openXsensor (oXs) on RP2040 board

For rc protocols : expressLRS / FRSKY (sport + Fbus) / HOTT / JETI Ex / JETI EXBUS/ MPX / FLYSKY / Futaba (SBUS2) / Spektrum (SRXL2)

This project can be interfaced with 1 or 2 ELRS, FRSKY , HOTT , MPX, FLYSKY , Futaba, Spektrum or Jeti receiver(s) (protocol has to be selected accordingly).

This project is foreseen to generate:

• telemetry data (e.g. when a flight controller is not used)
• PWM servo signals (based on Rc channel)
• Sbus signals
• PWM signals to stabilize a camera on pitch and roll
• different sequences of PWM signals (to control Servo or to generate an analog/digital voltage) based on Rc channel values
• data's (telemetry and/or PWM Rc channels) to be logged on a SD card
• localisation data's on a second Rf link in order to retrieve a lost model (= locator)

For telemetry, it can provide

• up to 4 analog voltages measurement (with scaling and offset) (optional); one voltage is normally used to measure a current and 1 or 2 (optionnaly) for temperature
• one RPM measurement; a scaling (rpmMultiplicator) can be used to take care e.g. of number of blades (optional)
• the altitude and the vertical speed when connected to a pressure sensor (optional)
• the airspeed when connected to a differential pressure sensor (and a pitot tube) (optional)
• compensated vertical speed when connected to a baro + a differentil pressure sensor
• Pitch/Roll and accelerations X/Y/Z when conncted to a MP6050 sensor (optional);
• GPS data (longitude, latitude, speed, altitude,...) (optional)
• rpm/volt/temp/current/consumption from some ESC (Hobbywing4, ZTW mantis, Kontronix, BlHeli) Note: vertical speed is improved when baro sensor is combined with MP6050 sensor.

It can also provide up to 16 PWM RC channels to drive servos from a CRSF/ELRS or from 1 or 2 Sbus/Fbus/Exbus/Ibus/SRXL2 signal (e.g Frsky,Jeti,Flysky,Spektrum). The refresh rate can be set between 50Hz(default) and 333Hz.

It can also provide SBUS signal (e.g. from 1 or 2 ELRS receivers).

When connected to 2 receivers, the output signals (e.g. PWM or Sbus) will be issued from the last received Rc channels. So this provides a kind of redundancy/diversity.

It can stabilize the plane (gyro). This requires

- to use a mp6050 device
- to configure oXs in order to get Rc channels and to generate PWM signals for the servos controling the camera
- to get from the handset the "original" (not mixed) sticks positions in 3 additional RC channels
- to get from the handset the gyro mode and general gain in 1 additional RC channels

It can stabilize a camera. This requires

- to use a mp6050 device
- to configure oXs in order to get Rc channels and to generate PWM signals for the servos controling the camera
- to edit the camera parameters in the config.h file and to compile the edited project.

It can generates some sequences on servos or e.g. led. This requires to configure oXs in order to get Rc channels.

It can log data's on a SD card. This require to build also another module with another RP2040: see oXs_logger project

It can transmit some localisation data on a separate rf link to another module in order to retrieve a model that has been lost.

Each function (telemetry/PWM/SBUS/gyro/logger/sequencer/localisation) can be used alone or combined with the others.

Note: when a mpu6050 is used (to improve vario reactivity, stabilize the plane and/or a camera), it is important to calibrate the mp6050 horizontally and vertically (see section below)

------- Hardware -----------------

This project requires a board with a RP2040 processor (like the rapsberry pi pico).

A better alternative is the RP2040-Zero or the RP2040-TINY (both have the same processor but smaller board)

This board can be connected to:

• a pressure sensor (GY63 or GY86 board based on MS5611, SPL06 or BMP280) to get altitude and vertical speed
• a MS4525D0_A or a SDP3X (x=1,2,3) or SDP8xx or XGZP6897D differential pressure sensor to get airspeed (and compensated vertical speed)
• a MP6050 (acc+gyro e.g. GY86) to improve reaction time of the vario or to get pitch/roll
• 1 or 2 ADS1115 if you want to measure more than 4 analog voltages
• a GPS from UBlox (like the beitian bn220) or one that support CASIC messages

  • note : a Ublox GPS can be re-configured automatically by oXs ( with own oXs param). It has then to use the default standard ublox config.

    It can also be configured manually (with U-center firmware) externally prior to be connected to oXs. Set up must then be:
    - 38400 baud (for a M10) or 9600 baud (for a M8)
    - output on uart1: only 4 UBX messages (no NEMA): UBX-NAV-PVT , UBX-NAV-POSLLH, UBX-NAV-VELNED (when supported) , UBX-NAV-SOL (when supported)

  • a CASIC gps has to be configured before use in order to generate only NAV-PV messages at 38400 bauds
    This can be done using a FTDI and the program GnssToolkit3.exe (to download from internet)

• some voltage dividers (=2 resistors) when the voltages to measure exceed 3V
  note : a voltage can be used to measure e.g. a current (Volt2) or a temperature (Volt3/4) when some external devices are used to generate an analog voltage
• a RPM sensor
• an ESC from Hobbywing (using V4 telemetry protocol), from ZTW mantis, from Kontronik or from BlHeli. Those ESC provide one voltage, one current (+ current consumption) + RPM + 1 or 2 temperatures.
• another rp2040 with an SD card to log huge volume of data's
• a LORA module SX1276/RFM95 to transmit the localisation on a long range rf link (see locator section)

About the SDP31, SDP32, SDP33 , SDP810: Those sensors are probably better than MS4525. They do not requires calibration (and reset) and are more accurate at low speed. Those sensors exists in 3 versions which differs by the maximum differential pressure (and so the max speed) they can measure SDP31 (or SDP810-500) can measure up to 500 Pa = 105 km/h SDP32 (or SDP810-125) can measure up to 125 Pa = 52 km/h SDP33 can measure up to 1500 Pa = 189 km/h The difference between SDP3x and SDP800 series is mainly the size of the sensor. SDP3x are very small (5mm) and require soldering on small pin SDP810 are bigger (25 mm) and have a 4 pin connector Currently oXs code is written for SDP3x serie but using a SDP810 requires only to change the I2C address in the config.h file.

--------- Wiring --------------------

FRSKY/ELRS/JETI/... receiver, MS5611, GPS and other sensors must share the same Gnd
Connect a 5V source to the Vcc pin of RP2040 board ( RP2040-zero or RP2040-TINY boards do not accept more than 5.5V on Vcc pin !! )
There is no default affectation of the RP2040 pins so user has to specify it with some parameters after flashing the firmware (see below)

Depending on the protocol, the pins used for PRIMARY/SECONDARY RC Channels and for Telemetry (TLM) varies

protocol	PRI pin is connectected to	SEC pin is connected to	TLM pin is connected to	Comment
C(ELRS)	(TX from Rx1)	(TX from Rx2)	(RX from RX1)
S(Frsky sport)	(Sbus from Rx1)	(Sbus from Rx2)	(Sport from RX1 or Rx2)	(1)
F(Frsky Fbus)	Fbus from Rx1	(Sbus from Rx2)	Not used	(2)
J(Jeti ex)	(Sbus from Rx1)	(Sbus from Rx2)	(Ex from Rx1 or Rx2)
E(Jeti Exbus)	Exbus from Rx1	(Sbus from Rx2)	Not used	(2)
H(Hott)	(Sbus from Rx1)	(Sbus from Rx2)	???(tlm from RX1 or Rx2)	(1)
M(Multiplex)	(Sbus from Rx1)	(Sbus from Rx2)	???(tlm from RX1 or Rx2)	(1)
I(Flysky Ibus)	(Sbus from Rx1)	(Sbus from Rx2)	( Ibus from RX1 or Rx2)	(1)
L(Spektrum Srxl2)	Srxl from Rx1	Not used	Not used	(2)
2(Futaba Sbus2)	Sbus2 from Rx1	(Sbus2 from Rx2)	Sbus2 from Rx1 via 1Kohm	(3)

Note: pins between () means that they are optional.

(1) for safety, insert a 1 kOhm resistor between TLM pin and Rx

(2) for safety, insert a 1 kOhm resistor between PRI pin and Rx

(3) For Futaba, TLM pin must be equal to PRI pin - 1 and insert 1 kOhm resistor between PRI and TLM

Up to 16 PWM signals can be generated on pin gpio 0...15 (to select in setup parameters).

Voltages 1, 2, 3, 4 can be measured on gpio 26...29. Take care to use a voltage divider (2 resistances) in order to limit the voltage on those pins to 3V max. V2 is normally used to measure a current (based on the analog voltage). V3 and V4 can be used to measure or a voltage or a temperature (based on a voltage). For each voltage being measured, you probably have to specify the offset and scale to be applied.

One RPM (Hz) can be measured

• Take care to limit the voltage to the range 0-3V; so if you use capacitor coupling, add diodes and resistor to limit the voltage
• All pulsed are counted (no debouncing); so use a hardware low pass filter (resistor/capitor) to avoid dummy pulses reading

When a MS5611/SPL06/BMP280 (baro sensor) and/or MP6050 is used:

• Connect the 3V pin from RP2040 board to the 5V pin of GY63/GY86 or the Vcc from other sensor
  Note: do not connect 5V pin of GY63/GY86 to a 5V source because the SDA and SCL would then be at 5V level and would damage the RP2040
• Connect SCL from baro sensor to the pin selected as SCL in parameter for RP2040
• Connect SDA from baro sensor to the pin selected as SDA in parameter for RP2040

When a differential pressure sensor is used, you should connect SCL/SDA like for a baro sensor.

• Vcc is connected to 5V or 3.3V depending on the chip you selected
• If the module you are using does not have pullup resistors and if you do not use other I2C modules, than you must use pullup resistor (4.7K) connected between SCL/SDA and 3.3V

When a GPS is used:

• Connect the 3V pin from RP2040 board to the Vin/5V pin from GPS
• Connect the RX pin from GPS to the RX pin selected in parameter for RP2040
• Connect the TX pin from GPS to the TX pin selected in parameter for RP2040
• So take care that wires TX and RX are not crossed (as usual in Serial connection)

When a Hobbywing, ZWT, Kontronik or BlHeli ESC is used:

• Connect the serial pin from ESC to the pin selected in parameter for RP2040 (for ESC_PIN)
• Connect GND from ESC to RP2040 GND
• do not define gpio's in RP2040 parameters for V1, V2, RPM and let TEMP parameter on 0. You can use V3 and V4 if you want. Note: SCALE1, SCALE2, OFFSET2 and RPM_MULT have to be defined based on your ESC and your motor.

About sequencers and locator, see below.

The affectation of the pins has to be defined by the user.
Here are the command codes and the pins that can be used are:
Note: pin 16 is reserved for an internal LED on RP2040-zero or RP2040-TINY and so should not be used with this board.

Command	used for:
C1 = 0/15 ... C16 = 0/15	PWM output
GPS_TX = 0/29	getting GPS data
GPS_RX = 0/29	sending configuration to GPS
PRI = 5 ,9, 21 ,25	primary RC channel input
SEC = 1, 13 , 17 ,29	secondary RC channel input
SBUS_OUT = 0/29	Sbus output
TLM = 0/29	telemetry data (! for futaba Sbus2, this pin must be equal to PRI pin - 1)
V1= 26/29 ... V4= 26/29	voltage (or current/temperatue) measurements
SDA = 2, 6, 10, 14, 18, 22, 26	I2C devices (baro, airspeed, MP6050, ADS115, ...)
SCL = 3, 7, 11, 15, 19, 23, 27	I2C devices (baro, airspeed, MP6050, ADS115, ...)
RPM = 0/29	RPM
LED = 16	internal led of RP2040-zero or RP2040-TINY
LOG = 0/29	data to be logged
ESC_PIN = 0/29	data provided by ESC (rpm, volt, current, temp)
SPI_CS = 0/29	Chip Select pin from RMF95 (locator)
SPI_SCK = 10, 14, 26	SCK pin from RFM95 (locator)
SPI_MOSI = 11, 15, 27	MOSI pin from RFM95 (locator)
SPI_MISO = 8, 12, 24, 28	MISO pin from RFM95 (locator)

--------- Software -------------------

This software has been developped using the RP2040 SDK provided by Rapsberry.

If you just want to use it, there is (in most cases) no need to install/use any tool.

• download from github the zip file containing all files and unzip them where you want.
• in your folder, there is a file named oXs.uf2; this is a compiled version of this software that can be directly uploaded and configured afterwards
• insert the USB cable in the RP2040 board
• press on the "boot" button on the RP2040 board while you insert the USB cable in your PC.
• this will enter the RP2040 in a special bootloader mode and your pc should show a new drive named RPI-RP2
• copy and paste (or drag and drop) the oXs.uf2 file to this new drive
• the file should be automatically picked up by the RP2040 bootloader and flashed
• the RPI_RP2 drive should disapear from the PC and the PC shoud now have a new serial port (COMx on windows)
• you can now use a serial terminal (like putty , the one from arduino IDE, ...) and set it up for 115200 baud 8N1. Set it up in order to let it send automatically CR/LF when you press ENTER.
• while the RP2040 is connected to the pc with the USB cable, connect this serial terminal to the serial port from the RP2040
• when the RP2040 start (or pressing the reset button), press Enter and it will display the current configuration.
• to list all the commands, send ?.
• if you want to change some parameters, fill in the command (code=value) and press the enter.
• you can enter severals commands without repowering the device
• Important note : when you enter usb commands to change parameters, they are not automatically applied. Most of the time, oXs will stop most functionalities. You have to save the changes using the SAVE command and then make a power off/on.

Developers can change the firmware, compile and flash it with VScode and Rapsberry SDK tools.
An easy way to install those tools is to follow the tutorials provided by Rapsberry.
In particular for Windows there is currently an installer. See : https://github.com/raspberrypi/pico-setup-windows/blob/master/docs/tutorial.md

Once the tools are installed, copy all files provided on github on you PC (keeping the same structure).
Open VScode and then select menu "File" + item "Open Folder". Select the folder where you copied the files.
In VScode, press CTRL+SHIFT+P and in the input line that appears, enter (select) CMake: Configure + ENTER
This will create some files needed for the compilation.
To compile, select the "CMake" icon on the left vertical pannel (rectangle with a triangle inside).
Move the cursor on the line oXs [oXs.elf]; an icon that look like an open box with some dots apears; click on it.
Compilation should start. When done a new file oXs.uf2 should be created.
For more info on VScode and SDK look at tutorials on internet.

Note : the file config.h contains some #define that can easily be changed to change some advanced parameters.

Note for ELRS:
The RP2040 send the telemetry data to the ELRS receiver at some speed.
This speed (=baud rate) must be the same as the baudrate defined on the receiver.
Usually ELRS receiver uses a baudrate of 420000 to transmit the CRSF channels signal to the flight controller and to get the telemetry data.
Still, ELRS receivers can be configured to use another baud rate. In this case, change the baudrate in parameters accordingly.

You have to compile your self the firmware if you want to change some values in the config.h file in order e.g. to:

• change the setup of the ADS1115
• allocate other slots for Sbus2 in Futaba protocol
• allocate another physical ID for Sport in Sport/Fbus protocols
• avoid or change priorities of some telemetry fields for Sport in Sport/Fbus protocols
• assign another sequence number and/of generate alarms for some telemetry fields in Multiplex protocol
• change the I2C address of some I2C sensors
• use other default paramaters in order to avoid using commands via the USB/serial monitor.
• change the sensitivity of the XGZP sensor (if defferent from XGZP6897D001KPDPN)

------------ Failsafe---------------

• For ELRS protocol, oXs does not received any RC channels data from the receiver(s) when RF connection is lost. If oXs is connected to 2 receivers (via PRI and SEC), oXs will generate PWM and Sbus signals on the last received data. If oXs does not get any data anymore from receiver(s), it will still continue to generate PWM and/or SBUS signals based on the failsafe setup stored inside oXs.

• For Frsky/Jeti... protocols where Sbus is used, the failsafe values are normally defined inside the receiver and the receiver continue to generate a Sbus signal even if the RF connection is lost. Still, when connection is lost Sbus signal contains some flags that say that some data are missing or that failsafe has been applied. When oXs is connected to 2 different receivers, it gives priority to PRI sbus signal except when SEC signal is OK and PRI is not OK (no signal, missing frame, failsafe). So for Frsky/Jeti, oXs does not have to take care of his own failsafe setup (except if oXs would not get any Sbus signal anymore - e.g due a wiring issue).

• For failsafe oXs has 3 options:

  • "Hold" = failsafe will be the last Rc channels values known just before connection is lost; to select this option, use the serial interface with the command "FAILSAFE=H"
  • store as failsafe the current RC channels values using the serial interface with command "SETFAILSAFE".
  • store as failsafe the current RC channels values using the "boot" button on the RP2040. To activate this option, doubble click the button. Led should become fixed blue. In the next 5 seconds, press and hold the "boot" button. Led will become white for 2 seconds confirming that values are saved in the config.

For the 2 last options, the handset must be on and generating the channels values that you want to save in oXs.

--- Telemetry fields being measured and transmitted ---

oXs tries to detect automatically which sensors are connected (based on the parameters being fill in the setup). It can display on the PC (on a serial terminal getting the messages via usb ) the current setup and the sensors that have been discovered.

oXs measures different fields depending on the sensors being detected.

Please note that the data being transmitted depends also on the protocol being used (Sport, ELRS, ...).

For more information, please look at document "fields per protocol.txt" in folder "doc"

When a baro sensor and an airspeed sensor are both used, oXs calculates 2 vertical speeds:

• the normal one based only on the baro sensor; this one is always transmitted
• an airspeed compensated Vspeed (=dte) that take care of the variation of airspeed.

You can use a channel to control the way airspeed compensated Vspeed is calculated and/or transmitted.

First you have to send a command ACC (via the PC) to specify the channel being used (1...16).

You must use a protocol/wiring (like Sbus, Fbus, Exbus, ...) that allows the Rx to communicate the RC channel values to oXs.

Then, depending on the value sent by the Tx on the selected channel, oXs manages the airspeed compensated Vspeed in different ways:

• if the value is around the center position, oXs uses a default coefficient (defined in config.h file as 1.15) to calculate compensated Vspeed and transmit it.
• if the value is largely positive, oXs uses it to adapt the coefficient (from 0.9 up to 1.4). Assigning e.g. a slider to this channel allows you adjust the coefficient while flying to find the best value.
• if the value is largely negative, oXs sent the "normal" Vspeed in the field foreseen for compensated Vspeed. So even if vario tone is based on compensated Vspeed telemetry field, you can switch while flying between the 2 Vspeed (with a switch on the TX).

Note: you can use the FV command to know the current coefficient. This allow you to check that your Tx sent a Rc channel value that match the expected goal and indeed required, adjust your Tx settings.

---------------- calibration of MP6050 ---------------

When an MP6050 is used, it is important to calibrate it and to let oXs knows his orientation in the plane. The mpu must be installed in the model in such a way that one axis of MPU6050 is vertical and that another one is aligned with main axis of the model (nose-queue). There are 24 possible orientations to match this. Most commercial gyro's require that the user declare the orientation of the MP6050 in the model. oXs does it automatically with a 2 steps calibration process (named here Horizontal and Vertical).

• Horizontal calibration calculates the acceleration and gyro offsets in the 3 directions. It identifies also partly the orientation of the MP6050. It requires that the plane is set horizontally (like when it flies and roll/pitch are both 0) and do NOT move at all. Then a usb command MPUCAL=H is sent. The horizontal calibration takes a few seconds (less than 5). The result is displayed.

• Vertical calibration completes the determination of MPU6050 orientation. It requires that the plane is set vertically with the nose up. It is not required to keep it totally still in this position. Then a command MPUCAL=V is sent. The horizontal calibration is done in less than 1 second.

Please note that, like other usb commands that change the configuration, you have to send afterwards a SAVE command to store the results in memory and so keep them after a power off or a reset.

The current configuration is displayed (like other parameters) with the ENTER command.

Once the MP6050 calibration process has been done and saved, it is normally not required to do it again. Still if you change the orientation of oXs in a model, you have or to perform the 2 steps calibration again or you can just change the orientation using 2 usb commands GOV (horizontal orientation) and GOV (vertical orientation). Those 2 commands can change the orientation but not the offset.

Note: at each power on, oXs performs automatically a new calibration of the gyro offsets (so nor acceleration offsets nor orientation). To get correct offset parameters, the model must stay still during the first 2 seconds. His attitude does not matter. It is possible to disable this automatic calibration with a parameter in file config.h.

Please note that when the MP6050 is used to stabilize the plane, you have also to perform a gyro mixer calibration. See gyro section.

---------------- Sequencers ---------------

With oXs, one single channel on the handset can control one or several SERVOS in sequences defined by the user (e.g. for landing gears with doors and wheels).

It can also generate one or several ANALOG signals in sequences (e.g. to blink leds or to start/stop motors).

One output is controlled by only one Rc channel.

You can use several Rc channels; each channel controls one or serveral outputs (SERVO and/or ANALOG).

oXs uses 3 concepts : sequencer, sequence and step.

• oXs uses ONE "sequencer" per GPIO to be controlled. There can be up to 16 sequencers (one per gpio 0...15).

• Each "sequencer" has several (min 2, max 21) "sequences". Each sequence is activated by a specific RC channel value (-100%,- 90%, ... 90%,100%)

• Each "sequence" has one or several steps. Each step correspond to one action (move servo to position X, set led on power Y)

• oXs can repeat continously one sequence or just stay on the last step waiting for a new Rc channel value to start a new sequence.

• Each "sequencer" is defined by 7 parameters:

  • The GPIO on which a PWM signal is generated ( must be in range 0/15); the same GPIO may not be used for another purpose or by another sequencer
  • The type of PWM signal
    • 0 = SERVO = PWM signal to control a servo (every xx msec a pulse in range 1ms/2ms is generated)
    • 1 = ANALOG = PWM signal to control a LED or analog voltage ( every xx msec, a pulse in range 0/xx ms is generated)
  • The duration of one "clock" in msec (must be greater than 20); this defines the base unit of the "smooth" and the "keep" delays used in steps definition (see below)
  • The Rc channel that control this sequencer (must be in range 1/16); the same Rc channel may be used in several sequencers
  • The default PWM value (to apply when no Rc channel has yet been received) (must be in range -100/100 for SERVO, 0/100 for ANALOG outputs)
  • The min PWM value (must be in same range as default PWM value); if a step requests a lower PWM value, the min PWM will be used
  • The max PWM value (must be in same range as default PWM value); if a step requests a greater PWM value, the max PWM will be used.
  • note: Min and Max defined at sequencer level can be usefull in order to define the end points of servo travel; they can avoid having to change many values at step level while using servos.

• Each "sequence" is defined by 5 parameters

  • The Rc channel value that activates this sequence. The value must be a multiple of 10 and in range -100...100 (so like -100, -90, -80... 0, 10, 20,...100);so there a 21 valid values. Note: the rc channel value sent by the handset can slightly differ from those values because oXs applies a tolerance of +/- 4%. So, if handset sent e.g. a value equal to -86, it will be handeld like -90.
  • an optional flag ("R" = Repeat) to say if the sequence must be automatically repeated after the last step; by default, sequence is not repeated
  • an optional flag ("U" = Uninterrupted) to say that the sequence may be interrupted before end of the last step; by default sequence may be interrupted
  • an optional flag ("O" = Only interrupted by priority sequence) to say that the sequence may be interrupted but only by a "priority" sequence
  • an optional flag ("P" = priority) to say that the sequence is a "priority" sequence (so it may interrupt a sequence with flag "O")

• Each "step" is defined by 3 parameters

  • The number of clocks (=delay) for a smooth transition from current PWM value up to the PWM value from this step (must be in range 0/255)
  • The PWM value to apply in this step at the end of the transition (in range -125/125 for SERVO, 0/100 for ANALOG pwm); a value 127 is also possible and has a special function: it means that oXs must keep the current value. This can be useful only when a running sequence must be interrupted and the position must remain unchanged.
  • The number of clocks the PWM value must be kept before applying next step (if any) or going back to the first step of this sequence. Must be in range 0/255

• For each sequencer, when the handset sent a different Rc value that matches the value of a sequence, oXs starts "playing" all steps of the related sequence. If this happens while oXs is already playing a sequence, oXs will continue or stop playing the current sequence depending on the flags U, O and P (see above). If the current step may not be interrupted, oXs, will delay the new requested sequence up to the end of the current sequence. When the current sequence reaches his end, oXs will or play the delayed sequence (if any), or repeat the current sequence (if flag = "R") or keep the last PWM output.

• For each sequencer, if the handset sent a Rc value that does not match a defined value (taking care of tolerance), the change of Rc value has no effect; oXs continues to "play" the current sequence.

• Sequencers, sequence and steps are defined sending a command via the USB port using a serial terminal. So there is no need to compile/flash to change some parameters.

• All sequencers are defined by only one command: SEQ=[...] (...) {...} {...} (...) {...} etc...

  • each [...] contains the 7 parameters of one sequencer (space delimited)
  • each (...) contains the parameters (Rc value + 4 optional flags) of one sequence (space delimited)
  • each {...} contains the 3 parameters of of one step (space delimited)
  • each [...] must be followed by (...) to specify the first sequence of this sequencer
  • each (...) must be followed by one or several {...} to specify the steps of this sequence
  • a sequencer must contain a least 2 sequences.
  • sequences of one sequencer must be in ascending order of RC channel values
  • e.g. SEQ=[3 0 100 15 -100 -100 +90] (-100 R O) {0 50 3} {10 100 2} (100 P) {0 -100 10} [4 1 500 16 0 0 100 ] (-100) {0 0 20} (-30) {0 100 1} (70 R) {0 100 20} {5 20 10}
  • this defines 2 sequencers:
    • Sequencer nr 1 [3 0 100 15 -100 -100 +90] has PWM output on gpio 3, for a servo , 100ms/clock, controlled by channel 15, PWM = -100 (default), -100 (min) and 90(max); it contains 2 sequences:
      • First sequence (-100 R O) is activated when Rc channel changes to -100%, may be repeated automatically and may be interrupted only by a priority sequence; it contains 2 steps:
        • first step {0 50 3} says that PWM must be set immediately(smooth=0) on 50%, stay on 50% for 3 clocks and then switch to step 2
        • second step {10 100 2} says that PWM must increase gradualy up to 100% over a timelaps of 10 clocks and then stay on 100% for 2 clocks before repeating the sequence
      • Second sequence (100 P) is activated when Rc channel change to 100% and is a priority sequence (so it may interrupt sequence 1)
        • step {0 -100 10} says that PWM must be set immediately (smooth =0) on -100% and stay on this value for at least 10 clock units
    • Sequencer nr 2 [4 1 500 16 0 0 100 ] has PWM output on gpio 4, for analog voltage, 500ms/clock, controlled by channel 16, PWM = 0 (default), 0 (min) and 100(max = Vcc); it contains 3 sequences
      • First sequence (-100) is activated when Rc channel change to -100% (does not repeat and may be interrupted)
        • step {0 0 20} says that PWM must be set immediately (smooth=0) on 0 and stay on 0; note: as the sequence may not be repeated automatically but may be interrupted by any new sequence, the delay of 20 clock units has in practice no effect
      • Second sequence (-30) is activated when Rc channel change to -30% (does not repeat and may be interrupted); it contains 2 steps:
        • step {0 100 1} says PWM must be set immediately (smooth=0) on 100% and stay on this value for 1 clock. note: as the sequence may not be repeated automatically but may be interrupted by any new sequence, the delay of 1 clock units has in practice no effect
      • Third sequence (70 R) is activated when Rc channel change to 70 and may be repeated automatically; it contains 2 steps;
        • step one {0 100 20} says that PWM must be set immediately(smooth=0) on 100 and stay on 100 for 20 clock and then next step is applied.
        • step two {0 20 10} says that PWM must be set smoothly (over 5 clock units) to 20 and stay so for 10 clock units before repeating the sequence.

• To delete all sequencers, enter SEQ=DEL

• Processing of SEQ commands

  • most controls on SEQ commands are performed before saving the parameters. In case of error, the command is just discarded and not saved
  • commands that fit those controls are saved in flash memory and oXs is rebooted. It can be that you have to make a manual reset (or power off+on)
  • After reset, oXs performs some more controls that could lead to an invalid config (e.g. if a gpio is used for several functionalities)

• Sequencers parameters are displayed as all other parameters when you press only ENTER. It is possible to make a complete copy/paste of the displayed SEQ to the input aera of the serial terminal in order to easily edit some parameters.

------------------ Logging -------------------

If the LOG Gpio is defined, all telemetry data and all PWM Rc channel values (usec) captured by oXs are transmitted on the LOG pin in a compressed format.

Each time a set of data is ready, oXs generates a packet (with only the newly generated data).

The packet is generated with an UART 8N1 (8 bits, no parity, 1 stop bit) at the defined logger baudrate.

Each packet starts with 0X7E (= synchro byte) followed by 4 bytes (number of milli sec since RP2040 start up) and by

• For telemetry : one or several data blocks; each one contains
  • one byte to identify the type of data (max 63 types e.g. Vspeed, Altitude, ...). The 2 most significant bits gives the number of "0" bytes that should be added to the data value in order to get a in32_t (code on 4 bytes)
  • 1, 2, 3 or 4 bytes with the value
• For Rc channels :
  • one byte equal to "41" (= type for Rc channels)
  • 32 bytes = 16 X 2 bytes; each 2 bytes (=uint16_t) represent the PWM values in microsec (1500 usec = neutral) of one Rc channel.

There is some stuffing mecanism (like in Frsky Sport protocol) in order to ensure that the value 0X7E can only be present at the begining of a packet.

This format allows to compress the data transmitted via the (quite slow) UART to the logger.

The logger will remove the stuff bytes, uncompress the data, combine the new data with previous one to create an "actual" set of data's, convert it in CSV format and finally store it on a SD card.

------------------ Gyro ------------------

When oXs get the Rc channels from a receiver (via Sbus, Ibus, ...) and when a MPU6050 is installed, oXs can apply gyro corrections on several servos. For more details, please read carrefully the file "gyro concepts.md" in the folder "doc".

Important note: at this stage, this is still experimental. It has not been intensively tested. So used it at you own risk.

------------------ Model Locator ----------------------------------------

oXs can be used to locate a lost model (if you add a LORA module).

The model is normally connected to the handset but when the model is on the ground, the range is quite limitted. So if a model is lost at more than a few hundreed meters, the handset will not get any telemetry data anymore. oXs allows to use a separate connection (with LORA modules) in order to have an extended range and have a chance to find back a lost model. This is possible because those modules use a lower frequency, a lower transmitting speed and a special protocol for long range. The LORA modules are SX1276/RFM95 that are sall and easily available (e.g. Aliexpress, ebay, amazon)

The principle is the following:

• You have to build 2 devices:

  • an oXs device with the sensors you want (ideally a GPS and optionally e.g. vario, voltages, current, ...) and a SX1276/RFM95 module
  • a "locator receiver" device with:
    • an Arduino pro_mini running at 8 mHz 3.3V
    • a second SX1276/RFM95 module
    • a display 0.96 pouces OLED 128X64 I2C SSD1306. It is small and is available for about 2€.;

• Normally:

  • the locator receiver is not in use.
  • oXs is installed in the model and transmits the sensor data's over the normal RC Rx/Tx link. The SX1276 module in oXs is in listening mode (it does not tranmit)

• When a model is lost:

  • the locator receiver" is powered on. It starts sending requests to oXs.
  • When the SX1276/RFM95 module in oXs receives a request, it replies with a small message containing the GPS coordinates and some data over the quality of the request signal.
  • the display on the locator receiver shows those data's as wel as the quality of the signal received and the time enlapsed since the last received message.

Note: the range of communication between two SX1276 modules is normally several time bigger then the common RC 2.4G link.
If oXs and locator receiver are both on the ground, it can be that there are to far away to communicate with each other. But there are 2 ways to extend the range:

• use a directional antena on the locator receiver. The advantage of this solution is that, if you get a communication, you can use the system as a goniometer (looking at the quality of the signal) to know the direction of the lost model. This even works if you have no GPS connected to oXs. The drawback is that a directional antenna is not as small as a simple wire.
• put the locator receiver (which is still a small device) on another model and fly over expected lost aera. In this case, the range can be more than 10 km and the chance is very high that a communication can be achieved between the 2 modules. Even if the communication is broken when the model used for searching goes back on the ground, you will know the location of the lost model because the display will still display the last received GPS coordinates.

An oXs device with a SX1276/RFM95 does not perturb the 2.4G link and consumes only a few milliAmp because it remains normally in listening mode and when sending it is just a few % of the time. So, in order to increase the reliability of the system, it is possible to power oXs with a separate 1S lipo battery of e.g. 200/500 mAh. This should allow the system to work for several hours.

Cabling : The SX1276/RFM95 module must be connected to the Rp2040 in the following way

• rp2040 SPI_CS <=> NSS from module
• rp2040 SPI_MOSI <=> MOSI from module
• rp2040 SPI_MISO <=> MISO from module
• rp2040 SPI_SCK <=> SCK from module
• rp2040 GRND <=> GRND from module
• external (or rp2040 ) 3.3V <=> 3.3V from module (!!! module does not support 5 Volt).

To be checked : perhaps you have to use an additional voltage regulator (cost less than 1€) to get the 3.3 V, because it is not sure that the rp2040 voltage regulator can provide enough current when module is transmitting (for just a small time)

To build the locator receiver, please check and use the project openXsensor for Arduino (on github)

------------------ Led -------------------

When a RP2040-Zero or RP2040-TINY is used, the firmware will handle a RGB led (internally connected to gpio16).

• when config is wrong, led is red and always ON.
• when config is valid, led is blinking and the color depends on RC channels being received ot not
  • Red = Rc frames have nerver been received, Sbus and/or PWM signals are not generated.
  • Blue = Sbus and/or PWM signals are based on failsafe values. Failsafe values are given by the receiver for Sbus or are configured in oXs for CRSF protocol
  • Yellow/oranje = Sbus and/or PWM signals are based on a valid RC channels frame received from PRI or SEC source but the orther source does not provided a valid RC channels frame.
  • blinking green = Sbus and/or PWM signals are based on valid RC channels frames (from one source; from both sources if both are foressen in the setup)
• when "Boot" button is used for setting the failsafe values, led becomes blue and white (see above)

Note: some users got a RP2040-zero or RP2040-TINY where red and green colors are inverted. If you got such a device and want to get the "normal" colors, you can enter a command LED=I to invert the 2 colors.

Please note that other boards do not have a RGB led on gpio16 and so this does not applies.