PWM Receivers

ExpressLRS now supports direct PWM output from receivers. This documentation refers to only receivers with native PWM output, not receivers attached to external CRSF to PWM converters such as the CRServoF or Matek CRSF-PWM-C.

Channel Mapping and Failsafe¶

The default channel mapping is straight through: CH1 from the TX goes to PWM Output 1, CH2 to Output 2, etc. To change this on ESP-based receivers, allow the receiver to go into WiFi mode then use the WebUI to configure the mapping. Any input channel can be mapped to any output channel, and the same input can be used for as many outputs as desired. AUX1/CH5 is always 1-bit, so you'll likely want to change this mapping to a channel with more resolution.

Failsafe values are set using this UI as well with values that can range from 988us to 2012us. Failsafe is entered if the receiver is connected and Link Quality (LQ) drops to 0, or 1 second has passed without a valid channels packet being received, whichever comes first. On startup, no pulses are generated until a transmitter connects, allowing ESC throttle calibration with the standard "raise the throttle before connecting" method. The default failsafe value is 1500us for all channels except Output 3, which defaults to 988us.

Channel Resolution¶

PWM output is still subject to the resolution of the ELRS protocol, which means by default there are still only 4x full resolution channels (10-bit CH1-CH4) and 8x switch channels (CH5-CH12). For the best resolution on the switch channels, use Switch Mode: Wide and a TLM Ratio of 1:8 to 1:256 for 7-bit (128 pos) switch channel resolution. Higher TLM Ratios (1:2 and 1:4) are reduced to 6-bit (64 pos) resolution. Remember that switch channels are sent one per packet in Wide mode, taking 8 packets to send all 7 channels (e.g. 150Hz mode 1:64 = 18.657Hz updates to CH6-CH12). AUX1/CH5 is sent in every packet in all switch modes but is only 1-bit (2-pos). See Switch Configs for more information.

Supported Output Modes¶

ELRS receivers support the following PWM output modes:

• PWM output frequencies: 50Hz, 60Hz, 100Hz, 160Hz, 333Hz, 400Hz
• Normal pulse width (988-2012us - center 1500us), extended pulse width (885-2115us - center 1500us), and also half pulse width servos (494-1006us - center 750us)
• 10kHz Duty Cycle 0-100% PWM (e.g. for driving a brushed motor FET)

On top of this, outputs can also be set to:

• Binary On/Off (High/Low signal output)
• DSHOT300 (for driving brushless motor ESCs; ESP32-based receivers only)

Serial Output¶

PWM receivers can also output any supported serial protocol, such as CRSF or SBUS. Select the desired output protocol using the ELRS lua, or on the Model tab in the receiver's WebUI. The default pins used for serial output vary by receiver. If your receiver has a dedicated serial port (e.g. RadioMaster ER6, ER8, ER8G(V)), serial output will be over this port. Otherwise, check the Model tab in the receiver's WebUI to see which pins can be mapped to Serial TX and RX (usually Ch2 and Ch3).