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I am doing some computations on Matlab and I need to send those values to an Arduino Leonardo through USB serial connection. I need to send 2 variables which can vary from -400 to +400. I'm saying their values because I was able to do this with small positive values (unsigned byte), but not larger and negative numbers. Please help! Thank you

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2  
Consider sending printable values just as if you had typed them into the serial monitor. It is not the most efficient solution, but it is one of the simplest, avoiding the format-matching and synchronization challenges of multibyte binary data. –  Chris Stratton Feb 13 at 7:20
    
I am analyzing images from webcam and continuously sending data to Arduino. I solved the part of sending data, but the problem is that the Serial.Available() is updated only once. It receives much of the data as one string. I need it to be updated after every sending from Matlab. There is no interrupt with each serial fprintf basically. –  Martin V Feb 17 at 23:13
    
Communications on the leonardo is packetized, so data will arrive in chunks that may or may not match your initial message size. –  Chris Stratton Feb 17 at 23:17
    
Turns out Matlab needed me to define a '\n' terminator in the serial fprintf function along with the type of data to be transferred. The problem was solved, thank you for the help. –  Martin V Feb 18 at 23:34

1 Answer 1

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You cannot send larger values because byte only covers the range from 0-255.

To send larger values, you can break your int variable into 2 byte variables. Here's an example:

// On Arduino
int myVar = -123;
byte myVar_HighByte = myVar>>8; // get the high byte
byte myVar_LowByte = myVar; // get the low byte
// x86 compatible machines are little-endian so we send the low byte first
Serial.write(myVar_LowByte); 
Serial.write(myVar_HighByte);

% On MATLAB
s = serial('COM10')
fopen(s)
myVar = fread(s,1,'int16')

Disclaimer: Syntax might not be entirely correct, since I have no MATLAB or Arduino near me when I typed this. But you should get the idea. ;-)

Edit: On second thought, it might be easier to use a pointer. 

// On Arduino
float myFloat = 3.14159265359;
byte* ptr = (byte*) (&myFloat);
Serial.write(*ptr++);
Serial.write(*ptr++);
Serial.write(*ptr++);
Serial.write(*ptr++);

% On MATLAB
s = serial('COM10')
fopen(s)
myVar = fread(s,1,'float')

Having said that, you will still have to take care of the endianness, if you use a different microcontroller.

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