Creating your own PWM to control a servo.

Other Hardware Motors, Mechanics, Power and CNC
1

Huy guys, I just got a package from Hobby King with servo's and they are HEXTRONIC HXT500 5gr minis'.

Iv'e tested them out with the Servo Library and they worked Perfetto! However i bought such little ones (they almost fit on your thumb!) for a special purpose. I want to make a light weight and little insect robot. If i want to build it and make it light then I have no option but remove the Arduino Uno from the project and use it only for prototyping. Instead, the robot's brain will be a Attiny 85.

The Attiny has a problem however, it doesn't support the Servo library for the Arduino. I know that there are Servo libraries for little Attinie's but I prefer to do the code by myself.

I wrote a code to control my servo by pulsing a pulse of between 1ms and 2ms and then a stop period of 19- 18ms (20 - 1 = 19 ms; 20 - 2 = 18 ms). The standard stop period is 20ms, which is 50HZ exactly suitable for the servo's. But the servo just sits still when i upload my sketch to my Arduino Uno. :frowning:

I have no idea how to alter the PWM signal from 500HZ (default) to 50HZ, so instead i decided to do it with the help of digitalWrite() and delay() functions.

Please suggest me how to create a suitable PWM signal for controlling a servo.

Here is the code:

int servo = 14;
 
void setup() { 
  
  pinMode(servo, OUTPUT); //OUTPUT setup
  
  for ( int i = 0; i > 5; i++){ // Set the possition to 90 degrees
   digitalWrite(servo, HIGH);
    delayMicroseconds(1500);
    digitalWrite(servo, LOW);
  }
  delay(2000);
} 
 
 
void loop() {
  for ( int i = 0; i > 50; i++){
    digitalWrite(servo, HIGH);
    delayMicroseconds(1000); // Set the possition to 0 degrees
    digitalWrite(servo, LOW);
    delay(19); 
  }
  for ( int i = 0; i > 50; i++){
    digitalWrite(servo, HIGH);
    delayMicroseconds(2000); //Set the possition to 180 degrees
    digitalWrite(servo, LOW);
    delay(18); 
  }
}

Thank you guys.

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2

? int servo = 14;

Have not looked at the pin def's for the tiny85 which is a 8 pin device but pin 14 sounds wrong.

3

I don't think he 50Hz rate is a requirement, particularly not a lower limit. I'd try just sending a pulse to each servo each time through loop(). If your servos don't like that you could use the BlinkWithoutDelay example to send pulses only every 20 ms.

4

for ( int i = 0; i > 5; i++){ // Set the possition to 90 degrees
digitalWrite(servo, HIGH);
delayMicroseconds(1500);
digitalWrite(servo, LOW);
}

this loop has no delay after digitalWrite(servo, LOW); so it is effectively a high for 7.5 ms ?

Are you familiar with the blink without delay sketch?
It is a way to do timing more precise and it removes the blocking delays.
You need several extra variables to hold state.

5

Hi,
If you decide to follow this approach, for each servo that you add, you will use up 10% of your processing power, it also means that you will not be able to respond to inputs while your timing code is running. For a small robot that moves slowly and has few sensors to respond to, it should be fine, but its worth mentioning these limitations.

Duane B

rcarduino.blogspot.com

6

There is this servo library made specific for the Attiny 45/85

http://www.cunningturtle.com/servo8bit-library-version-0-6-released/

7

Thank you for your responses guys! there were many helpful ones! :))

8

I succeded to control a servo on an attiny 84 and attiny 85 with the following code

//Tested on attiny 84 with servo towerpro microservo 90 SG90
// 08/jan/2014 servo data on www.servodatabase.com
int servoPin = 6; // servo connected to digital pin 6

void setup()
{
pinMode(servoPin, OUTPUT); // sets the servoPin to be an output
}

void loop() // run over and over again
{
servowrite(servoPin, 0, 500, 2400);
servowrite(servoPin, 90, 500, 2400);
servowrite(servoPin, 180, 500, 2400);
// servotest();
}

void servowrite(int pin, int angle, int microzero, int micro180)
{
// pin for servo, angle to move, microseconds for zero degrees, microseconds for 180 degrees
int microvalue= map(angle, 0, 180, microzero, micro180); //map angle to microseconds
int i;
for (i = 0; i < 50; i++) //Send 50 pwm pulses with width microvalue uS
{
analogWrite(servoPin,255); //Pulse high for microvalue uS
delayMicroseconds(microvalue);
analogWrite(servoPin,0); //Pulse low for 15mS
delay(15);
}
delay(10); //Pause
}

void servotest()
// test servo
{
int i;

for (i = 0; i < 50; i++) //Send 50 pulses with width 500 uS
{
analogWrite(servoPin,255); //Pulse high for 500uS
delayMicroseconds(500);
analogWrite(servoPin,0); //Pulse low for 15mS
delay(15);
}
delay(100); //Pause for 100mS

for (i = 0; i < 50; i++) //Send 50 pulses with width 1.25mS
{
analogWrite(servoPin,255);
//Pulse high for 1.4 mS
delayMicroseconds(1400);
analogWrite(servoPin,0);
delay(15);
}
delay(100);

for (i = 0; i < 50; i++) //Send 50 pulses with width 2.4ms
{
analogWrite(servoPin,255);
delayMicroseconds(2400);
analogWrite(servoPin,0);
delay(15);
}
delay(100);

}

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