Embedded Systems Design 1 Lecture Set B Interfacing the MCS-51 to: –7 and 16 segment displays –a Multiplexed 7-Segment Display –a Keypad –an LCD.

Embedded Systems Design 1

Lecture Set B

Interfacing the MCS-51 to:– 7 and 16 segment displays– a Multiplexed 7-Segment Display– a Keypad– an LCD


The 7-Segment Display

• The 7-segment display is nothing but 7 LEDs arranged to form the figure 8.– By turning on and off the appropriate segments

(LEDs), different combinations can be produced.– The 7-segment display is useful for displaying the

digits 0 through 9, and some characters.• However, it would be difficult to distinguish between

things like a D and a 0.a

b

c

f

e

g

d


The 7-segment Display (Contd.)

• 7-segment displays come in 2 configurations:

Common Anode Common Cathode

• As we have seen when we discussed interfacing the basic LED, it would be preferable to connect the cathode of each diode to the output pin.– Therefore, the common anode variety would be

better for our interfacing needs.


Interfacing a 7-segment display to the 8051

• Also, as seen with interfacing the LED, a resistor will be needed to control the current flowing through the diode.– This leaves two possibilities:

• Case 2 would be more appropriate as case 1 will produce different brightness depending on the number of LEDs turned on.


The 16-segment Display

• Similar to the 7-segment display except that it contains 16 LEDs.– With the 16 segments, it is possible to

represent all digits and letters in the alphabet (English).

– With 16 LEDs, the 16-segment display requires 16 pins and 16 bits to determine the exact character.

• All 16 values must be present at the same time.– Therefore, interfacing to a 16-segment display will require:

» Either external hardware such as a latch.

» Using two of the 8051 ports for connecting to a single display.


Multiplexed Displays

• 7-segment displays require a lot of power (~10 mA per LED) and a large number of pins (8 per LED).– Suppose that a battery-powered system has 10

displays.• If the system is displaying “8888888888”.

– All LEDs are on in all displays.

– The total current required just to activate the displays is ~700 mA.

• This would reduce the useful battery lifetime to a few hours at best.

– The solution is not to activate all displays at once.


Multiplexed Displays

• The figure below show a 4 character multiplexed display.– The LED inputs are common to all 4 but each

character has a separate “enable” input.• When a particular display is to be activated, its enable

input is turned on, the display now uses the values on the LED pins to determine which LEDs to activate.

– Total number of pins: 8 + # of characters.

abcdefg


Interfacing to a Multiplexed Display• To use a multiplexed display, each character will be

flashed for a short period of time,• the first character is activated for a brief amount of time, then it

is turned off.• Then the next character and so on through the four characters.• Back to the first character.

– Only one character is activated at any point in time.• Worst case current requirement: ~70 mA.

– If the multiplexing is done fast enough, the human eye will filter out the flickering and it would look like all the displays are one constantly.

• Most humans cannot distinguish flashes at more than 20 per second. They would consider that to be steady light.

• The scanning frequency should be at least 30 Hz.– Each character is on for 1/30 of a second.


The ProgramMOV B, #11H ; Display sequence

LOOP: MOV R2, #04 ; Number of characters

MOV R0, #20H ; Location of message

NEXT: MOV P0, #00H ; Turn off the displays

MOV A, @R0 ; Get the next character

LCALL CONVRT ; Binary to 7-segment

MOV P1, A ; Display the first character

INC R0 ; Next character in the message

MOV P0, B ; Turn on the appropriate display

MOV A, B ; Go on to next display

RL A

MOV B, A

LCALL DELAY ; On-time (1/30 seconds)

DJNZ R2, NEXT ; Next character

JMP LOOP ; Start next scan


Interfacing a Keypad

• A 16-key keypad is built as shown in the figure below.– 16 keys arranged as

a 4X4 matrix.– Must “activate”

each row by placing a 0 on its R output.

• Then the column output is read.

• If there is a 0 on one of the column bits, then the button at the column/row intersection has been pressed.

• Otherwise, try next row.

– Repeat constantly.

C 1C 2C 3C 4

R 1R 2R 3

R 4


Interfacing a Keypad

• Algorithm:– Drive a “0” on a row– Read all the columns– If any key had been pressed, its column will be “0”,

else 1– Keep repeating in a loop for each successive row

• Example:– Switch 4 is pressed

• R1 0, C1:C4 = 1111• R2 0, C1:C4 = 0111

– Switch 2 is pressed• R1 0, C1:C4 = 1101


Bouncing Contacts

• Push-button switches, toggle switches, and electromechanical relays all have one thing in common: contacts.

• These metal contacts make and break the circuit and carry the current in switches and relays. Because they are metal, contacts have mass.

• Since at least one of the contacts is on a movable strip of metal, it has springiness.

• Since contacts are designed to open and close quickly, there is little resistance (damping) to their movement.


Bouncing

• Because the moving contacts have mass and springiness with low damping they will be "bouncy" as they make and break.

• That is, when a normally open (N.O.) pair of contacts is closed, the contacts will come together and bounce off each other several times before finally coming to rest in a closed position.

• The effect is called "contact bounce" or, in a switch, "switch bounce”.


Why is it a problem?

• If such a switch is used as a source to an edge-triggered input such as T0 or INT0, then the MCS-51 will think that there were several “events” and respond several times.

• The bouncing of the switch may last for several milliseconds.– Given that the MCS-51 operates at microsecond

speed, a short ISR may execute several times in response to the above described bounciness.


Hardware Solution

• The simplest hardware solution uses an RC time constant to suppress the bounce. The time constant has to be larger than the switch bounce and is typically 0.1 seconds.

• The buffer after the switch produces a sharp high-to-low transition.– If the button is pushed again

before the expiration of the time constant, the second press will not be recognized.

V c c

O U T


Software Solution

• It is also possible to counter the bouncing problem using software.

• This would require one of two approaches:– Delays: When the first edge is noticed, an

“appropriate” delay is executed, then the value of the line is checked again to make sure the line has stopped bouncing.

• Actually, it is also necessary to execute a similar delay on the button’s release to counter the release bouncing.

– Interrupts and Timers: When the first edge is noticed, a timer is started. When following edges appear, the value of the counter is checked. If the edges appear “too quickly”, they will be rejected.


LCD Interfacing

• LCDs have become a cheap and easy way to get text display for an embedded system – Various configurations (1 line by 20 characters upto 8 lines

by 80 characters), starting from around $5.– Graphics LCDs are also available

• Intelligent LCDs have internal ASCII decoders, Character Generators and LCD control circuitry

• Some also have custom character generation capacity– User defined character RAM– Program this RAM with the character pattern– Then use it like ordinary ASCII characters

• Usually MSB decides between std ASCII and custom characters


Alphanumeric LCD Interfacing• Pinout (statistical average)

– 8 data pins D7:D0– RS: Data or Command

Register Select– R/W: Read or Write– E: Enable (Latch data)– VEE : contrast control

• RS – Register Select– RS = 0 Command Register– RS = 1 Data Register

• R/W = 0 Write, R/W = 1 Read

• E – Enable– Used to latch the data present on the data pins.– A high-to-low edge is needed to latch the data.

• D0 – D7– Bi-directional data/command pins. – Alphanumeric characters are sent in ASCII format.

E

R/W

RS

DB7–DB0

LCD controller

communications bus

Microcontroller

8

LCD Module


LCD Commands• The LCD’s internal controller can accept several commands and

modify the display accordingly. These commands would be things like:– Clear screen– Return home– Decrement/Increment cursor– Shift display right/left

– Check the data sheet from the manufacturer.

• After writing to the LCD, it takes some time for it to complete its internal operations. During this time, it will not accept any new commands or data.– We can check if the LCD is busy or not by reading the busy flag.

• Set R/W = 1 and RS = 0, then read from the LCD– If D7 = 1, then the LCD is still busy, wait.


Example LCD Interface ProgramORG 100H

MOV A, #38H ; Initialize, 2-lines, 5X7 matrix.

ACALL Command

MOV A, #0EH ; LCD on, cursor on

ACALL Command

MOV A, #01H ; Clear LCD Screen

ACALL Command

MOV A, #06H ; Shift cursor right

ACALL Command

MOV A, #86H ; Cursor, line 1 position 6

ACALL Command

MOV A, #’N’ ; Character N

ACALL Data

MOV A, #’O’ ; Character O

ACALL Data

Wait: SJMP Wait

Command: ACALL Ready

MOV P1, A

CLR P2.0 ; RS=0

CLR P2.1 ; R/W=0 (Write)

SETB P2.2 ; E = 1

CLR P2.2 ; E = 0 – complete the 1-to-0 pulse

RET

Data: ACALL Ready

MOV P1, A

SETB P2.0 ; RS = 1

CLR P2.1 ; R/W = 0

SETB P2.2 ; Enable pulse

CLR P2.2

RET

Ready: SETB P1.7 ; bit 7 of port 1 set to input

CLR P2.0 ; RS = 0 – Command Reg.

SETB P2.1 ; RW = 1 – Read

Back: SETB P2.2 ; Enable pulse

CLR P2.2

JB P1.7, Back ; Wait till not busy

RET


The Cursor Location

• Whenever a new data character is sent to the LCD, the cursor position will be automatically updated one position to the right.

• It is also possible to place the cursor at a particular location using the “set address” command.– Set RS = 0 and R/W = 0– Set D7 = 1 – (set address command)– Set D6 = 0 for line 1 and D6 = 1 for line 2– Set D5 – D0 for the position on the line (in binary).

• 1st position (extreme left) is 0.


Interfacing an 8031 to an LCD

Source: Intel Application Brief AB-39

Embedded Systems Design 1 Lecture Set B Interfacing the MCS-51 to: –7 and 16 segment displays –a Multiplexed 7-Segment Display –a Keypad –an LCD.

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