Controlling a servo with an AVR microcontroller

Welcome to my first AVR tutorial on this site! We’ll be doing something basic today, namely controlling a servo. There are a lot of tutorials on how to control it with an Arduino, but less tutorials using only a bare AVR chip. In this tutorial we’ll be using the ATTiny44, a small and cheap microprocessor, which also contains a 16 bit timer, which will make our life a bit easier.

Servos are often used to move robot arms and things alike, because they can rotate a specific amount of degrees very precisely, depending on the pulsewidth you feed it with the microcontroller. They can also be used as a motor (you’ll need special ‘continuous rotation’ servos for that), you’ll often find them in RC cars.

So lets get started, and see how you actually control a servo!

Pulse width modulation

To move the servo, you need to send it a pulse. Depending on the duration of the pulse, the servo positions itself in a fixed position. This position is unique to the pulse duration. In other words, no matter what the current position of the servo is, it always rotates to the same position for a certain pulse duration. The following pulse durations result in the following positions:

• 1ms: 90 degrees to the left
• 1.5ms: Center position
• 2ms: 90 degrees to the right

For most servos the maximum frequency is around 50 Hz, which means a time period of 20ms. The servo is of course not limited to the above corners, the corner is proportional to the pulse duration, with a minimum of 0.7ms and a maximum of 2.3ms.

Implementing it with an AVR microcontroller

Lets start with the circuit, which is really simple.

In this case, I use a 9V battery to power my AVR, and 4 AA batteries (total of 6V), to power my servo. It’s better to use a different power source for your servos, because they can draw high currents when they’re rotating, and therefore possibly trigger your reset of the AVR resulting from the voltage drop.

I use the LM7805 to transform the 9V to 5V for the AVR, some capacitors to remove any AC components of the voltage, and a 4.7k resistor as a pull up for the reset pin. As you can see, nothing special here. More interesting is the source code running on the AVR.

AVR source code

/* -*- Mode: C; indent-tabs-mode: t; c-basic-offset: 4; tab-width: 4 -*- */
/*
 * main.c - Auto-generated by Anjuta's Makefile project wizard
 * 
 */

#ifndef F_CPU
#define F_CPU 1000000L
#endif

#include <avr/io.h>
#include <avr/interrupt.h>
#include <stdint.h>

#define SERVO_PWM_TOP 10000UL // 50 Hz PWM
#define SERVO_LEFT 500UL // Capture value to position the servo to the left (1.0ms)
#define SERVO_CENTER 750UL // Capture value to position the servo in the center (pulse width 1.5ms)
#define SERVO_RIGHT 1000UL // Capture value to turn the servo (pulse width 2ms)

int main(void)
{
	// Setup ports and timers
	DDRA = 0xFF; // All output
	PORTA = 0;

	// Configure timer/counter1 as phase and frequency PWM mode
	TCNT1 = 0;
	TCCR1A = (1 << COM1A1);
	TCCR1B = (1 << WGM13) | (1 << CS10);
	ICR1 = SERVO_PWM_TOP;
	OCR1A = SERVO_LEFT;

	while(1)
	{
	}
}

So, what are we doing here? We’re going to use the PWM hardware to generate the pulse we want. The ATTiny44 contains a 16 bit timer, which has a special mode called ‘Frequency and phase correct PWM’. Grab the ATTiny44 datasheet, and read about it.

To summarize what is does: you start with providing two values: a TOP value, and a compare value. The AVR will then count to the TOP value, and after it reached that, it counts back to zero. When the counter has the same value as in OCR1A, something will happen: in the case of up counting, it will reset the pin value of OCA1 to 0, and when down counting, it sets the pin value to 1. This behaviour can be changed by setting the right bits in the TCCR1A/B/C registers. Read the datasheet for more information.

To visualize a bit what’s happening, here’s a drawing:

Some things you can conclude from the above drawing and description: The value in ICR1 defines the frequency, the value in OCR1A defines the duty cycle.

Calculating the values for ICR1 and OCR1A

So why does the above code has 10000 as value for ICR1, and 500/750/1000 for OCR1A? With a few simple calculations, you can get the values you need:

1. Check at which clockfrequency your AVR runs, in our case it’s 1 MHz.
2. Because it runs on 1 MHz, if we had an unlimited number of bits for our counter, it would count to 1000000 in one second.
3. Almost all servos operate at a frequency of 50 Hz, which means a time period of 20 ms.
4. You know this: 1000000 for one second, so 1000000 * (20*10^-3) = 20000 counts for 20 ms.
5. In phase and frequency correct mode, it counts up AND down, to we devide the above number by 2, which results in 10000, the value for ICR1. This is why a 16 bit timer is useful, because 10000 easily fits in 16 bits, and definitely not in 8 bits.

The same can be done to position the servo at the left (a pulse of 1 ms), or at the right (a pulse of 2 ms).

Please note that the above code will constantly pulse every 20 ms. If the servo is already in the right position, it will not move. And it’s probably better to disable the timer, when you’ve sent the pulse. That’s left as an exercise for the reader. ;)

Troubleshooting

• In general, it’s better to use a separate power source for the servos, to avoid triggering the reset of the AVR. But remember: the grounds of each power source should be connected!
• If you want your servo to rotate continuously, remember to buy a servo which is capable of doing that. Most servos can’t rotate a full 360 degrees. However, there are mods to modify a non-continuous rotation servo to one that can rotate the full 360 degrees. Using Google, you can find a lot of tutorials to do that. And it’s quite easy.

Thanks for reading, and I hope you enjoyed the article!