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33.3.1 RSSI Embedded in Collective PPM/Total Signal Channel
33.3.2 Analog Voltage RSSI Feeds to a Dedicated Pin
33.3.3 PWM Type RSSI Feed to a Dedicated Pin
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The following is an example of a typical RC receiver RSSI connection scheme:
33.3.1 RSSI Embedded in Collective PPM/Total Signal Channel
It is common practice to use a cPPM or sBus type sum signal on one signal line to feed all RC channels from the RC receiver to the autopilot . Most UHF type remote control systems, like EZUHF , OpenLRS , Crossfire or DragonLink , support a dedicated channel that embeds RSSI into this aggregate/sum signal stream. According to ArduPilot default channels 1-4 are used as inputs for main controls (roll, pitch, throttle, yaw), any channel number above 4 can be used to embed RSSI . Typically, these systems use a GUI to assign the RSSI to a dedicated channel slot on the RC receiver, or default to a standard channel. Please refer to the user manual of the respective system.
If you already have a RC receiver connected to the autopilot , no additional wiring is required to use the embedded RSSI :
The example below shows how to assign channel 5 embedded RSSI on a DragonLink RC receiver :
The RSSI value will be output via the PPM data stream on signal pin 1 and the sBus data stream on pin 4 . Both can be fed to the RCInput pin of the autopilot .
Now set the corresponding RSSI parameters:
1. RSSI_TYPE = 2 (if previously set to the default value of 0 , the parameter needs to be reloaded).
2. RSSI_CHANNEL = The channel on which the RSSI is embedded on the RC receiver (channel 5 in the example above).
3. RSSI_CHAN_HIGH = The channel value when the received signal is the strongest.
4. RSSI_CHAN_LOW = The channel value when the received signal is the weakest.
Displaying the input value of the corresponding RC channel while rxssi is displayed in GCS helps to adjust the RSSI value range to match the 0-100% RSSI range :
In the strongest receiving state, Ch5 shows that the PWM value is 1904us . After turning off the transmitter, the value will drop to 1093us . Setting RSSI_CHAN_HIGH and RSSI_CHAN_LOW respectively can extend the used PWM range to 0 - 100% and get the best resolution.
！Note
Certain sBus protocol differences may cause channel values not to update during receiver failsafe. In this case, you will see the last valid RSSI value displayed during the failsafe . Since received signal strength usually gradually decreases under flight conditions, this probably won't have any negative effects. However, during setup and testing, when your RC transmitter is actually off, the RSSI is frozen at a rather high value, which can be confusing.
33.3.2 Analog Voltage RSSI Feeds to a Dedicated Pin
If your RC receiver outputs an RSSI type analog voltage range, you can feed that into the autopilot 's analog RSSI input pin. Typical voltage range is 0 - 3.3V or 0 - 5V . For details, refer to the remote control system manual or check with a multimeter.
！Note
Note that while most autopilot input pins are theoretically 5V tolerant , most ADCs can only measure 3.3 - 3.6 V unless equipped with a voltage divider to reduce the higher input voltage. Please check your RC receiver's RSSI voltage as well as the autopilot 's specs to ensure compatibility.
If the existing RC receiver connection to the autopilot includes supply voltage and ground, only one additional signal line is required. The example below shows how to connect the receiver's analog voltage type RSSI to the sBus output pin of the pixhawk autopilot :
Now set the corresponding RSSI parameters:
1. RSSI_TYPE = 1 (if previously set to the default value of 0 , the parameter needs to be reloaded).
2. RSSI_ANA_PIN = 103 ( PixHawk SBUS output/ RSSI input pin).
3. RSSI_PIN_HIGH When the signal is the strongest, the voltage received on RSSI_ANA_PIN is usually 3.3V .
4. RSSI_PIN_LOW When the signal is the weakest, the voltage received on RSSI_ANA_PIN , usually 0V .
！Note
The RSSI type options depend on your RC system specifications. Some systems require additional workarounds or additional hardware translations of non-standard proprietary protocols.
33.3.3 PWM Type RSSI Feed to a Dedicated Pin
If your RC receiver outputs PWM type RSSI on a dedicated pin or pad , you can feed that to one of the autopilot 's GPIO pins. See GPIOs to set PWM output pins as GPIOs . On pixahwk type hardware, this refers to the AUX pins, since they connect to the board's MCU , bypassing the IOMCU coprocessor.
| HW Pin (AUX) number |
alternative GPIO number |
| 1 |
50 |
| 2 |
51 |
| 3 |
52 |
| 4 |
53 |
| 5 |
54 |
| 6 |
55 |
In the example above, set BRD_PWM_COUNT to 4 , set pins 1-4 as PWM outputs, and free pins 5 and 6 for use as GPIO 54 and 55 .
If the existing RC receiver connection to the autopilot includes supply voltage and ground, only one additional signal wire is required. The example below shows how to connect the receiver's PWM type RSSI output to the pixhawk autopilot 's AUX 6 pin group for use as GPIO 55 :
Now set the corresponding RSSI parameters:
1. RSSI_TYPE = 4 (if previously set to default value 0 , you need to reload the parameter).
2. RSSI_ANA_PIN = GPIO pin number used ( 55 in the above example).
3. RSSI_CHAN_HIGH = PWM value when the received signal is the strongest.
4. RSSI_CHAN_LOW = PWM value when the received signal is the weakest.