SYMAX Receiver

Thanks to

• execuc for the original code (https://github.com/execuc/v202-receiver)
• Deviationtx team for hacking the symax protocol (https://bitbucket.org/deviationtx/deviation/src/1194044d116b9611a015837226729e26de7e8365/src/protocol/symax_nrf24l01.c)
• Special thanks to hexfet from deviationtx forum for the invaluable support

Goal

This code decodes frames from the X5C-1, X11, X11C, X12... transmitter (blue or green led) with an arduino and a nrf24L01+ chip. NOT the old x5c! This code has not been tested enough and it is not super reliable. So don't use it with dangerous rc model as planes, helicopters, cars...

Hardware

Arduino, nrf24l01+ chip Connect SCK, MISO, MOSI on pins D13, D12 and D11. Then connect CE and CS on digital pins you have chosen in the code (see the wireless.setPins() method below). Finally connect nrf24l01 VCC and GND to arduino 3.3V (if you plan to NOT use the chip in low power mode, get a proper 3.3v power supply or you may destroy the Arduino) and GND pins.

Use

There are two classes :

• nrf24l01p : handle the spi protocol to communicate with the nrf24l01p chip
• symax_protocol : handle the symax protocol

symax_rx.ino is an example showing how use theses classes.

These two classes are instantiated :

nrf24l01p wireless; 
symaxProtocol protocol;

The nrf24l01 pins must be defined in arduino setup function. setPins method arguments define the nrf24L01 CE (chip enable) and CS (SPI chip select) pins in this order. In my example, I did not use SS arduino pin as CS but SS pin must be set to output to activate the SPI mode to master.

void setup() {
  ...
  // SS pin must be set as output to set SPI to master !
  pinMode(SS, OUTPUT);

  // Set CE pin to 10 and CS pin to 9
  wireless.setPins(10,9);
  
  // Set power (PWRLOW,PWRMEDIUM,PWRHIGH,PWRMAX)
  wireless.setPwr(PWRLOW);
  
  protocol.init(&wireless);
  ...
}

SPI wrapper (nrf24l01p class) are linked to the protocol in the setup function

protocol.init(&wireless);

In the loop function, symaxProtocol Run() method must be called AT MOST every 4ms with & rx_values_t structure:

uint8_t value = protocol.run(&rxValues);

This function has several kind of returns from an enum :

enum rxReturn
{
   BOUND_NEW_VALUES = 0,   // Bound state, frame received with new TX values
   BOUND_NO_VALUES,        // Bound state, no new frame received
   NOT_BOUND,              // Not bound, initial state
   BIND_IN_PROGRESS,       // Bind in progress, first frame has been received with TX id, wait no bind frame.
   UNKNOWN                 // Not used for moment
};

When a frame is received (BOUND_NEW_VALUES), rx_values_t structure can be read :

typedef struct __attribute__((__packed__)) {
  uint8_t throttle; // 0...255
  int8_t yaw; // 127...-127
  int8_t pitch; // 127...-127
  int8_t roll; // 127...-127
  int8_t trim_yaw; // 31...-31
  int8_t trim_pitch; // 31...-31
  int8_t trim_roll; // 31...-31
  bool video; // true means button is down
  bool picture; // true means button is down
  bool highspeed; // true means high speed mode selected
  bool flip; // true means button is down
} rx_values_t;

Four axis, 3 trims values and buttons are available.

Improvements

Some improvements :

• Handle missing frame : for each frequency, transmitter send two frames in 8 ms. If there are lost, received will wait 4 frequency switching * 8ms to get frame. It would be interesting to jump some frequencies to retrieve the channel switching faster.