Delay in Communication Between Arduino and RPi

Question

I already asked a question regarding communication between a Arduino and RPi through a USB serial connection and was able to solve major issues. Now my RPi and Arduino are talking to each other and getting along swimmingly... well almost.

I am using the following code for a simple task that could be almost completed without a Raspberry Pi (or without the Arduino). I have a ultrasonic sensor and a led hooked up to the Arduino and I am feeding the results to the RPi. The RPi is taking the results of the ultrasonic sensor and depending on the distance telling the Arduino to turn on the led. I am currently using the RPi as a way to start the process.

I have a raw input with the RPi "To start the ultrasonic sensor, please type "Start":". This works but after the two start communicating there is approximately a five second delay from when an object is close enough to light the led and when the led actually turns on. I would like to eventually use this on a robot and need a quicker reaction time.

Here is the Arduino Code:

int led = 10;
const int pingPin = 2;
long duration, distanceInches, distanceCm;


void setup()
{
  pinMode(led, OUTPUT);
  Serial.begin(9600);
}

void loop()
{
  pinMode(pingPin, OUTPUT);
  digitalWrite(pingPin, LOW);
  delay(2);
  digitalWrite(pingPin, HIGH);
  delay(5);
  digitalWrite(pingPin, LOW);

  pinMode(pingPin, INPUT);
  duration = pulseIn(pingPin, HIGH);

  distanceInches = microsecondsToInches(duration);
  distanceCm = microsecondsToCentimeters(duration);
  Serial.print(distanceInches);
  Serial.println();

  delay(200);

  char ch = Serial.read();
  if (ch == 'a') digitalWrite(led, HIGH);
  else if (ch == 's') digitalWrite(led, LOW);

  delay(10);
}

long microsecondsToInches(long microseconds)
{
  return microseconds / 74 / 2;
}

long microsecondsToCentimeters(long microseconds)
{
  return microseconds / 29 / 2;
}

Here is the code for the Raspberry Pi in Python 2.7:

import serial
import time
import sys

chip = '/dev/ttyACM0'
baud = 9600

ser = serial.Serial(chip, baud)
ser.flush()

def run():
     while True:
         x = ser.readline()
         y = int(x)
         if y < 14 :
             ser.write('a')
         else :
             ser.write('s')
         time.sleep(0.010)

def start():
    while True:
        on = raw_input('To start the ultrasonic sensor, please type "Start": ')
        if on == "Start":
            run()
        else :
            print('Invalid command. Please try again')


start()

Thanks in advance for any help and let me know if you need any clarification.

Robb

Answer 1

I had a similar problem once and I believe it was due to the fact that the serial data was overflowing a buffer on the RPi side. You may want implement a simple 'handshaking'. For example the Rpi sends a character (say the integer '1') on the serial port, when the Arduino sees a '1' it sends back the data from the ping sensor.

The way your code is currently is a bit of a problem because the Arduino is constantly sending data when it doesn't need to. Try it out and let me know if it works.