left shift problem

Projects Programming
1

Hello, I am working on a binary communication between arduino.
and I have faced some problem, I can't retrieve the correct data by using left shift function.

The following code is what I do for sending and receiving binary data, to make troubleshoot easy, i made it in one program.

First, I break the data to 4 byte from first arduino
Second, I will collect back the data using second arduino and combine the 4bytes back into one value.

I am totally have no idea why is it not giving me back my value, please let me know if you can solve it, thanks!

long val = 12980992;
byte buffer[4];
long retrieve;

void setup(){
  Serial.begin(38400);
}

void loop(){
  /*to send out from first arduino*/
  buffer[0] = val&0xff;
  buffer[1] = (val >> 8)&0xff;
  buffer[2] = (val >> 16)&0xff;
  buffer[3] = (val >> 24)&0xff;
  

  long num;
  int start = 0;
 
 /*to retrieve by second arduino*/      
  num = buffer[start];
  num = (buffer[start + 1] <<8) | num;
  num = (buffer[start + 2] <<16) | num;
  num = (buffer[start + 3] <<24) | num;
        
  Serial.println(num);
  delay(500);
}

Best,
Jack

2

I'm not positive, but I think it's because you're shifting 8bit values (which are likely handled by 16bit registers) more than 16 bits left. So after buffer[1] you are just ORing in 0x0000;

Try This:

num = buffer[start+3];
  num = (num<<8)|buffer[start + 2];
  num = (num<<8)|buffer[start + 1];
  num = (num<<8)|buffer[start];

Edit:

To be more clear, here is what's happening line by line:
your initial value is 0x00C61300, so your buffers are:

buffer[0]: 0x00
buffer[1]: 0x13
buffer[2]: 0xC6
buffer[3]: 0x00

Here's what happens:

num = buffer[start];
//num = 0x00000000
num = (buffer[start + 1] <<8) | num;
//num = (0x13<<8) | 0x00000000
//num = 0x1300 | 0x00000000
//num = 0x00001300
num = (buffer[start + 2] <<16) | num;
//num = (0xC6<<16) | 0x00001300
//num = 0x0000 | 0x00001300  <-this is because the 0xC6 was shifted off the top of the 16bit working register
                                          // You expected 0xC60000 but you get 0x0000
//num = 0x00001300
3

thanks hillridge, it works perfectly!

4

@Hillridge,
Very well explained ! +1