Python serial between Arduino and Raspberry Pi: Data is modified upon reception

Question

I'm using an Arduino for sensor readings and sending them to a Raspberry Pi via USB, using PySerial for data reception.

It works great, except for the fact received data is awkwardly modified (And set as constant). For example, I'm reading voltages and calculating currents. The results on the Arduino serial are as follows:

Volt   Current
4.93   0.38
4.92   0.37
4.92   0.37
4.92   0.36
...    ...

However, on the Raspberry Pi, it's constantly read as follows (Notice how the digits are changed to zero):

  Volt   Current
    4.99   0.30
    4.99   0.30
    4.99   0.30
    4.99   0.30
    ...    ...

I've tried several turnarounds, but with no luck. I'm not sure where the problem lies, as I am very confident my code is flawless. I even converted the readings to string before sending and yet the constant readings and zero'd digits keep appearing. I appended a counter integer which was sent correctly with no problems.

Has anyone ever tried this before? Any thoughts on how to solve this?

Raspberry Pi Code:

from time import gmtime, strftime
import time
import serial
import struct
ser = serial.Serial('/dev/ttyACM1', 19200)
f = open('results.txt','w')

while 1:
        temp=strftime("%Y-%m-%d %H:%M:%S", gmtime())+'\t'+ser.readline()
        print(temp)
        f.write(temp)
        f.close()
        f = open('results.txt','a')
        time.sleep(5)

Arduino Code:

...

  double volt = 5.0*(analogRead(A0))/1023.0;

  double current = 5.18 - temp;   //Resistance ~= 1 Ohm if you are wondering

  buffer += d2s(volt,2)+'\t'+d2s(current,2)+'\t'+ d2s(count,0) +'\t' + d2s(minCount,0);

  Serial.println(buffer);

...

//I got this from the web

String d2s(double input,int decimalPlaces){

  String string;

  if(decimalPlaces!=0){
    string = String((int)(input*pow(10,decimalPlaces)));

       if(abs(input)<1){
          if(input>0)
              string = "0"+string;
          else if(input<0)
              string = string.substring(0,1)+"0"+string.substring(1);
  }

  return string.substring(0,string.length()-
decimalPlaces)+"."+string.substring(string.length()-decimalPlaces);
}

   else {
     return String((int)input);

}
}

Answer 1

The arduino is not well suited to doing floating point math, nor is it partiularly well suited to doing string manipulation.

You would be better off by sending the value read from the analog input directly to the python code on the Pi and do the math and string manipulation in Python.

On the arduino side just do something like this:

int value;

value = analogRead(A0); // reads a 12bit integer value from the Analog input
Serial.println(value);  // converts the integer to a string and sends it over the serial port

Then on the Pi side:

str = ser.readline()  # read a string from the serial port

value = float(str)    # convert a string to a floating point number

volt = 5.0 * value / 1023.0  # compute the voltage

Answer 2

I found a way to send the information correctly using Serial.write(), but apparently it works byte-by-byte. Thus, I'l have to write some more code to translate each and every byte.

I'm sure there's a better and easier way though.

Answer 3

If you're suspecting the PySerial library, I have to say I'd be really suprised if the issue is there.

I've used PySerial pretty extensively, for both binary and ascii data without issues (though not on a Raspberry Pi).

One thing I can think of is it's possible the port is opening in a weird mode. Try manually specifying the parity and stop-bits mode:

ser = serial.Serial('/dev/ttyACM1', baudrate=119200, bytesize=8, parity='N', stopbits=1)

The other thing I can think of is that you're somehow sending an ASCII control-code, which PySerial is interpreting properly, and the Arduino terminal isn't.
If possible, try an additional serial terminal, and see if it agrees with one or the other.

---

Honestly, this really smells like a RAM issue to me. You're using a lot of C++ functions (std::string), which are very RAM hungry. You should really think of the arduino as a C device, and avoid C++ abstractions, if possible.

Also, Why the heck are you using doubles? The ADC precisions is 12 bits! A plain-old float is 32 bits on the arduino.

Also you're confident your code is "flawless", which smacks of inexperience to me (the only perfect code is code that doesn't exist).
Together with that (very sloppy) hack of a double to string conversion function (that you found on the internet!), makes me suspect the arduino code more.

Please post your whole arduino source, not just a snippet, too.

---

Another thing to try is just to store and print some of your results as pure strings:

Serial.println("4.93   0.38")
Serial.println("4.92   0.37")
Serial.println("4.92   0.37")
Serial.println("4.92   0.36")

If that fails, it is indeed likely it's a problem in PySerial. If it doesn't, it's the arduino code.

Answer 4

Actually, pyserial reads data from the Arduino as bytes, e.g.: b234\r\n. So you need to strip those unnecessary values before you can use them:

 valuebtw0to1023= serialObj.readline().strip('\r\n').strip()

Answer 5

if you plan to read serial data from al APC220 RF antenna with python, you will have to add the line :

ser = serial.Serial('/dev/ttyACM1', 19200)
ser.setRTS(0) #  <------------------- this line solve the problem

I found it after a whole day of try-error.