march 2014 - site.iugaza.edu.pssite.iugaza.edu.ps/wp-content/uploads/lab3_keypad... · 4 embedded...

March 2014

3

2 Embedded Systems Lab

Objectives

- To be familiar with interfacing Keypads to PIC18F4550 as input/output

devices.

- To deal with new components such as BCD 7-segment and sounder.

- To be able to start building real useful applications.

Keypads

Matrix keypads are simply an extension to the simple tact switch inputs. They

consist of keys interconnected in the shape of a matrix. Each key is a simple

mechanical switch located at the crossing between the matrix rows and

columns. When a key is pressed, its row and column form an electrical contact.

The rows and columns can be connected to the pins of microcontroller ports. The

big advantage of using a matrix keypad is that it allows interfacing a large

number of keys with a relatively small number of microcontroller pins. For

example, a 12-key keypad requires only 7 (instead of 12, if interfaced

individually) I/O pins of the microcontroller if organized into a 4 rows and 3

columns matrix.

Keypad


Let's consider the case of a 12 keys matrix as the one shown in the figure and we

want to interface this keypad to PIC18F4550 microcontroller. In order to detect

that one key is pressed we will use the following strategy:

1. The 12 keys are arranged in 3 columns and 4 rows. Each column and row is

connected by a separated wire.

2. Pushing a key merely connects the corresponding row to the corresponding

column.

3. We connect the columns to 3 output pins of the PIC and the rows to other 4

input pins.

4. For each of the columns we output logic '1' holding other columns to logic '0'

and scan the values of the rows, if one of the rows is logic '1', this means that

the key located at the intersection of the current column and the current row

is pressed. If no row is one, the logic '1' should be moved to the next column

and the rows then rescanned.

5. The PIC should keep repeating this process every time it's required to detect

a key press.


Notice

In order to start, you must know some important notes:

- Keypad at Proteus: write keypad << choose Keypad-Phone (2nd choice).

- It is preferred to use ports efficiently by reserving just one port for keypad.

- For all next parts, you will deal with some new components and new codes,

all of that will mainly depend on the main concept of keypad’s work.

So, think .. then start working

In case the microcontroller does not have the internal pull-down

feature on its I/O port, the pull-down resistors on the input lines

must be connected externally. With our keypad, resistors must be

connected with rows.

1. Write an assembly and basic programs that displays the

value of the pressed key on a 7-segment.

2. Simulate the circuit using Proteus ISIS program.

3. Connect the circuit at the hardware kit.

Lab Work 1


Proteus

How to think …

- Strategy of checking pressed number will depend on the previous

description of keypad structure and work.

- Pins that are connected to the columns are configured as outputs

and to the rows as inputs.

- Checking will be by:

Enabling the first column by entering 1 0 0 for the pins that are

connected with columns.

Check the four cells (pins) at the first column.

If any of rows pins is pressed, load the 7-seg representation value

of pressed number to the connected port of 7-seg.

Repeat the previous steps for the second and the third columns

with changing enabling way at the first step (0 1 0 .. then .. 0 0 1).

- With Proteus, remember the order of columns and rows: 1-3 from

left to right for columns, 1-4 from top to bottom for rows.


Mikrobasic


Assembly

How to think …

- The same work will be here with some differences: Enable column <<

test rows, but with external loop that contains the repeated test

operation.

- The operation of loading numbers into 7-seg will be done as an

external codes (out of loop) using goto instruction (just for code

management).

- Every pressed number is displayed at 7-seg by loading its value and

calling delay subroutine.


New instructions are used:

Byte-oriented File Register Operations

- Clrf portx: clear f: portx = 0x00

Bit-oriented File Register Operations

- Bsf portx,b: bit set f: sets bit ‘b’ of portx (bit=1).

- Bcf portx,b: bit clear f: clears bit ‘b’ of portx (bit=0).

- Btfsc portx,b: bit test file, skip if clear:

If bit ‘b’ in register ‘f’ is 0, then the next instruction is

skipped

Start the loop


Zero label denotes to the code of

loading the 7-seg representation of

zero to the port with delay << then

goto x to continue checking pressed

numbers.

To make a continues loop


1. Edit your programs and simulation in order to display the

value of the pressed key on a BCD 7-segment.

Lab Work 2

- Binary coded decimal 7-segment display takes the numbers

from 0-9 and displays them normally. Its internal work

converts the entered decimal to binary by representing the

number with four digits then views it as normal 7-seg.

- At Proteus: bcd display

- It has four pins 1-4 from right to left << connected to the pins

of your port.


Proteus

One of the edited parts at Assembly (the same for other numbers)

At Mikrobasic, just change the loaded value of numbers to the ports for all

columns and rows.


For more of scientific interest and to be more integrated in the practical

applications of programming PIC (Mikrobasic) …

Starting from this lab, Application part will be allocated at the work; it will

be divided into parts through the labs. For its end, we will get a useful

application similar to one of important application in the world that is

possible for you to develop or expand it for wider use for the future.

Access control System

(Secure Electronic Lock)

I It has a distinct evaluation of the lab report, so try to reserve a

place of your work for our new task, which is .. Application.

Note

1. Write a Mikrobasic program that enable the user to enter his

password then press “check” button, if the password matches

the stored one, the LED will shine.

Application - Part 1


Proteus

How to think …

- Checking the pressed numbers will be the same of the previous

parts, but instead of loading the value into the port when the

number is pressed, we will make the following steps:

Password will be 3-digits << the original password will be entered

into the code as an array of integers.

The user must press 3 numbers, every time the pressed number is

stored in another array with different index.

Index will be determined as a counter of pressed numbers: ex:

If the user pressed 1: counter will start from 0: array[0] will store

1, then counter is incremented, and the same work happened for

2nd and 3rd numbers.

- After the checking of the pressed numbers code, we must check if

the button is pressed and if the entered password matches the

stored one. If the both conditions are achieved, LED will shine.


Array of password digits:

declared as a constant

with integer type.

Array of the entered

password by the user.

Counter of pressed

numbers by user, as an

index of pass counter

array

We can use while ‘condition’ instead of using

‘delay_ms’ in order to be sure for getting one

press during long press. This happened using

empty while loop that make the PIC remains at

this state through a long press.


Index of array starts with 0,

condition check is done by ‘and’

operation.


1. Construct the circuit shown in the figure below and

write a mikrobasic program that will generate a

different sound for each keypad key.

- Sound will be generated by “sounder” device that

is connected to the pin and to the ground by its

two sides.

- Sounder has two functions that will be used:

Sound_init

- Sound_init(port, pin) ..ex: sound_init(portb,0):

configure the pin (0 here) of port (b here) for sound

generation.

- It is placed before „while true‟ as a part of configuration section with tris configuration, .. etc.

Homework


… Good Luck …

Sound_play

- Sound_play(frequence_in_hz, duration_in_ms).

ex: sound_play(1900,400): generates a sound with

1900hz freq. and 400ms duration.

- It is placed instead of loading numbers into 7-seg

port with the same code.

- In order to generate a mixed series of tones, you

can repeat this function for several times for

different frequencies and durations.

Feel free to build your own tones

2. Develop your Application using:

- Sounder, by making different tunes for warning or

welcoming for the user (for in/correct password).

- Green led shining for correct password (3-digits

only) and red led shining for incorrect password.

- Use red led shining also to alarm the error for

entering more than or less than 3 digits for

password (1234 is incorrect!).

3. Include your labwork at the report (all work of

labwork1 and just edited parts at the other labworks

or application with their simulation results).

march 2014 - site.iugaza.edu.pssite.iugaza.edu.ps/wp-content/uploads/lab3_keypad... · 4 embedded...

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