Raspi I2C communication with Arduino issues with wiringPi - arduino

Question

I'm experiencing unexpected results using wiringPi's wiringPiI2CWriteReg16() function, and I'm not sure if it's due to incorrect usage, or something else. This is the declaration for the function:
extern int wiringPiI2CWriteReg16 (int fd, int reg, int data);
There are notes within the wiringPiI2C.c file that state it resembles Linux's SMBus code, if that helps.
On my Arduino (both an Uno R3 and a Trinket Pro), I am running this pared-down sketch:
#include <Wire.h>
#define SLAVE_ADDR 0x04
void receive_data (int num_bytes){
Serial.print("bytes in: ");
Serial.println(num_bytes);
while(Wire.available()){
int data = Wire.read(); // tried char, uint8_t etc
Serial.println(data);
}
Serial.print("\n");
}
void setup() {
Serial.begin(9600);
Wire.begin(SLAVE_ADDR);
Wire.onReceive(receive_data);
}
void loop() {
delay(500);
}
I would think that Wire.read() would break things apart at the byte boundary, but that's not occurring in my case. Perhaps this is my issue... a misunderstanding.
Nonetheless, I have this C code (requires wiringPi v2.36+ to be installed):
// word.c
#include <wiringPiI2C.h>
void main (){
int fd = wiringPiI2CSetup(0x04);
wiringPiI2CWriteReg16(fd, 0x00, 255);
wiringPiI2CWriteReg16(fd, 0x01, 256);
}
Compiled like this:
gcc -o word word.c -lwiringPi
When run, ./word, I receive the following on my Arduino's serial output:
bytes in: 3
0
255
0
bytes in: 3
1
0
1
In the first call to wiringPiI2CWriteReg16(), I expect the first byte in the output to be zero (0x00) as that's the register address I'm requesting. The second byte (255) is also correct. The third byte (0) is meaningless from what I can tell (as I'm only sending in one byte as data).
However, in the second call to that function, I do get the correct output for the register (first byte as 0x01 == 1), but the second byte is zero, and the third byte has what appears to be the correct remainder (255 == one byte, + 1). The problem is, is that the second byte is 0.
The exact same effect happens if I pass in 511 or for that matter, any number as the data in the call.
My question is whether I'm missing something glaringly obvious (I'm relatively new to C and Arduino), and/or if I can get some pointers on how to troubleshoot this more thoroughly.

Answer 1

I found that the problem was in my Arduino code. In the official Raspi forums, Gordon tipped me off that the bytes are read in separately, LSB first. Throughout all of my searching, I hadn't come across that, and really didn't quite understand what was happening. After changing my I2C read loop code in my Arduino sketch to this:
while(Wire.available()){
Wire.read(); // throw away register byte
int16_t data = Wire.read(); // LSB
data += Wire.read() << 8; // MSB
Serial.print("data: ");
Serial.println(data);
}
...everything works. In effect, that's at least one way to read two-byte values over I2C on the Arduino and put the bytes back together.