Reading Bytes from Arduino to MATLAB

I've been trying to read data from ESP32S-Dev module to MATLAB sending over the data in bytes to make the transmission quicker. As seen below, I convert float data into bytes and then write the data. MATLAB sees the data and then creates a 1x8 array for each float value instead of just 1 value for each float. I've tried other methods, shown in 2nd part of MATLAB code, and it creates 1 value for each float but their wildly off (7-34 orders of magnitude)

I've tried different baud rates and even asked for help from the almighty ChatGPT but have not been able to crack the code. Any thoughts.

Arduino IDE code:

#include <SPI.h>

const int CS_SiPM_pot = 34;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);  //Starting
  delay(1000);
  SPI.begin();
  pinMode(CS_SiPM_pot, OUTPUT);
}

void loop() {
  for (int d_pot2 = 55; d_pot2 <= 55; d_pot2 = d_pot2 + 1)  //57
  {
  digitalWrite(CS_SiPM_pot, LOW);  //LOW
  delay(10);                       //10
  SPI.transfer(d_pot2);
  digitalWrite(CS_SiPM_pot, HIGH);
  // put your main code here, to run repeatedly:
    float newTmp[4] = { 01.00, 02.00, 03.00, 04.00 };
    for (int i = 0; i < 4; i++) {
      byte *byteData = (byte *)&newTmp[i];
      Serial.write(byteData, sizeof(float));  // Send each float as 4 bytes
      delay(10);
    }
  }
}

MATLAB code:

esp = serialport('COM5',152000);
%%
flush(esp)
% Read 16 bytes of data (4 floats * 4 bytes per float)
numBytes = 16;
data = read(esp, numBytes, 'uint8');

% Convert each 4-byte sequence to a float
float1 = typecast(data(1:4)), 'single');
float2 = typecast(data(5:8), 'single');
float3 = typecast(data(9:12), 'single');
float4 = typecast(data(13:16), 'single');

disp([float1, float2, float3, float4]);


% Convert the bytes back to float values
floatValues = typecast(uint8(data), 'single');


% Display the result
disp('Float values:');
disp(floatValues);

disp('Raw bytes received:');
disp(data); % Print the raw bytes

Results from MATLAB:

Float1: 0 0 0 0 0 0 0 0

Float2: 0 3.7480 0 0 0 0 0 0

Float3: 0 2.1250 0 3.7480 0 0 0 0

Float4: 0 0 0 0 0 3.7480 0 0

data: 0 0 0 0 255 0 0 0 3 255 0 0 0 0 255 0

[Walter Roberson]

float1 = typecast(data(1:4)), 'single');

You have an extra ) in there.

[Nicholas Ross]

@Walter Roberson thanks for the heads up!

Answer 1

[dpb]

I dunno how to fix the Arduino side, but the data bytes you're returning aren't the right values for the single float values 1 thru 4.

x=single(1:4).';
format hex
disp(x)

   3f800000
   40000000
   40400000
   40800000

bytes=reshape(typecast(x,'uint8'),4,[]).'

bytes = 4x4

00 00 80 3f 00 00 00 40 00 00 40 40 00 00 80 40

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format short
bytes

bytes = 4x4

0 0 128 63 0 0 0 64 0 0 64 64 0 0 128 64

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arrayfun(@(i)typecast(bytes(i,:),'single'),[1:height(bytes)].')

ans = 4x1

1 2 3 4

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The data array you received back should have been

data=reshape(bytes.',1,[])

data = 1x16

0 0 128 63 0 0 0 64 0 0 64 64 0 0 128 64

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Answer 2

[Walter Roberson]

https://www.mathworks.com/help/instrument/serialport.read.html

data = read(device,count,datatype) reads the number of values specified by count in the form specified by datatype from the serial port connection device. For all numeric datatype types, data is a row vector of double values.

You still need the 'uint8' to specify how to read the data, but you should be using

data = uint8(read(esp, numBytes, 'uint8'));