read 'long' type data from I2C

Projects Programming
1

I am using arduino as a I2C master to read a 'long' type data from the I2C bus. I found every byte I receive is right. But I encounter some difficulties when I want to 'recover' the original 'long' type data. The data is transmitted MSB first.

I read the data on the I2C bus into a four bytes unsigned char array and then convert it. This is my code.

void ReadData4(unsigned char cmdSend, long* data)
{
  unsigned char d[4]; 
  // command 
  cmd[0] = cmdSend;
  // send command 
  Wire.beginTransmission(0x50); // transmit to device, 0x50 is the address of neurons motor module
  Wire.write(cmd,1);            // sends command 
  Wire.endTransmission();       // stop transmitting
 
 //read
  Wire.requestFrom(0x50, 4);    // request 4 bytes from slave
  if (Wire.available())
  {
    d[0] = Wire.read();    // receive a byte as character    
  }
  if (Wire.available())
  {
    d[1] = Wire.read();    // receive a byte as character
  }
  if (Wire.available())
  {
    d[2] = Wire.read();    // receive a byte as character
  }
  if (Wire.available())
  {
    d[3] = Wire.read();    // receive a byte as character
  }
  
   Serial.print(d[0], HEX);         // print the character
   Serial.print('\n');
   Serial.print(d[1], HEX);         // print the character
   Serial.print('\n');
   Serial.print(d[2], HEX);         // print the character
   Serial.print('\n');
   Serial.print(d[3], HEX);         // print the character
   Serial.print('\n');
   
  *data = 0;
  *data |= (d[3]);
  *data |= (d[2] << 8);
  *data |= (d[1] << 16);
  *data |= (d[0] << 24);
  
  Serial.print(*data, HEX);         // print the character
  Serial.print('\n');

  return;
}

and this is my output from uart:

FF
D1
AA
DC
FFFFAADC

It looks the single bytes are read correctly. But the second 'FF' in the 'FFFFAADC' should be D1.
then I tried a really simple code:

unsigned char d[4]; 
  d[0] = 0xFF;
  d[1] = 0xD1;
  d[2] = 0xAA;
  d[3] = 0xCD;

   Serial.print(d[0], HEX);         // print the character
   Serial.print('\n');
   Serial.print(d[1], HEX);         // print the character
   Serial.print('\n');
   Serial.print(d[2], HEX);         // print the character
   Serial.print('\n');
   Serial.print(d[3], HEX);         // print the character
   Serial.print('\n');
   
  *data = 0;
  *data |= (d[3]);
  *data |= (d[2] << 8);
  *data |= (d[1] << 16);
  *data |= (d[0] << 24);
  
  Serial.print(*data, HEX);         // print the character
  Serial.print('\n');

And the output is just the same.
I am wondering why the 2nd MSB always wrong. Looking forward for any help.

2

What happens to n 8 bit value when you shift it 8, 16, or 24 bits? The type on the left side of the equal sign has nothing to do with the type on the right.

This question is asked once a week here. You clearly made no effort to search.

3

Thanks. I assign the unsigned char to a long type data, then shift it. It works. I will search the fourm for better ways. And if you have some links, I will be grateful.