Session 9: servo motor, PWM, and ADC

Goal

Bonus

TODO: fill

Servo Motor

Servo motor is a type of electeric motor that is designed for precise control. It is widely used in robotics. We can control the position of the servo motor by sending a PWM signal to it. In this session, we will be using a simple DC servo motor wich only take angles in range of [-90, 90]. -90 when we send 1000us pulse, and 90 when we send 2000us pulse. The desired PWM frequency for this servo motor is 50Hz. This servo motor has three pins: VCC, GND, and signal. We are connecting those pins like this:

• VCC -> 5V (Fixed voltage)

• GND -> GND

• Signal -> PD5 of ATmega32

PWM

PWM (Pulse Width Modulation), is a technique for controlling the power delivered to a component.
In this technique we use different width of pulses in a signal. These signals switch between 0 and 1. The percentage of the time that a pulse is 1 is called duty cycle. The pictures below show two examples of 100Hz PWM, one with the 30% duty cycle and the other 60%.

Create PWM on ATmega32

We can create PWM using Timers on ATmega32. We have three timers on Atmega32:

• Timer0: 8bit

• Timer1: 16bit

• Timer2: 8bit

We can create PWM with all three timers. As you can see on the picture below, each corresponding pin for each timer is shown.

• Timer0:

  • OC0 -> PB3

• Timer1:

  • OC1A -> PD4

  • OC1B -> PD5

• Timer2:

  • OC2 -> PD7

In Timer0 and Timer2 for generating the desired frequency we only can use a prescaler which could only take values of: [1, 8, 64, 256, 1024]. As you can see this numbers are limiting. But for Timer1 in addition to a prescaler, we have another variable called ICR1 that can be used to control the desired freqeuency which we are going to explain more about it later. So to generate the desired frequency for our servo motor we will be using Timer1.

Timer1

As we discussed in the previous session, we have three timers in Atmega32. The only timer that is 16bit is Timer1. We have some essential registers that we should use them to control the timer. Also to put Timer1 in PWM mode, we need these registers that we are going to talk about them.

TCNT1A, TCNT1B

These registers are used to store the current value of the timer.

OCR1A, OCR1B

These registers are used to compare the value of the timer with them. We can use these registers in different modes, specially in PWM mode to control the duty cycle.

ICR1

This register is used to control the frequency of the timer. In Fast PWM mode, we have this formula:

\[ f_{PWM} = \frac{f_{clk}}{N \times (1 + ICR1)} \]

• \(f_{PWM}\): desired frequency

• \(f_{clk}\): clock frequency

• \(N\): prescaler

• \(ICR1\): value of ICR1 register

TCCR1A and TCCR1B

Timer/Counter Register Description (TCCR) is a way that we control our timer setup. For Timer1 we have two registers TCCR1A and TCCR1B.

TCCR1A	7	6	5	4	3	2	1	0
name	COM1A1	COM1A0	COM1B1	COM1B0	FOC1A	FOC1B	WGM11	WGM10
Read/Write	R/W	R/W	R/W	R/W	R/W	R/W	R/W	R/W
initial value	0	0	0	0	0	0	0	0

TCCR1B	7	6	5	4	3	2	1	0
name	ICNC1	ICEC1	-	WGM13	WGM12	CS12	CS11	CS10
Read/Write	R/W	R/W	R/W	R/W	R/W	R/W	R/W	R/W
initial value	0	0	0	0	0	0	0	0

COM1A1/COM1B1	COM1A0/COM1B0	Description (in fast PWM mode)
0	0	Normal port operation, OC1A/OC1B disconnected.
0	1	WGM13:0 = 15: Toggle OC1A on Compare Match, OC1B disconnected (normal port operation). For all other WGM13:0 settings, normal port operation, OC1A/OC1B disconnected.
1	0	Clear OC1A/OC1B on compare match, set OC1A/OC1B at BOTTOM, (non-inverting mode)
1	1	Set OC1A/OC1B on compare match, clear OC1A/OC1B at BOTTOM, (inverting mode)

Because we need to clear OC1A which is connected to PD6 get cleared every time it reaches the value of OCR1A, we put it in the non-inverting mode.

TCCR1A |= (1 << COM1A1)

Mode	WGM13	WGM12 (CTC1)	WGM11 (PWM11)	WGM10 (PWM10)	Timer/Counter Mode of Operation	TOP	Update of OCR1x	TOV1 Flag Set on
0	0	0	0	0	Normal	0xFFFF	Immediate	MAX
1	0	0	0	1	PWM, Phase Correct, 8-bit	0x00FF	TOP	BOTTOM
2	0	0	1	0	PWM, Phase Correct, 9-bit	0x01FF	TOP	BOTTOM
3	0	0	1	1	PWM, Phase Correct, 10-bit	0x03FF	TOP	BOTTOM
4	0	1	0	0	CTC	OCR1A	Immediate	MAX
5	0	1	0	1	Fast PWM, 8-bit	0x00FF	BOTTOM	TOP
6	0	1	1	0	Fast PWM, 9-bit	0x01FF	BOTTOM	TOP
7	0	1	1	1	Fast PWM, 10-bit	0x03FF	BOTTOM	TOP
8	1	0	0	0	PWM, Phase and Frequency Correct	ICR1	BOTTOM	BOTTOM
9	1	0	0	1	PWM, Phase and Frequency Correct	OCR1A	BOTTOM	BOTTOM
10	1	0	1	0	PWM, Phase Correct	ICR1	TOP	BOTTOM
11	1	0	1	1	PWM, Phase Correct	OCR1A	TOP	BOTTOM
12	1	1	0	0	CTC	ICR1	Immediate	MAX
13	1	1	0	1	Reserved	–	–	–
14	1	1	1	0	Fast PWM	ICR1	BOTTOM	TOP
15	1	1	1	1	Fast PWM	OCR1A	BOTTOM	TOP

We need to put it on Fast PWM mode when we can control the frequency using ICR1. So we have:

TCCR1A |= (1 << WGM11);
TCCR1B |= (1 << WGM12) | (1 << WGM13);

CS12	CS11	CS10	Description
0	0	0	No clock source (Timer/Counter stopped).
0	0	1	clkI/1 (No prescaling)
0	1	0	clkI/8 (From pre-scaler)
0	1	1	clkI/64 (From pre-scaler)
1	0	0	clkI/256 (From pre-scaler)
1	0	1	clkI/1024 (From pre-scaler)
1	1	0	External clock source on T1 pin. Clock on falling edge.
1	1	1	External clock source on T1 pin. Clock on rising edge.

To make our calculation easier, we put our prescaler to 8 using the code below:

TCCR1B |= (1 << CS11);

Setup our simulation

We do the following steps to setup our simulation for pwm.

• Put an ATmega32 on the board.(make sure to connect its reset to a fixed volatage.)

• set the clock frequency to 8MHz (in properties).

• Then put a servo motor from Outputs/Motors/Servo Motor on the board.

• Connect the V+ pin of the servo motor a 5V (fixed voltage).

• Connect the GND pin of the servo motor to the ground.

• Connect the Signal pin of the servo motor to the PD5 of the ATmega32.

Now our setup ready to write our code. But to make sure that we are making the right frequency and to see the pulses we also add an oscilloscope to our setup.

• Put an oscope from Meters/Oscope on the board.

• Connect the CH1 of the oscope to the PD5 of the ATmega32.

• Connect the GND of the oscope to the ground.

So you are going to have something like this:

Write our code

So at first let’s calculate the value of ICR1 to get the desired frequency. We know our clock frequency is 8MHz and the desired frequency is 50Hz. (Make sure to put board_build.f_cpu = 8'000'000L in the platformio.ini file.) Also we put the prescaler to 8. So we have:

\[\begin{split} f_{PWM} = \frac{f_{clk}}{N \times (1 + ICR1)} \\ \Rightarrow 50 = \frac{8 \times 10^6}{8 \times (1 + ICR1)} \\ \Rightarrow 50 = \frac{10^6}{1 + ICR1} \\ \Rightarrow 1 + ICR1 = 20,000 \\ \Rightarrow ICR1 = 19,999 \end{split}\]

Now we can write a function to intitiate our pwm.

void pwm_init()
{
  // Set PD5 as output
  DDRD |= (1 << PD5);

  // Make sure that there is no setup other than the one we are going to put
  TCCR1A = 0;
  TCCR1B = 0;

  // non-inverting mode
  TCCR1A |= (1 << COM1A1);

  // Fast PWM mode
  TCCR1A |= (1 << WGM11);
  TCCR1B |= (1 << WGM12) | (1 << WGM13);

  // Set prescaler to 8
  TCCR1B |= (1 << CS11);

  // Set the frequency to 50Hz
  ICR1 = 19'999;
}

Then we can control our servo motor using OCR1A. So OCR1A=1000 would be -90 and OCR1A=2000 would be 90.

ADC

ADC (Analog to Digital Converter) converts an analog signal to a digital signal. We used to have only 1 or 0 in the digital world. For example if our highest voltage was 5V, we could only have 5V or 0V. But with ADC we can have a range of values between 5V and 0V. In ATmega32 the resolution of the ADC is 10bit. So we can have 1024 different values between 5V and 0V. The formula to calculate the value of the analog signal is:

\[ V_{digital} = \frac{V_{analog} \times 1023}{V_{ref}} \]

If our reference voltage would be 5V and we want to calculate digital voltage when our analog voltage is 5V, we have:

\[ V_{digital} = \frac{5 \times 1023}{5} = 1023 \]

And for 0V we have:

\[ V_{digital} = \frac{0 \times 1023}{5} = 0 \]

We have some registers that we should set in order to get the desired value from the ADC.

ADMUX

ADC Multiplexer Selection Register (ADMUX) is used to select the channel of the ADC.

ADMUX	7	6	5	4	3	2	1	0
name	REFS1	REFS0	ADLAR	MUX4	MUX3	MUX2	MUX1	MUX0
Read/Write	R/W	R/W	R/W	R/W	R/W	R/W	R/W	R/W
initial value	0	0	0	0	0	0	0	0

REFS1	REFS0	Description
0	0	AREF, Internal Vref turned off
0	1	AVCC with external capacitor at AREF pin
1	0	Reserved
1	1	Internal 2.56V Voltage Reference with external capacitor at AREF pin

We should set the reference voltage to AVCC using the code below:

ADMUX |= (1 << REFS0);

• ADLAR: ADC Left Adjust Result

MUX4	MUX3	MUX2	MUX1	MUX0	Single Ended Input
0	0	0	0	0	ADC0
0	0	0	0	1	ADC1
0	0	0	1	0	ADC2
0	0	0	1	1	ADC3
0	0	1	0	0	ADC4
0	0	1	0	1	ADC5
0	0	1	1	0	ADC6
0	0	1	1	1	ADC7

By setting the MUX bits we can select the channel of the ADC. For example if we want to select ADC1 we should set the MUX bits to 00000 using the code below:

ADMUX &= 0xF0;
ADMUX |= 1;

ADCSRA

ADC Control and Status Register A (ADCSRA) is used to control the ADC.

ADCSRA	7	6	5	4	3	2	1	0
name	ADEN	ADSC	ADATE	ADIF	ADIE	ADPS2	ADPS1	ADPS0
Read/Write	R/W	R/W	R/W	R/W	R/W	R/W	R/W	R/W
initial value	0	0	0	0	0	0	0	0

• ADEN: ADC Enable

• ADSC: ADC Start Conversion

• ADATE: ADC Auto Trigger Enable

• ADIF: ADC Interrupt Flag

• ADIE: ADC Interrupt Enable

ADPS2	ADPS1	ADPS0	Division Factor
0	0	0	2
0	0	1	2
0	1	0	4
0	1	1	8
1	0	0	16
1	0	1	32
1	1	0	64
1	1	1	128

To enable the ADC we should set the ADEN bit to 1 using the code below:

ADCSRA |= (1 << ADEN);

Then we set the prescaler to 128 using the code below:

ADCSRA |= (1 << ADPS0) | (1 << ADPS1) | (1 << ADPS2);

ADC

This variable is 10bit and is used to store the value of the ADC. It contains of two registers ADCL and ADCH.

Connect a potentiometer

In order to create different voltages we can use a potentioemeter.

• Put a potentiometer from Passive/Resistors/Potentiometer on the board.

• Connect the V+ of the potentiometer to the 5V (fixed voltage).

• Connect the GND of the potentiometer to the ground.

• Connect the Signal of the potentiometer to the ADC0 of the ATmega32.

To see what voltage we are making we can add a voltmeter to our setup.

• Put a voltmeter from Meters/Voltmeter on the board.

• Connect the red pin of the voltmeter to the fixed voltage connected to the potentiometer.

• Connect the back pin right beside the red pin to the ADC0 of the ATmega32.

Now we need to connect our reference voltage on ATmega32.

• Connect the AREF of the ATmega32 to the 5V (fixed voltage).

• Connect the AVcc of the ATmega32 to the 5V (fixed voltage).

So you are going to have something like this:

Write our code for ADC

To initialize the ADC we can write the following function:

void adc_init()
{
    // Set reference voltage to AVCC
    ADMUX |= (1 << REFS0);

    // Enable ADC
    ADCSRA |= (1 << ADEN);

    // Set prescaler to 128
    ADCSRA |= (1 << ADPS0) | (1 << ADPS1) | (1 << ADPS2);
}

To read the value of the ADC we can write the following function:

uint16_t adc_read(uint8_t channel)
{
    // Set the channel
    ADMUX &= 0xF0;
    ADMUX |= channel;

    // Start the conversion
    ADCSRA |= (1 << ADSC);

    // Wait for the conversion to finish
    while (ADCSRA & (1 << ADSC))
        ;

    // Retrun the value
    return ADC;
}

Now we can use the adc_read function to read the value of the ADC (which is between 0 and 1023). Then cast it to 1000 to 2000 and put it in the OCR1A to control the servo motor. (we can use map function to do this.)