Sending a struct over i2c

Question

How would you send a struct from an i2c slave when requested by an i2c master?

Slave Struct Respdonder Sketch (Seeeduino Xiao)

#include <Wire.h>
struct TransmitData
{
 int a;
 int b;
 int c;
 int d;
 float e;
 float f;
 float g;
 float h;
};
TransmitData data;
void setup()
{
 Serial.begin(9600);
 Wire.begin(2);
 Wire.onRequest(requestEvent);
 data.a = 1;
 data.b = 2;
 data.c = 3;
 data.d = 4;
 data.e = 5.1;
 data.f = 6.1;
 data.g = 7.1;
 data.h = 8.1;
}
void loop()
{
 delay(100);
}
void requestEvent()
{
 Serial.print("sending ("); Serial.print(sizeof data); Serial.println(" bytes)");
 Wire.write((byte *)&data, sizeof data);
}

Master Struct Requester/Reciever Sketch (Arduino MEGA)

#include <Wire.h>
struct TransmitData
{
 int32_t a;
 int32_t b;
 int32_t c;
 int32_t d;
 float e;
 float f;
 float g;
 float h;
};
TransmitData data;
void setup() {
 Wire.begin();
 Serial.begin(9600);
}
void loop() {
 Serial.print("requesting ("); Serial.print(sizeof data); Serial.print(" bytes)... ");
 if (Wire.requestFrom(2, sizeof data)) {
 Wire.readBytes((byte*) &data, sizeof data);
 Serial.println("done");
 Serial.println(data.a);
 Serial.println(data.b);
 Serial.println(data.c);
 Serial.println(data.d);
 Serial.println(data.e);
 Serial.println(data.f);
 Serial.println(data.g);
 Serial.println(data.h);
 } 
 else {
 Serial.println("could not connect");
 }
 delay(500);
}

The serial output on the slave device is:

sending (32 bytes)

The serial output on the master device is:

requesting (24 bytes)... done
1
0
2
0
0.00
0.00
5.10
6.10

• chrisl: You can try the I2C_Anything library by Nick Gammon

Answer 1

int isn't guaranteed to be the same size across platforms. Use fixed width integer types instead.

https://en.cppreference.com/w/cpp/types/integer

struct TransmitData
{
 int32_t a;
 int32_t b;
 int32_t c;
 int32_t d;
 float e;
 float f;
 float g;
 float h;
};