V202 Receiver

Goal

This code decodes frames from the v202/v222/v262/v282... transmitter with an arduino and a nrf24L01 chip. I want to thank Rivig for sharing the v202 protocol with his transmitter code (https://bitbucket.org/rivig/v202/src). I have included his decode functions. This code has not been tested enough and it is not reliable. So don't use it with dangerous rc model as planes, helicopters, cars...

Hardware

I use a simple DIP arduino on a prototype board and nrf24l01+ chip to test it. But an arduino UNO or other with the same chip makes the job. 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 and GND pins.

Use

There are two classes :

• nrf24l01p : handle the spi protocol to communicate with the nrf24l01p chip
• v202_protocol : handle the v2xx protocol

v202_rx.ino is an example showing how use theses classes.

These two classes are instantiated :

nrf24l01p wireless; 
v202Protocol 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 D7 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);
  Serial.begin(115200);
  // Set CS pin to D7 and CE pin to D8
  wireless.setPins(8,7);
  protocol.init(&wireless);
  ...
}

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

protocol.init(&wireless);

In the loop function0, v202Protocol 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.
   ERROR_SIGNAL_LOST,      // Signal lost
   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;
  int8_t yaw;
  int8_t pitch;
  int8_t roll;
  int8_t trim_yaw;
  int8_t trim_pitch;
  int8_t trim_roll;
  uint8_t flags;
} rx_values_t;

Four axis, 3 trims values and flags are available. Last value depends on buttons pushed on the transmitter.

Improvements

There are many improvements to do :

• Make more reliable code, first.
• Handle signal lost : return to initial state for example.
• Make a better state machine for this protocol with a re-factor of the code and a smaller footprint.
• Reduce execution time. State machine takes 120us when a frame arrived and 32ms when there are nothing to do.
• Implement this protocol in the multiwii project, if it useful.
• Make an arduino example with some RC servos.