Connection Pi -> Arduino through nRF24l01, problem with integers

Question

I want to send integers via RF-module nRF24l01 from my Pi (library: https://github.com/jpbarraca/pynrf24) to my Arduino Pro Mini (library: http://maniacbug.github.io/RF24/index.html).

I am able to get a a basic connection and send a char 'a' and interpret it right on Arduinos side.

RPi code:

#!/usr/bin/python
# -*- coding: utf-8 -*-

import time, sys
from nrf24 import NRF24

pipes = [ [0xF0, 0xF0, 0xF0, 0xF0, 0xE1], [0xF0, 0xF0, 0xF0, 0xF0, 0xD2] ] #40-bit hex adressen der pipes

#nRF24L01 Setup
radio = NRF24()
radio.begin(0,0,22, 23) #Set CE and IRQ pins
radio.setRetries(15,15) #zeit zwischen retries (x * 250mikro s) und anzahl an retries
radio.setPayloadSize(1) #32-byte payload
radio.setChannel(0x4c) #0-127 in hex
radio.setDataRate(NRF24.BR_250KBPS) 
radio.setPALevel(NRF24.PA_MAX) #power amplifier level
radio.openWritingPipe(pipes[0]) 
radio.openReadingPipe(1,pipes[1])
radio.printDetails() #Debug
print(time.strftime("%d.%m.%Y %H:%M:%S", time.localtime()) + " ------ PyNRF start ------")

try:
    # main code
    while True:

        data = ['a']

        result = radio.write(data)

        print(time.strftime("%H:%M:%S", time.localtime()) + " data send --- " + str(result))

        time.sleep(5)

except KeyboardInterrupt:
    sys.exit(0)

Arduino code:

#include <SPI.h>
#include "nRF24L01.h"
#include "RF24.h"

//nRF24 stuff
RF24 radio(8,9);
const uint64_t pipes[1] = { 0xF0F0F0F0E1LL };

void setup(void) {
  Serial.begin(57600); //Debug  

  //nRF24 stuff
  radio.begin();
  radio.setRetries(15,15);
  radio.setDataRate(RF24_250KBPS);
  radio.setPayloadSize(1);
  radio.openReadingPipe(1,pipes[0]);
  radio.startListening();
}

void loop() {
  if ( radio.available() ) {
    char data[1];
    bool done = false;
    while (!done) {
      done = radio.read( &data, sizeof(data) );
    }
    Serial.println(data[0]);
  }
}

But when i try to send an integer (lets say 5), i get as output 31493

RPi code: same as above, except data = [5]

Arduino code: same as above, except int data[1];

My first thought was changing payload size to 2 byte on both sides, since integers on arduino side are 16-bit and on RPi too (?), but that just gave me as output 31749

Where am I struggling here?

Answer 1

There are a couple of things involved in this.

First, integers in Python are not 2 bytes and you should not relay on their representation being the same at both ends. You should define a format for send integers as bytes "over the air" and break/reconstruct on both sides. For example, you could send two bytes, the fist being (val>>8)&0xFF and the second val&0xFF. This convention will work for positive integer from 0 to 32767.

Second, you need to understand how a sequence of bytes is send/read at each side. In pynrf24 you should use a list:

data = [ (val>>8)&0xFF, val&0xFF ]
result = radio.write(data)

On the Arduino, you will use a array of bytes

int val;
char data[2];
...
done = radio.read( &data, sizeof(data) );
val = (data[0] << 8) + data[1]

Answer 2

Try sending this in the Python code

data = ['1']

Then you can use

Serial.println(data[0]);

in the Arduino to print it.

Answer 3

From the Sending Data example:

status = radio.write("HELLO")

it looks like you can send a primitive down the pipe rather than a list; that may be causing the problem. Try changing your code to this and see if it works:

data = 1
result = radio.write(data)