON may also be needed within Betaflight 🐝 for compatibility

Network Options

⚠️ Must be defined if you plan to update your RX over wifi without using a button on the RX ⚠️ This will automatically turn the wifi 📶 on for any module that has an ESP8285 on it if no TX connection is established after N seconds from boot (the 30 is the time). This enables pushing firmware updates to the RX by connecting to its wifi network and visiting


These options set Home Network Access for your Wifi-enabled hardware. With these set, the devices will try connecting to your existing WiFi Network when you click on "(Wifi) Update" on the ExpressLRS Lua script (for some Tx Modules) or automatically after your set interval time. Once the devices connect to your Home WiFi, the Update page can be accessed anywhere, from any device on the same network. Tx Module Wifi update page can be reached using the address http://elrs_tx.local, while receivers' update page can be reached via http://elrs_rx.local.

Wifi mode will first try to connect to the network specified before falling back and creating a new wifi network. The Home Network can also be modified from the webui.

Thermal Options

New in version 2.1

For TX devices with fans, FAN_MIN_RUNTIME keeps the fan running even after the power level has dropped below the configured fan threshold. This prevents the fan from turning on and off every few seconds if the power level is constantly changing. Default is 30 seconds if not defined, value can be 0-254 (in seconds).


Other Options

MY_STARTUP_MELODY="<music string>|<bpm>|<semitone offset>" -or-
MY_STARTUP_MELODY="<rtttl string>"
For TXes like the R9M, this sets if the TX only beeps one-time versus playing a startup song. Currently, it is set to play the startup song 🎼 , but if you don't prefer it, uncomment this to turn it off. ✖️

For all your customization needs, use DMY_STARTUP_MELODY to define your own startup melody using the BlHeli32 or RTTTL syntax. For BLHeli32, the parameters music string and bpm are required, whereas semitone offset is optional to transpose the entire melody up or down by the defined amount of semitones.

Example BlHeli32 melodies are available on Rox Wolfs youtube channel, some experimentation may be required though. 🎵 To write your own melody, this (Sheet Music 101) and this (BLHeli Piano) are useful resources.

The build process also supports RTTTL-formatted ringtone strings. RTTTL melodies are delimited by colons : and start with a description versus the BLHeli style with have pipes |. e.g. Mario:d=4,o=5,b=100:32p,16e6,16e6,16p,16e6,16p,16c6,16e6,16p,16g6,8p,16p,16g

Disables beep sequence at startup of TX, but the TX will still beep when CRSF connection is acquired

Disables all TX buzzer beeps at any state

Enables code for talking to a connected ESP8266 backpack on the TX module, and associated Lua params. The device target should enable this automatically for devices that come with this built-in, but can be added to any device. The TX backpack allows wireless integration with VRx modules and planned telemetry mirroring over wifi.

Obsolete user_defines

See Obsolete user_defines