trouble with Serial.read()...

Using Arduino Programming Questions
1

Hey Guys, i am kinda new to Arduino so i will try to explain everything so you can get a better picture of my question:

First i am using an Arduino Mega to controll a pump by using a PWM. So I am at the point where i am sending a Constant from MatLab Simulink with a serial send block. I want to read it in the Arduino code that i have uploaded in the board and for now use it in the analogWrite() function to set this constant as the value for the PWM (0-255).

So this is the code:

float h;
int SensorVal;
float res;
char Arr[4];

void setup() {
// open a serial connection
  Serial.begin(9600);
// put your setup code here, to run once:
  pinMode(11, OUTPUT);
}

void loop() {
if (isnan(Serial.read())){
  analogWrite(11, 0);
}else{
  for( byte i=0;i<4;i++){
    Arr[i]=Serial.read();
  }   
  res = *((float *)&Arr);
  analogWrite(11, res);
}
// read A10
SensorVal = analogRead(A10);
h=(float)(SensorVal)*100.0/600.0;
Serial.write((unsigned char *)(&h),4);

delay(500);
}

So my question is: Am I not converting the value correctly or am I not getting it rite from the serial port? OR is the whole logic gone down the drain?

Thanks in advance!

2 
if (isnan(Serial.read())){

So now you know it wasn't a nan but you will never know what it was. The next read will be something new and it might be nan even though that one wasn't. Reading from serial consumes the characters out of the buffer. If you want to be able to use them then read them into a variable.

Besides, nan is something a float variable might be. Reading from serial gives characters. It will never be nan. You're just throwing away half your transmission with that line.

3

You should also read up on using available to check if there is serial data to be read before you start reading data that might or might not be there.

4

Serial.read() is non blocking. if byte is not available the it returns -1 (that is why the return type is int).
so you must check if the returned value isn't -1 or check if data are available before read.

Serial Input Basics

5

Thank you both for your replies.

Yes, i just gave it a check to see if Serial.available is bigger than 0 so i know that i have something. I tried to see if i am getting something, or nothing or the same thing all over again. As you said I added the reading to a variable and used it to see what is happening and i wrote a check see if I am getting a -1. But i also changed the analogWrite function to see if i am actually getting to it or i am stuck in my ifs, and no i am actually getting to the analogWrite function.

...
   Arr[i]=Serial.read();
    }
    res = *((float *)&Arr);
    analogWrite(11, 255);//<---analogWrite(11, res)

So i am getting a Serial.available bigger than 0, and my reading is a number value and it is not -1. My Guess is that maybe the code is okay, but maybe i am sending wrong data from simulink :/.

6

Are you printing back the number you read? Are you receiving raw bytes with numbers or are you receiving ascii text from matlab? That makes a HUGE difference.

7 
res = *((float *)&Arr);

This also looks odd to me. Arr is already a pointer so I'm not following why there is an & there.

Why not:

res = *((float*)Arr);

Cast Arr from a char* to a float* and then dereference the float*.

Alternatively you could have:

res = *((float *)&Arr[0]);

But that's a bit redundant to dereference the pointer just to take the address of it.

8

Read the updated serial input basics if it's text based.

Also contains an example to convert text to integer. If you really need a float instead of an integer, use atof instead of atoi.