How to read data from arduino bluetooth?

I am successful in sending data from matlab to arduino via Bluetooth, but I am unable to read the data sent by Arduino-bluetooth(JY-MCU).

% serial communication between matlab and bluetooth     

delete(instrfind);

b = Bluetooth('BTslave',1);

fopen(b); %write and read function

fwrite(b,'K');

A = fread(b,5)

fclose(b);

clear('b');

I am able to read 5 values but they are just random values not the right values.

Answer 1

[Walter Roberson]

Watch out for the default on fread() being 'double' . If the bluetooth device is sending data in some other datatype you need to adjust your fread() to know that.

[kintali narendra]

Thank you Walter,

my sensor is connected to analog pin on arduino, when I check the output of sensor on serial of arduino its value varies from 0 -1023. which data type I need to use double give me very small value of 1.0e-56*0.2793.

[Walter Roberson]

Please show the (C) code that is used to send the value.

I suspect that what you are transmitting is either short int or int or long int datatype, and that you need to use one of

A = fread(b, 5, 'int16->double')
A = fread(b, 5, 'int32->double')
A = fread(b, 5, 'int64->double')

However, it would not surprise me if the values are integer values being transmitted in a different byte order than your host is using, so it might turn out to be necessary to use swapbytes()

To probe this further you could use

Atemp = fread(b, [1 5*8], '*uint8');
Atemp
dec2hex(Atemp)

and show us the output; that would allow us to test some hypotheses about byte orders and word lengths

[kintali narendra]

Hi Walter, I tried all clues which u have given, but still there is no success. here is my Arduino code.

 char val; // variable to receive data from the serial port
int ledpin = 8; // LED pin1
int ledpin2 = 9; // LED pin2
void setup() {
  pinMode(ledpin, OUTPUT);  // Onboard LED as OUTPUT
  pinMode(ledpin2, OUTPUT);
  Serial.begin(9600);       // start serial communication at 9600bps
}
{
void loop() {
  if( Serial.available() )       // if data is available to read
  { val = Serial.read();  }       // read it and store it in 'val'
  if( val == 'K' )               // if 'K' was received
  {
    digitalWrite(ledpin, HIGH);  // turn ON the LED
    digitalWrite(ledpin2, LOW);
    int potvalue = analogRead(A5);
    Serial.print(potvalue);
  } else { 
    digitalWrite(ledpin, LOW);   // otherwise turn it OFF
    digitalWrite(ledpin2, HIGH);
    int potvalue = analogRead(A5);
    Serial.println(potvalue);
  }
 delay(1000);                    // wait 100ms for next reading
}

I even tried reading values of the sensor from Com15 where I can see sensor values in Arduino-IDE but it didn't work. It gives an error "com port is unable to open".

[Walter Roberson]

Change the Serial.print for 'K' to Serial.println (which you are already using for the non-'K' case).

Now, on the MATLAB side,

A = fscanf(b, '%f');

Also, your comments need to explain why your code compares to 'K' but your comment talks about 'H', because that is confusing.

[kintali narendra]

Hi Walter, sorry, I didnt see the mistake in the comment section. It worked perfectly with fscanf() thanku. Can u say why it didn't work for fread() and how to make matlab to read the value continuously.

[Walter Roberson]

Earlier I indicated that the default for fread was double. That was incorrect: the default is uint8=>double . Each character of the printable values was being interpreted as a separate value, but you needed a group of printable values to be read together as a single value.