Serial communication between python and arduino doesn't work

Projects Programming
1

I'm trying to make simple serial communication between python and arduino. But it seems like I can't send data to arduino without receiving data after writing data to arduino in while loop. Or arduino isn't receiving that data, because python code doesn't freeze between those write commands

python code

import numpy as np
from mss import mss

import time
import serial


ser = serial.Serial(
    port='COM14',
    baudrate=515200,
    timeout=5,
)

time.sleep(2)
bounding_box = {'top': int(2160/3), 'left': 0, 'width': 3840, 'height': int(2160/3)}

sct = mss()
count = 1
while True:

    sct_img = sct.grab(bounding_box)
    average = np.mean(np.array(sct_img), axis=(0, 1))
    averageString = str(int(average[0])).zfill(3) + "%" + str(int(average[1])).zfill(3) + "%" + str(int(average[2])).zfill(3) # + "" + str("A")
    ser.write(averageString.encode('utf-8'))
    print(averageString)

    # have to do this to send more data to arduino, but it's slowing down too much communication
    #msg = ser.readline()
    #msg = msg.decode('utf-8')
    #msg = msg.rstrip()
    #print("Message from arduino: ")
    #print(msg)

arduino code

int r = 9;
int g = 10;
int b = 11;

void setup() {
  Serial.begin(515200);
  delay(500);
}

void loop() {
  String data;
  while (Serial.available() == 0) {}
  data = Serial.readString();

  data.trim();
  //delay(100);
  Serial.println("data" + data);

  int delimiter, delimiter_1, delimiter_2, delimiter_3;
  delimiter = data.indexOf("%");
  delimiter_1 = data.indexOf("%", delimiter + 1);
  delimiter_2 = data.indexOf("%", delimiter_1 + 1);
  delimiter_3 = data.indexOf("%", delimiter_2 + 1);

  String second = data.substring(delimiter + 1, delimiter_1);
  String first = data.substring(delimiter_1 + 1, delimiter_2);
  String third = data.substring(delimiter_2 + 1, delimiter_3);

  analogWrite(r, 255 - first.toInt());
  analogWrite(g, 255 - second.toInt());
  analogWrite(b, 255 - third.toInt());
}
2

you sure about this ?

3

I have tested many different speeds and they don't effect. I just found out it need one second sleep between sends to work correctly at speed of 115200. But why It need that one second delay. I would like to update between 10-100ms

4

you need to start off with some code that's very simple like ( adjust the sleep timer in the code )

while True:
        dataToSend = 'data from python'
        dataToSend.encode()
        ser.write(dataToSend)
        time.sleep(0.2)  # 0.2 secs == 200 milliseconds

and on arduino side you have this ( please use 115200 as baud at both ends )

void loop() {

   if (Serial.available()) {
     String buffer = "";
       while(Serial.available())  {
         delay(1);  // slow down a bit
         char c = Serial.read();
         buffer += c;
      }  // end while

   if ( buffer.length() > 1 ) {
         Serial.println("data: "+ buffer);
     }
  } // end if


}

If all is good then you can proceed injecting the rest of your code slowly

by the way you can have your own millis() in Python so that you won't have to use time.sleep()


def millis():
    return int(round(time.time() * 1000))
5

how does your python code know when the Arduino is ready?

6 
Serial.readString();

readString() will read characters and if no more characters have been received in 1 second (default timeout) returns the string read
this may be where your 1 second delays comes frome
if the Python program terminates the transmitted string with a newline '\n' you could use readStringUntil()

Serial.readStringUntil('\n');

will read characters into the String until a '\n' is received or timeout occures (1 second)

7

I used millis() from arduino to measure how long it take to read sent data from pc. When sending from arduino ide serial port it takes about 1ms to run that code but when I sent it from python it took 1052ms. That 1000ms is that serial.readStringUntil() function timeout. I'm using 'A' as the end of data so '\n' won't mess up my python console. I think problem is that arduino doesn't understand ser.writelines(averageString.encode('utf-8')) encode to utf-8. So what decoding does arduino use?

8

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