Arduino - ServoMotor Control and SoftwareSerial

Question

My two arduino communicating serially with XBee. The receiver arduino also controls the servo motor. Servo works stably when there is no xBee connection. But when Arduino's serial communication, there is a problem in the operation of the servo motor. Servo motor stuttering while rotate, so stuttering occurs.

Can you help me?

My Coordinator Code:

#include <SoftwareSerial.h>
#define xBeeRxPin 10
#define xBeeTxPin 9
int deger3;
int deger4;
String d1,d2,d3;
SoftwareSerial xBeeSerial(xBeeRxPin, xBeeTxPin);

void setup() { 
  Serial.begin(38400);
  xBeeSerial.begin(38400); 
}void loop() {
  
  deger3 = analogRead(A0);
  deger4 = analogRead(A2);
  
  String deger5 =  (String)deger3 + ";" + (String)deger4 ;

  xBeeSerial.println(deger5);
  
  delay(50);
}

My Receiver Code:

#include <SoftwareSerial.h>
#include <Servo.h>
#define xBeeRxPin 10
#define xBeeTxPin 9
SoftwareSerial xBeeSerial(xBeeRxPin, xBeeTxPin);
String d1,d2,d3;

const int trigPin = 4;
const int echoPin = 3;
long duration;
int distance;
Servo myServo;

void setup() {
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  xBeeSerial.begin(38400);
  Serial.begin(38400);
  myServo.attach(11); 
}
void loop() { 
  for(int i=15;i<=165;i++){  
    myServo.write(i);
     delay(10);
    digitalWrite(trigPin, LOW); 
    delayMicroseconds(2);
    digitalWrite(trigPin, HIGH); 
    delayMicroseconds(10);
    digitalWrite(trigPin, LOW);
    duration = pulseIn(echoPin, 11200); 
    distance= duration*0.034/2;

    
    if(xBeeSerial.available()>0){
      d1=xBeeSerial.readString();
    }
  }
  
  for(int i=165;i>15;i--){  
    myServo.write(i);
     delay(10);
    digitalWrite(trigPin, LOW); 
    delayMicroseconds(2);
    digitalWrite(trigPin, HIGH); 
    delayMicroseconds(10);
    digitalWrite(trigPin, LOW);
    duration = pulseIn(echoPin, 11200); 
    distance= duration*0.034/2;
  

    if(xBeeSerial.available()>0){
      d1=xBeeSerial.readString();
  }
 }  
}

What exactly is this "problem in the operation of the servo motor"? Please edit that into your question. — ocrdu
– ocrdu, Commented Dec 22, 2020 at 13:02

timemage · Accepted Answer · 2020-12-22 14:10:00Z

1

When SoftwareSerial enters an interrupt each time it receives a character and busy-waits (delays) to time in each data sample as the bits in the character come in. No other interrupt service routines can run while this is happening. It also turns off interrupts when it sends.

The servo library for the AVR uses timer interrupts to generate the servo position signals.

When characters are being received, SoftwareSerial will interfere with the generation of the servo position signal because only one interrupt service routine can run at a time.

You may be able to mitigate the problem adding significant delay between characters sent, reducing the total number of characters sent. This would limit the interference and give servo time to get back to its correct position from however far it managed to deviate over the course of an inbound serial character.

There are also alternative libraries that use the AVR's Input Capture feature on an ICP pin to do some of the work of serial in hardware, and that may work for you. Here's one example library.

@ocrdu has rightly prompted me to add that there are servo libraries that will do the servo signal generation in hardware.

edited Dec 22, 2020 at 14:10

answered Dec 22, 2020 at 13:19

timemage

5,7191 gold badge15 silver badges27 bronze badges

1

I removed some material regarding I2C/Wire because this was probably completely useless advice with XBee being involved.

timemage
– timemage

2020-12-22 13:29:55 +00:00
Commented Dec 22, 2020 at 13:29
1

Maybe a servo library that doesn't use interrupts can be found also?

ocrdu
– ocrdu

2020-12-22 13:31:31 +00:00
Commented Dec 22, 2020 at 13:31
1

Yeah, I had a thought to add that, but don't off-hand know of one. If you have a recommendation I'll put it in, or if you put one in a separate answer I'll up-vote it.

timemage
– timemage

2020-12-22 13:33:06 +00:00
Commented Dec 22, 2020 at 13:33
1

Perhaps I'll add in arduino.cc/reference/en/libraries/servo-hardware-pwm if you add an answer, I'll remove it and specifically call out your answer.

timemage
– timemage

2020-12-22 13:35:50 +00:00
Commented Dec 22, 2020 at 13:35

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