Samuel Farayola Kolawole1, D.S. Nyitamen1, M.K Barau2 ...

Design and Development of Low-Cost …………….. Samuel Farayola.Kolawole, D.S. Nyitamen, M.K Barau

website: www.academyjsekad.edu.ng

DESIGN AND DEVELOPMENT OF LOW-COST HANDHELD OSCILLOSCOPE

Samuel Farayola Kolawole1, D.S. Nyitamen1, M.K Barau2 1Department of Electrical//Electronic Engineering, Nigerian Defence Academy, Kaduna

2Kaduna Refining and Petrochemical Company, Kaduna

Abstract

The paper presents the design and development of a portable and-held digital oscilloscope using Atmel

Atmega644p Microcontroller Unit operating at 20MHz & 20MIPS on a mono chrome Graphical Liquid

Crystal Display. The oscilloscope has two channels through which the controller samples the analog input

signals. AC & DC signals are fed to the controller’s ADC for conversion into its digital equivalents. The

sampled signals undergo processes of pre-scaling, filtration, storage, reconstruction and then displayed on the

GLCD. The microcontroller is programmed using BASCOM-AVR, a BASIC programming language-based

Integrated Development Environment (IDE) The Oscilloscope can display the scaling factor used in readable

format in Volts/div for the vertical axis and Time/div for the horizontal axis. It can also display measured

signals from the two channels simultaneously and has channels offset control and trigger mode adjusting. The

handy Oscilloscope has the capability of displaying the measured signal frequency value in the range of 0-

250kHz. Some signal types were measured and compared with the laboratory oscilloscope and produced an

error of less than 5%. The cost of the hand-held oscilloscope was N12,410, which is less than $50, hence it is

low-cost compared with commercially available digital oscilloscope.

Keywords: A/D Conversion, ATMega644P Microcontroller, Oscilloscope, Pre-scaling, Sampling,

Transducer.

1.0 INTRODUCTION

In many applications, observing certain

voltage waveforms in a circuit plays a crucial

role in understanding the operation of the

circuit. For that purpose several measuring

instruments are used like voltmeter, ammeter,

or the oscilloscope. An oscilloscope is a

voltage sensing electronic instrument that is

used to visualize certain voltage waveforms

(Banf, 2011). An oscilloscope displays the

variation of a voltage waveform in time on the

oscilloscope’s screen graphically. Digital

oscilloscopes are quickly replacing Cathode

Ray Oscilloscopes (CRO) because of their

inherently high input impedance and small

size. (Chandan, 2004). Most oscilloscopes

have at least two input channels and each

channel can display a waveform on the screen.

Multiple channels are useful for comparing

waveforms. For example, one can observe the

voltage waveforms at the input and the output

terminals of a circuit simultaneously, by using

a two channel oscilloscope.

Special-purpose oscilloscopes may be used for

such purposes as analysing an automotive

ignition system or to display the waveform of

the heartbeat as in Electrocardiogram.

Cathode Ray Oscilloscopes are expensive and

bulky, there is always a need to develop

systems that are portable. Celma (1992),

Chandan (2016) and Shilpa (2016) developed

PC-based Oscilloscopes which can be used

instead of the expensive CRO, however, these

will require a computer. Recent research

efforts are on developing small size

oscilloscopes. (Additi, 2014; Bhagashree

2015;Hua. 2011; Ishitak 2014; Shepherd 2001;

Yogesh 2014.)

This paper is aimed at designing and

developing a low cost Microcontroller-based

‘Hand-held Oscilloscope’, using Graphic

Liquid Crystal Display (GLCD) to display the

output.

http://en.wikipedia.org/wiki/Electrocardiogram


Academy Journal of Science and Engineering 10 (1), 2016 Page 53

2.0 METHODOLOGY

Fig.1 shows the block diagram for the

handheld Oscilloscope.A dc source is used to

power the system. The input signals on two

channels are voltage signals. If the physical

quantity is not in voltage form,

transducers/sensors would have been used to

convert the physical quantity into a voltage

signal. Pre-scaling and filtering takes place on

the signal before it is fed into an ATmega644P

8-bit AVR Microcontroller. The micro-

controller has ADC which samples and codes

the analog signal. The microcontroller sends

digital form of the analog signal to the GLCD

where the signal is reconstructed and

displayed. A Key-pad is attached to the

microcontroller for the purpose of choosing

channel and scale and other operations of

moving the waveform up-down or left-right.

2.1 HARDWARE DESIGN

2.1.1 Input Signal Attenuator/Scaler

The attenuation of any input signal voltages

particularly those with higher amplitude

voltages was achieved using the combination

of input resistors network providing potential

divider arrangement with CMOS analog

multiplexer IC [TC4052B] (U1,U2).

Toshiba TC4052B is a high speed, differential

4-channel CMOS analog Multiplexer/De-

multiplexer ICthat has two binary control

inputs (A and B) and an inhibitinput. The two

binary input signals select one of four pairs

(X0Y0, X1Y1, X2Y2, X3Y3,) of channels to be

turned on and connect the analoginputs to the

respective outputs (X-out & Y-out).

Also anotheranalog multiplexer IC (U2)

provides another function of switching DC/AC

signals. The analog multiplexer IC requires a

voltage supply of ±5V in order to respond to

the AC waveform.

Input

Fig. 1 Block Diagram of Handheld Oscilloscope

Micro

controller Unit

Analog-to-

Digital

Converter GLCD Controller

Driver

Graphic LCD

Screen

I/P Signal Attnuator

Power Supply Unit

Crystal Oscillator

Key Board Unit



2.1.2 Channel Selection and Scaling

In order to provide a calibrated trace on the

display, input attenuators must have very

accurate resistive networks that have a flat

response over the whole oscilloscope

bandwidth. Three pairs of potential divider

arrangements of resistors were established at

the oscilloscope input for scaling the signals

connected across the probes. The control

inputs A, B of the TC4052B analog

multiplexer IC are connected to the

microcontroller which select the needed (train

of potential divider) factor to scale input

signal.

Table 1 Truth Table of TC4052B IC (U1)

X=Don’t care

The combination of the resistors with

TC4052Banalog multiplexer switch is to

configure the attenuator input sensitivity to

supports 10V, 25V and 50V on a scale factors

of 1/2, 1/5, and 1/10 respectively, whereas the

pre-scaling for lower input voltages 0.25V,

0.5V & 1V is done through the software.

2.1.2.1 DC Input Signal Scaling (Channel X)

The input signal to be measured under goes a

pre-scaling, (Fig. 2) before it is fed to the

microcontroller’s ADC. The resistors network

formed three (3) independent potential divider

configurations, with each connected to the (3

out of 4) of Multiplexer’s channels to the

ground via the IC’s internal resistance (Ron)

and the common output.

For input X3

Fig. 2 Pre-Scaling Resistor Divider

Control Inputs Output Channel Scale Factor

B A

(PA0) (PA1)

INH XoutYout

0 0 0 X0 , Y0 NC

0 1 0 X1 , Y1 X 1/2

1 0 0 X2 , Y2 X 1/5

1 1 0 X3 , Y3 X 1/10

XX 1 None, None None

TC4052B

PA1 PA0

To PA4, ADC

R1

R2

X com

X1

X2

X3

Signal input (V)

Ron

1

10

50V max



Using a scale factor of 1

10

Let R1 be = 10 kΩ

And Ronfrom TC4052B data sheet was given

to be 60Ω (at 5V, 40oC)

Therefore,

Using Voltage Divider Rule, 1

10𝑉 =

R2 + Ron

R1 +R2 + Ron𝑉

R2 = 1051.11Ω ≈ 1051Ω or 1.051kΩ, i.e.

1000Ω + 51Ω

For input X2

Let the scale factor be = 1

5

R2 = 2440Ω ≈ 2400Ω or 2.4kΩ (the nearest

standard value).

For input X1

Let the scale factor be = 1

2

R4 = 9940Ω ≈ 10000Ω or 10kΩ (being the

nearest standard value).

2.1.2.2 DC Input Signal Scaling (Channel Y)

The same analysis applied for oscilloscope

channel 2 using Y- channel of the analog

Multiplexers Q1 and Q2, for the D.C. and A.C

waveforms. The exception here is that, Ycom

output of IC Q2 goes to microcontroller port

PA5, (See the truth table above).

2.1.2.3AC/DC Signal Selection (Channel X),

IC[TC4052B] U2

In order to read and reconstruct A.C.

waveform another multiplexer [TC4052B]

(U2) is used via capacitor connected in series,

and also for A.C. / D.C. selection. For D.C.

input signal the MUX is switched to by-pass

the capacitor, while for the A.C. signal type it

goes straight through the capacitor. In addition

to the AC input signal, the MUX was also

equipped with a fixed offset voltage of 2.5V

via a 1.0 MΩ resistor for current limiting.

Table 2 is the truth table for the selection.

Table 2: Truth Table for AC/DC selection, TC4052B IC (U2)

X = Don’t care

2.1.3Analog-to-Digital Converter ADC

In the real world, most data is characterized by

analog signals. In order to manipulate the data

using a microprocessor, we need to convert

the analog signals to its digital equivalent, so

that the microprocessor will be able to read,

understand and manipulate the data.3The main

goal of A/D Converter is to digitize the analog

signals, which means to read, store and

produce at its output the analog signals in

numbers (binary).

2.1.3.1Sampling Ratefs

The analog signal is continuous in time and it

is necessary to convert this to a flow of digital

values. It is therefore required to define the

rate at which new digital values are sampled

from the analog signal. The rate of new values

is called the sampling rate or sampling

frequency of the converter (the number of

samples taken in a second).

Control Inputs Output Channel Scale Factor

B A

(PA2)(PA3)

INH XoutYout

0 0 0 X0 , Y0 XDC, YDC

0 1 0 X1 , Y1 Xoffset , YDC

1 0 0 X2 , Y2 XDC , Yoffset

1 1 0 X3 , Y3 Xoffset , Yoffset

XX 1 None, None None

http://en.wikipedia.org/wiki/Continuous_function


Academy Journal of Science and Engineering 10 (1), 2016 6

A continuously varying, band limited signal

can be sampled (that is, the signal values at

intervals of time T, the sampling time, are

measured and stored) and then the original

signal can be exactly reproduced from the

discrete-time values by an inter-

polation formula. The accuracy is limited by

quantization error. However, this faithful

reproduction is only possible if the sampling

rate is higher than twice the highest frequency

of the signal. This is essentially what is

embodied in the Shannon-Nyquist sampling

theorem

2.1.4 Selection of the Microcontroller

The ATmega644 AVR microcontroller was

chosen for the microcomputer unit of this

work instead of a PIC (Programmable

Interface Controller) or Arduino module due

to its wide range of applications such as higher

memory size and better processing speed (one

instruction/clock cycle), pulse width

modulation (PWM) capabilities, digital signal

processing (DSP), multiple I/O ports (up to 5

I/O ports),. Other factors that have driven the

selection of the AVR microcontroller

(Atmega644P) are low cost, versatility, ease of

programming, and small size. The

ATmega644P architecture is based on the

Reduced Instruction Set Computer (RISC)

concept which allows the processor to

complete 20 million instructions per second

(MIPS) when operating at 20MHz.

Atmel's AVR microcontrollers use a new

RISC architecture which has been developed

to take advantage of the semiconductor

integration and software capabilities of the

1990's. The resulting micro controllers offer

the highest MIPS/mW capability available in

the 8-bit micro controllers market today.

The architecture of the AVR microcontrollers

was designed together with C-language

experts to ensure that the hardware and

software work hand-in-hand to develop a

highly efficient, high-performance code. The

family of AVR microcontrollers includes

differently equipped controllers - from a

simple 8-pin microcontroller up to a high-end

microcontroller with a large internal memory.

2.1.5 The Key Board Unit

The normal execution of the program follows

a designated sequence of instructions.

However, some times this normal sequence of

events must be interrupted to respond to high

priority faults and status both inside and

outside the micro controller. When these

higher priority events occur, the

microcontroller must temporarily suspend

normal operation and execute event specific

actions called an interrupt service routine.

Once the higher priority event has been

serviced, the microcontroller returns and

continues processing the normal program.

Those higher priority interrupts and other

unconditional calls are provided by the use of

some set of press button switches called the

Key Board Unit. The microcontroller used in

this project is equipped with a complement of

26 interrupt sources. Two of the interrupts are

provided for external interrupt sources while

the remaining interrupts support the efficient

operation of peripheral subsystems aboard the

microcontroller.

2.1.6 The Graphic Liquid Crystal Display

(GLCD) JHD12864E Unit

The graphical LCD type chosen for this work

was the JHD12864E. It is a 128 x 64 pixel

graphical LCD with backlight and driven by

two 64 x 64 pixel Samsung NT7108C and

NT7107C display drivers. This type was

chosen especially for two reasons; its low cost,

and its NT7108/07 drivers can be easily used.

Instruction is received from MPU to NT7108

for the display control. The JHD12864E

Liquid Crystal Display is basically driven by

three categories of display drivers and

controllers. These ICs control the complex AC

voltage requirements for the LCDs, and they

need an LCD controller to keep refreshing the

individual pixel information to their drive

circuitry.

The schematic diagram of the low-cost

handheld oscilloscope is as shown in Fig. 3.



Fig. 3 Schematic Circuit Diagram of the Handheld Oscilloscope

2.2 SOFTWARE DESIGN

2.2.1 The System Software

The assembly language used to programme

the ATmega644 consists of 135 commands

that control all functions of the

microcontroller. BASCOM allows one to mix

BASIC with assembly commands. This can be

very useful in some situations when one need

full control of the generated code. Almost all

assembly mnemonics are recognized by the

compiler. The exceptions are: SUB, SWAP

and OUT. These are BASIC reserved words

and have priority over the ASM mnemonics.

To use these mnemonics precede them with

the ‘!’sign.

On power ON, the MCU loads and executes

the first commands on the program register

which in this case is initialisation of the

system followed by loading a welcoming

message.

Constantly the processor scans/reads the

keyboard port register and branches to the

subroutine of the particular key depressed and

executes the command found there.

If we recall in the hardware design, we

computed resistance values to be used in

potential divider mode for scaling of the input

signal voltage/amplitude at different scale

factors, and that selection of the scale factor

channels were to be achieved by the use of a

Multiplexer IC [TC4052B] (U1). The control

pins B & A of multiplexer for this selection

are connected to the bits 0 & 1 of Port A (PA0

& PA1) respectively. Fig 4 shows the flow-

chart of the program.

PC6/TOSC1/PCINT2225

PC5/TDI/PCINT2124

PC4/TDO/PCINT2023

PC3/TMS/PCINT1922

PC2/TCK/PCINT1821

PC1/SDA/PCINT1720

PC0/SCL/PCINT1619

AVCC27

AREF29

PC7/TOSC2/PCINT2326

PA6/ADC6/PCINT631

PA5/ADC5/PCINT532

PA4/ADC4/PCINT433

PA3/ADC3/PCINT334

PA2/ADC2/PCINT235

PA1/ADC1/PCINT136

PA0/ADC0/PCINT037

PA7/ADC7/PCINT730

PB6/MISO/PCINT142

PB5/MOSI/PCINT131

PB4/SS/OC0B/PCINT1244

PB3/AIN1/OC0A/PCINT1143

PB2/AIN0/INT2/PCINT1042

PB1/T1/CLKO/PCINT941

PB0/XCK0/T0/PCINT840

PB7/SCK/PCINT153

PD6/ICP/OC2B/PCINT3015

PD5/OC1A/PCINT2914

PD4/OC1B/XCK1/PCINT2813

PD3/INT1/TXD1/PCINT2712

PD2/INT0/RXD1/PCINT2611

PD1/TXD0/PCINT2510

PD0/RXD0/PCINT249

PD7/OC2A/PCINT3116

RESET4

XTAL18

XTAL27

U3ATMEGA644P

CS

11

CS

22

GN

D3

VC

C4

V0

5

RS

6

R/W

7

E8

DB

09

DB

110

DB

211

DB

312

DB

413

DB

514

DB

615

DB

716

RS

T17

-Vo

ut

18

LCD1AMPIRE128X64

C110u

X012

X114

X215

X311

Y01

Y15

Y22

Y34

A10

B9

INH6

X13

Y3

U1

4052

X012

X114

X215

X311

Y01

Y15

Y22

Y34

A10

B9

INH6

X13

Y3

U2

4052

R11M

R210k

R31k

R42.4k

R510k

R6

10

0k

R7100k

R81M

R91M

R10100k

R111M

R1210k

R131k

R142.4k

R1510k

C2

100n

C3100n

C4100n

D1BAT43W

D2BAT43W

D3BAT43W

+5V

-5V

+5V

-5V

D4BAT43W

+5V

D5BAT43W

+5V

-5V

D6BAT43W

D7BAT43W

+5V

-5V

D8BAT43W

RV110K

RV210K

+5V

TRIGGER

LEVEL

CHANNEL

OFFSET

CHANNEL 1

CHANNEL 2

C522p

C622p

X1

CRYSTAL

+5V

RV3

10K

-5V

+5V

R1610K

L110u

C7100n

Reset

SW1

SW-SPDT

Display

Ch1/Ch2

SW1

SW2

SW3

SW4

SW5

SW6

SW7

SW8

SW9

SW9

Auto/Single

Run

READY

(Single)

SLOPE + / -

TIME/DIV

[SLOW]

TIME/DIV

[FAST]

VOLT/DIV

[DOWN]

VOLT/DIV

[UP]DC

/AC

SE

LE

CT

CH1

CH2

D9

BAT43W

D10BAT43W

RE

AD

FR

EQ



No Yes

No

Load 1st Screen Message

SRAM space all occupied?

Clear Memory Space

Stop

Any Call from Keyboard?

Send Fetched data to GLCD Drivers

Increment SRAM Addr. by 1

Start

Initialise System

Configure Ports, Timers & ADC

Start ADC Conversion

Execute Key board call

Fetch stored data array for reconstruction & Display

Process Converted Signal & Store to SRAM

A

A

Yes

Fig. 4: System Flow Chart



3.0 RESULTS AND DISCUSSION

Plate 1 shows the designed and constructed

hand held oscilloscope before packaging. Plate

2-4 show the displays of various waveforms as

measured by the constructed Oscilloscope

alongside the ones as measured and displayed

by the conventional Oscilloscope (CRT) in the

laboratory to read and compare the amplitude

voltage as well as the frequency.

1

Plate 1 Constructed Handheld Oscilloscope

3.1.3.1 Triangular Waveform

Plate 2: Display of Triangular Waveforms on the constructed and Laboratory oscilloscopes

Voltage and frequency reading from CRO

V = 5V/div x 3.1div = 15.5Vp-p

Frequency = 1/(5x1x10-3) = 200Hz

Voltage reading from handheld oscilloscope

Frequency = 1/(2.4x2x10-3) = 208.33Hz

Comparing the handheld and CRO % error is

as follows

%Error voltage reading = ((16-15.5)/16) x 100

= 3.15%

% Error in frequency reading = ((208-

200)/200)x100=4.16%



3.1.3.2 Square Waveform

Plate 3: Display of Square Waveforms on the constructed and Laboratory oscilloscopes


V = 5V/div x 1.5div = 7.5Vp-p

Frequency = 1/(5.2x20x10-3) = 9.615Hz


V=2V/div x 3.7div = 7.4V

Frequency = 1/(2.3x50x10-3) = 8.696Hz


as follows

%Error voltage reading = ((7.7-7.4)/7.5) x 100

= 1.33%

% Error in frequency reading = ((9.615-

8.696)/9.615)x100=9.56%

3.1.3.3 Sinusoidal Waveform

Plate 4: Display of Sinusoidal Waveforms on the constructed and Laboratory oscilloscopes


V =10V/div x 3.6div =36Vp-p


V =10V/div x 3.8div =38Vp-p


as follows

%Error voltage reading = ((38-36)/36) x 100 =

5.5%



3.1.4 Frequency Measurements

The device constructed device has the

capability of measuring and displaying the

frequency of the measured signal. Some

samples of these signals were measured using

a function generator, and the results obtained

were very good. The accuracy was very high

to the nearest hundred hertz, only at higher

values above 200 kHz that the accuracy tent to

depreciate significantly.

Comparison with the laboratory Frequency

meter was not possible up to the time of filing

this report, as such conclusion cannot be met

as per the discrepancy and reliability of the

system is concerned. The measurement was

only known from the selected or dialled value

on the function generator which could show

other minute details. Other

discrepancies/deviation from the actual values

might be as a result of unstabilised power to

the frequency generating device, excessive

temperature of the environment and etc.

Table 3: Tuned Frequencies and the Displayed Values

Tuned Frequency Value

(Hz)

Reading Displayed by the

Oscilloscope (Hz)

10 10

50 51

150 153

450 447

890 897

1140 1138

12330 12299

40500 40522

100250 100321

150000 150089

200000 199882

550000 549753

870000 871541

1000000 Not stable

5000000 Not stable

15000000 Not stable

4.0 CONCLUSION

The AVR microcontroller based handheld

Oscilloscope has beendesigned ,constructed

and tested. It is well suited for field and

laboratory uses. The program was designed

and written using a modified basic language

for AVR and its compiler called Bascom-avr.

Many waveforms were measured and

reconstructed accurately up to the speed of 20

kHz This limitation could be as a result of the

ADC’s clock speed requirement of 50 kHz to

200 kHz in order to get its maximum

resolution and the conversion of the analog to

digital signal takes 13 ADC clock-cycles to

complete, which further divides the ADC

speed by 13 making the sampling frequency to

be 200/13 kHz (15.38 kHz). By the Shannon-

Nyquist criteria with this sampling rate only

7.69 kHz signal can be fully reconstructed.

“By default, the successive approximation

circuitry of the ADC requires an input clock

frequency between 50kHz and 200 kHz to get

maximum resolution. However, If a lower

resolution than 10 bits is needed, theinput

clock frequency to the ADC can be higher

than 200 kHz to get a higher sample rate.The

ADC module contains a prescaler, which

generates an acceptable ADC clock frequency



from any CPU frequency above 100 kHz”.The

ADC’s pre-scaler was adjusted up to get

higher sampling rate and at the detriment of

the resolution, this made it possible to measure

up to 20 kHz.

The device is handy, that can be physically

justified, and has a low-cost because the total

cost of all the components put together was

only ₦12,410.

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