Sending large structs via nrf24l01 leads to cliping of some structs members

Question

I started with building of the wireless temp/hygro/other values probes. I use 2 arduinos, both with nf24l01+ wireless transceivers, library used is rf24.h (https://github.com/maniacbug/RF24). Basically ping/pong test works for me - so each arduino communicates properly via nrf24l01+. I added dht22 temperature sensor and just for testing I started with sending structure:

typedef struct{
  int A; // just counter to see if receiving new data
  float B; // temperature
} data;

data payload;

I send it from Arduino1 like this:

radio.write( &payload, sizeof(payload) );

Other Arduino2 receives the payload:

radio.read( &payload, sizeof(payload) );

This works perfectly and I am able to print the counter value as well as the temperature value on the oled display o Arduino2. The problem is when I add float C to the struct for sending/receiving hygrometer values. Then Arduino2 does not receive/display any valid hygro value, but there is just value of 0.00.

I think I know where my problem is - nrf24.h has possibility to set payload size for nrf24l01+ - both Arduinos have to use the same payload size (default is 16, min is 8, max is 32). I use 8 - as this looks to be the most reliable value when using larger distances between Arduinos. When I tried to set payload size to 32 on both Arduinos:

radio.setPayloadSize(32);

Then it works and I can receive and display counter, temp, hygro values. So it looks the nrf24l01+ sends a payload of either 8 or 32 bytes and when sending struct with INT, FLOAT, FLOAT - 8 bytes is not sufficient and 32 bytes of payload is needed. But when I will use many other values, I can reach also limit of 32 bytes payload.

I do not have much skills in C programming and also I do not fully understand how data packets are being transferred between nrf24l01+ modules- maybe I just need to adapt my send/receive code to handle sending of the larger structures. Maybe somebody could give me a hint.

Answer 1

Your payload is too big. You will have to split it to smaller pieces.

NOTE: If you'd use a library like RF24Mesh then you'd not have to worry about it.

Answer 2

as stated above, it all has to do with the size of your payload. To clarify things, you need to keep in mind how many bytes a certain data type uses/needs:

char: 1 byte int: 2 bytes float 4 bytes

(above is based on the Arduino reference, see https://www.arduino.cc/en/Reference/Float )

Thus, if you set payload size to 8 bytes in the NRF24L01 and then try to send a struct with int, float, float you will have 2 + 4 + 4 = 10 bytes, which does not fit in the payload size.