I've been hitting against a wall for over a week now. My current project is to use a Teensy 3.6 to send data over an SPI bus to a chip which is effective going to be a signal generator. I've made a GUI in Matlab to control this function generator in 'real-time' by sending over values such as frequency and amplitude. For my initial case, I just want to output a sine wave. to do this I am using the built in math function sin and just getting the amplitude and frequency from Matlab. However, when I send over my data I'm getting nothing on the output, but when I hard program in the values, it works just fine. From this I assume I am not sending the data over correctly OR not converting it correctly upon receiving it.

My order of operations are:

  1. send my variable to the Teensy 3.6 using fwrite in Matlab

    serialID = serial('COM5','BaudRate',115200);                    
    val = 2.55;
    fwrite(serialID, val, 'double');
  2. receiving the data on the Teensy 3.6 using Serial.read()

    byte temp[8];
    for (int c = 0; c<8; c++){
        temp[c] =Serial.read();
  3. converting my byte array to a double using a union

    double byteToDouble(byte DoubleInBytes[8]) {
        //Receives 8 bytes values and return them into a double value
        union {
           double   Out;                                                      
           byte   In[8];                                                      
        } U ;
        for (int c = 0; c < 8; c++) {
            U.In[c] = DoubleInBytes[c];
        return U.Out;

I'm at a loss as to what I might be doing wrong. I've tried to use the serial monitor to monitor variable states but I can't use that while Matlab is using the COM port.

Any helpful tips would be appreciated.

  • 1
    Have you verified byte-order? And read when there is data available, i.e. check Serial.available(). – Mikael Patel Feb 21 '19 at 18:53
  • Yes, they are both little endian. And yes, I used Serial.available to see if data is available. I'm currently reading in the data and sending back some debugging text to Matlab to see where I am in the code. When I hard code the numbers in I actually still read in data but don't use it to get into the if(Serial.available()) portion of the code. – Ryne Feb 21 '19 at 20:08
  • You mean if (Serial.available() >= 8). – Edgar Bonet Feb 21 '19 at 20:09
  • I did write it wrong, but not >= 8 but rather if(Serial.available() > 0) I just want to know something is ready to be pulled before starting. – Ryne Feb 22 '19 at 20:23

I got a bit more traction on this issue on the Arduino forums and was able to answer it over there. I'm posting the link here to complete this post with a working answer. Hope it helps!



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