Instructor: Paul Gordy Office: H-115 Phone: 822-7175 Email: PGordy@tcc

Instructor: Paul GordyOffice: H-115Phone: 822-7175Email: [email protected]

EGR 262Fundamental Circuits Lab

Presentation for Lab #5Pulse Width Modulation

1Lab 5 EGR 262 – Fundamental Circuits Lab

Pulse Width ModulationA periodic waveform can be described by v(t) = v(t + T) for some positive value of T, called the period of the waveform. The pulse waveform below is a periodic waveform with period T. The frequency, f, of the waveform is 1/T. T is measured in seconds and f is measured in Hertz.

0 TH T 2T 3T

V

v(t)

t

A pulse-width modulated (PWM) signal is one where TH can vary. The duty cycle, D, of the waveform is defined below. D is usually expressed as a percentage. VAVG or VDC is the average or DC value of the waveform.

TT D H

T1 f


DV V V DCAVG

Examples: Determine the period, frequency, duty cycle, and average value of each waveform below.

T = ____________F = ____________D = ____________VDC = ____________0 2 4 8

5V

v(t)

t (ms)6 10

0 4 20 40

5V

v(t)

t (us)24 44

0 65 80 160

5V

v(t)

t (ns)145 205


T = ____________F = ____________D = ____________VDC = ____________

T = ____________F = ____________D = ____________VDC = ____________

Applications: Pulse-width modulated signals are used in many applications. A few are listed below. Many devices essentially respond to the average value of the waveform.

Motor control. Motor speeds up or slows down as pulse width (or duty cycle) changes.

V

v(t)

t

Motor turns fast

V

v(t)

t

Motor turns slow

Microwave Oven. The microwave is essentially turned on and off as the power setting is changed.


V

t

80% power setting

V

v(t)

t

20% power setting (defrost)

v(t)

Servo control. A servo is a combination motor & gear box where the motor position can turn 0 to 180 degrees (typically) as pulse width varies. Servos are used for steering RC cars.

V

v(t)

t

Turn Left

V

v(t)

t

Turn Right

V

t

Capacitor charges to larger analog voltage

V

v(t)

t

Capacitor charges to smaller analog voltage

v(t)

Digital-to-Analog Converter. In Lab 6 we will improve on the DAC built earlier using an R-2R ladder network by using the PWM signal to charge a capacitor to the desired analog voltage.


Generating a PWM signal using the Arduino UNOPWM signals can be generated using the analogWrite( ) command:• form: analogWrite(pin, value)• Available on digital outputs 3,5,6,9,10,11 (note the ~ symbol on the UNO

below)• 0V to 5V square wave has duty cycle based on value• value varies from 0 (off) to 255 (on)• Uses set frequency of approximately 490 Hz• It is not necessary to use pinMode( ) to specify a pin as an output when it will

be used with analogWrite( )• PWM waveform will be maintained until the next occurrance of

analogWrite( ), digitalWrite( ), or digitalRead( ).• The analog value is the average

value or DC value of the waveform


DCAVG V V valueanalog

Example: Create a PWM signal on D11 with a 25% duty cycle.


ArduinoUNO

D11

0 0.51 2.04 4.08

5V

v(t)

t (ms)2.55 4.62

0VVDC = 1.255 V

V 1.255 (5V)(D) V Vms 0.512 ms) 04(0.251)(2. TD T so

, TT D

25.1%)(or 0.251 25564 D

ms 2.04 4901

f1 T

Hz 490 f

DCAVG

H

H

Example: (continued) The sketch on the previous slide was compiled and uploaded to an Arduino UNO in lab. The output on D11 was observed using an oscilloscope. The values shown are very close to the expected results.


f = 489.234 Hz

VDC = 1.29 V

1V/div so waveform is 0V to 5V

5 V

0 V

T = 2.044 ms

Ground (0V) for CH1

Example: (continued) The duty cycle, D, can be measured on another oscilloscope screen using two cursors. The values shown are very close to the expected results.


Input to program: value = 64D predicted by program: 25.1%D observed on oscilloscope:D = TH/T*100% D = 500us/2.044ms*100D = 24.46%

Cursor 1 Cursor 2

Delta = Cursor 2 – Cursor 1= TH = 800.0 us

f = 489.234 HzT = 1/f = 2.044 ms

Suppose that we wanted the user to enter the value for a PWM signal (from 0 to 255)? It is useful to begin with a little background on reading and writing using serial communication.


Serial Communication using the Arduino UNOSerial communication is used by the Arduino UNO to communicate with the computer and other devices.• Serial communication uses pins D0 (RX or Receive)

and D1 (TX or Transmit).• Serial communication takes place at a rate of 9600

baud (bits per second). The following command should be included in setup( ):

Serial.begin(9600);• Serial communication takes place via the USB cable

and requires no additional wiring.• The Arduino software has a built-in monitor for

displaying information sent to or from the computer.• Serial communication is also used when you compile

and upload a sketch to the Arduino UNO.

11Lab 5 EGR 262 – Fundamental Circuits LabSerial Communication (continued)Data is transmitted one byte (8 bits) at a time using serial communication. When a key is pressed on a keyboard, an 8-bit ASCII code is transmitted. The ASCII code for letter A is 010000012 = 4116 = 6510

If you enter the characters ABC on the keyboard, 3 bytes of information are transmitted to the Arduino UNO as illustrated below.

01000011 01000010 01000001

C B A

USB cable

12Lab 5 EGR 262 – Fundamental Circuits LabSerial Communication (continued)Several useful functions that work with Serial are:• Serial.write(x); // writes character corresponding to the ASCII code so the

argument should be from 0 to 255• Serial.print(x); // displays the corresponding symbol(s) based on the ASCII

value• Serial.println(x); // similar to print( ) but advances to the next line• Serial.print(“Text”); // displays the exact text shown in double quotes• Serial.write(“Text\n”); // same as the line above• \n and \t can be used within text for tabs and newlines, respectively

Some of these functions may not work as you would expect, so examples are shown on the following slide.

Example using Serial.print( ) and Serial.write( ):13Lab 5 EGR 262 – Fundamental Circuits Lab

Notes:• Serial.write( ) only worked correctly when the argument was text or a value

between 0 and 255. Using a floating point argument yields a compiler error.• Serial.print( ) worked correctly for text or numerical values, but rounds off

floating point values to 2 digits after the decimal point.

14Lab 5 EGR 262 – Fundamental Circuits LabSerial Communication (continued)Additional useful functions that work with Serial are:• Serial.available( ); // Used to check for input values from keyboard• Serial.read( ); // used to read one byte of information. The function returns

this value.• Serial.parseInt( ); // used to read an integer number (1 or more digits)• Serial.parseFloat( ); // used to read a floating-point number (1 or more

digits)

Some of these functions may not work as you would expect, so examples are shown on the following slide.

Example using Serial.available( ), read( ), write( ) and print( ):15Lab 5 EGR 262 – Fundamental Circuits Lab

Note: When 3 was entered on the keyboard, the ASCII code (001100112 or 5110 ) was read.

Enter inputs here and then click Send or press Enter

Let’s repeat the last example using a multiple digit input:16Lab 5 EGR 262 – Fundamental Circuits Lab

Notes:• When 345 was entered on the keyboard, each digit was read separately, so 3

different integers were read. We could try to build the number 345 from these digits (for example, int N = 3*100 + 4*10 + 5), but this is a lot of work.

• A better solution: Use the parseInt( ) function•

Let’s repeat the last example using a multiple digit input:17Lab 5 EGR 262 – Fundamental Circuits Lab

Note:• Much better. Serial.parseInt( ) allows us to correctly read an integer input

from the keyboard.

Example: Create a PWM signal where the value (from 0 to 255) can be entered by the user to set the duty cycle.


Now let’s return to our earlier topic of PWM. Recall that we wanted to enter the width of the pulse. We now know how to do it using parseInt( ).

If pin D11 was connected to an oscilloscope, we would see the waveform change every time a new value was entered.

Digital to Analog Conversion:Note that a PWM signal has a digital input (0 to 255) and has an analog output (or DC value), so it is essentially a Digital to Analog Converter (DAC).However, the output has a large ripple voltage, VR, which is not suitable for some applications.Example: An illustration is shown below for the last program (with value = 64):


0 0.51 2.04 4.08

5V

v(t)

t (ms)2.55 4.62

0VVDC = 1.255 V

ArduinoUNO

D11Digital Input= 6410

= 010000002

Analog Output= 1.255 VDC

(with VR = 5V or 398% )398% 100

1.2555 V

100%VV Vor

5V 0V - 5V VV - V V

R

DC

RR

R

minmaxR

Looking ahead to Lab 6In Lab 6 we will add an RC circuit to a PWM output and will be able to reduce the ripple significantly (we will design it for 10% max ripple voltage).

20Lab 5 EGR 262 – Fundamental Circuits Lab Arduino

UNO

D12

D11

D13 V0 = analog output2R

2R

2R

R

R

2R

ArduinoUNO

D11

V0 = analog output (PWM)

t

ArduinoUNO

D11V0 = analog output (PWM)

t

R

C

Lab 4: 3-bit R-2R DAC – only 8 analog values, but VR = 0

Lab 5: 6-bit DAC – 256 analog values, but VR = 5V (or hundreds of percent in some cases)

Lab 6: 6-bit DAC – 256 analog values with VR = 10% max

Lab 4

Lab 5

Lab 6

Documents

Instructor: Paul Gordy Office: H-115 Phone: 822-7175 Email: PGordy@tcc