Display Interface

Display Interface

1. LED Interface

• Sourcing current : It refers to the maximum current that the 8051 port pin can supply to drive an externally connected device. The device can be an LED, a buzzer or TTL logic device. For TTL family of 8051 devices the sourcing current is approximately 60 µA.

• Sinking current : If refers to the maximum current that the 8051 port pin can absorb through a device which is connected to an external supply. Pins of P1, P2 and P3 can sink a maximum current of 1.6 mA. Port 0 pins can sink current upto 3.2 mA.

• A typical LED consumes 10 to 15 mA. Thus we cannot drive LED directly in current source mode as shown in the Fig. 17.2.1 (a). However, we can drive LED directly in current sink mode, as shown in the Fig. 17.2.1 (b). Here, sinking current of 8051 is not enough to drive LED, so the brightness of LED is poor.

• We can use driver transistor to solve the problem of current sinking and sourcing. The Fig. 17.2.2 shows the LED interface using driver transistor.

• In Fig. 17.2.2 (a), the source current of 8051 port pin is amplified by the npn transistor to drive the LED. In Fig. 17.2.2 (b), the pnp transistor is used to amplify the sinking current.

R = V_CC - V_LED / I_LED     

RB = V_out – V_BE / (I_LED / β)

where V_LED is the voltage across LED

I_LED is the current through LED

V_out is the voltage at output pin

β is the current gain (Beta) of the transistor

Example 17.2.1 Write an ALP to flash the LED connected to port P2.0.

AU : Dec.-13, Marks 2

Solution : Algorithm 1

Step 1: Make P2.0 High

Step 2: Wait for some time

Step 3: Make P2.0 LOW

Step 4: Wait for some time

Step 5: Repeat steps 1 through step 4

Algorithm 2

Step 1: Complement P2.0

Step 2: Wait for some time

Step 3: Repeat steps 1 and 2

Program 1:

ORG 0000H

BACK: SETB P2.0 ; Make P2.0 high

ACALL Delay ; Wait for some time

CLR P2.0 ; Make P2.0 low

ACALL Delay ; Wait for some time

SJMP BACK ; repeat

Program 2:

ORG 0000H

BACK: CPL P2.0 ; Complement P2.0

ACALL Delay ; Wait for some time

SJMP BACK

2. Multiplexed 7-Segment Display Interfacing

Seven-segment display : Seven-segment displays are generally used as numerical indicators and consists of a number of LEDs arranged in seven-segments as shown in the Fig. 17.2.3.

Any number between 0 and 9 can be indicated by lighting the appropriate segments.

The seven-segments are labelled a to g and dot is labelled as h. By forward biasing different LED segments, we can display the digits 0 through 9. For instance, to display 0, we need to light up a, b, c, d, e, and f. To light up 5, we need segments a, f, g, c and d.

These 7-segment displays are of two types :

• Common anode type

• Common cathode type

In common anode, all anodes of LEDs are connected together as shown in Fig. 17.2.5 (a) and in common cathode, all cathodes are connected together, as shown in Fig. 17.2.5 (b).

Static display

Fig. 17.2.6 shows a circuit to drive a single, seven-segment, common anode LED display. For common anode, when anode is connected to positive supply, a low voltage is applied to a cathode to turn it on. Here, BCD to seven-segment decoder, IC 7447 is used to apply low voltages at cathodes according to BCD input applied to IC 7447. To limit the current through LED segments resistors are connected in series with the segments. This circuit connection is referred to as a static display because current is being passed through the display at all times.

The value of the resistor in series with the segment can be calculated as follows :

We know, V_CC - Drop across LED segment - IR = 0

Drop across LED segment is nearly 1.5 V.

ஃ IR - V_CC - 1.5 V = 5 - 1.5 V

= 3.5 V

Each LED segment requires a current of between 5 and 30 mA to light. Let's assume that current through LED segment is 15 mA.

ஃ  R = 3.5 V / 15mA = 233 Ω

In practice, the voltage drop across the LED and the output of IC 7447 are not exactly predictable and the exact current through the LED is not critical as long as we don't exceed its maximum current rating. Therefore, a standard value 220 Ω can be used.

The static display circuits work well for driving just one or two LED digits. However, these circuits are not suitable for driving more LED digits, say 8 digits. When there are more number of digits, the first problem is power consumption. For worst case calculations, assume that all eight digits with all segments are lit. Therefore, worst case current required is

1 = 8 (digits) × 7 (segment) × 15 mA (current per segment)

= 840 mA

A second problem of the static approach is that each display digit requires a separate BCD to 7-segment decoder.

Multiplexed display

To solve the problems of the static display approach, multiplexed display method is used. Fig. 17.2.7 shows the 4 seven-segment displays connected using multiplexed method. Here, common anode seven-segment LEDs are used.

Anodes are connected to +5 V through transistors. Cathodes of all seven-segments are connected in parallel and then to the output of 7447 IC through resistors. Looking at the Fig. 17.2.7 the question may occur in our mind that,"Aren't all of the digits going to display the same number?" The answer is that they would show the same number only if all the digits are turned on at the same time. However, in multiplexed display the segment information is sent for all digits on the common lines (output lines of IC 7447), but only one display digit is turned on at a time. The PNP transistors connected in series with the common anode of each digit act as an ON and OFF switch for that digit. Here's how the multiplexing process works.

The BCD code for digit 1 is first output from port 1, to the IC 7447. The IC 7447, BCD to seven-segment decoder outputs the corresponding seven-segment code on the segment bus lines. The transistor Q₁ connected to digit 1 is then turned on by outputting a low to that bit of port 3. All of the rest of the bits of port 3 are made high to ensure no other digits are turned on. After 2 ms, digit 1 is turned OFF outputting all highs to port 3. The BCD code for digit 2 is then output to the port 1, and bit pattern to turn on digit 2 is output on port 3. After 2 ms, digit 2 is turned off and the process is repeated for digit 3 and digit 4. After completion of turn for each digit, all the digits are lit again in turn.

With 4 digits and 2 ms per digit we get back to digit 1 every 8 ms or about 125 times a second. This refresh rate is fast enough that, to our eye and due to persistence of vision all digits will appear to be lit all the time.

In multiplexed display, the segment current is kept in between 40 mA to 60 mA so that they will appear as bright as they would if not multiplexed. Even with this increased segment current, multiplexing gives a large saving in power and hardware components.

Example 17.2.2 Interface an 8-bit 7-segment LED display to 8051 through port 1 and port 3 and write an 8051 assembly language program to display message on the display.

Solution : Hardware

The Fig. 17.2.8 shows the multiplexed 8-digit 7-segment LED display connected in 8051 system using port 1 and port 3. In this circuit port 1 and port 3 are used as a latch, i.e. output port. Port 1 provides the segment data inputs to the display and port 3 provides a means of selecting a display position at a time for multiplexing the displays. Here, instead of BCD to seven-segment decoder (IC 7447) transistors are used to drive the LED segments. Due to this we can also display HEX characters on the display; however in this case we have to send the proper 7-segment code of a particular digit that is to be displayed on the port 1.

 Subroutine to Display Message

MOV R0, #8H ; Initialize counter

MOV R1, #7FH ; load select pattern

MOV DPTR, #6000H ; Starting address of message to be displayed

AGAIN : MOV P3, R1 ; select digit

MOVX A, @DPTR ; Get data

MOV P1, A ; Send data

LCALL DELAY ; Wait for some time

MOV A, R1 ; [ Adjust

RR A ; selection

MOV R1, A ; pattern]

INC DPTR ; Increment message pointer

DJNZ R0, AGAIN ; Decrement R0 and check

RET ; if for zero; if not, goto AGAIN

Note This subroutine must be called continuously to display the 7-segment coded message stored in the memory from address 6000H.

3. LCD Interfacing

Now-a-days, many LCD modules are available which have built-in drivers for LCD and interfacing circuitry to interface them to microprocessor/microcontroller systems. These LCD modules allow display of characters as well as numbers. They are available in 16 × 2, 20 × 1, 20 × 2, 20 × 4 and 40 × 2 sizes. The main advantage of using LCD is modules is that they require very less power. The first figure represents number of character in each line and second figure represents number of lines the display has. In this section, we see the interfacing of 20 × 2 LCD display module to microprocessor/microcontroller system through 8255. In this module the display is organized as two lines, each of 20 characters. The module has 14-pins. The function of each pin is given in the Table 17.2.1.

The Fig. 17.2.9 shows the interfacing of a 20 character × 2 line LCD module with the 8051. As shown in the Fig. 17.2.9, the data lines are connected to the port 1 of 8051 and control lines RS,  and E are driven by 3.2, 3.3 and 3.4 lines of port 3, respectively. The voltage at V_EE pin is adjusted by a potentiometer to adjust the contrast of the LCD.

The display can be controlled by issuing proper commands to the LCD module. The Table 17.2.2 lists the command available for LCD module.

Abbreviations used in the table

DD RAM : Display data RAM

CG RAM : Character generator RAM

ACG : CG RAM address

ADD : DD RAM address

AC : Address counter used for both DD and CG RAM address

1/D = 1 : Increment        1/D = 0 : Decrement

S = 1 : Accompanies display shift

S/C = 1 : Display shift   S/C = 0 : Cursor move

R/L = 1 : Shift to right R/L = 0 : Shift to left

DL = 1 : 8-bits      DL = 0 : 4-bits

N = 1 : 2 Lines      N = 0 : 1 Line

F = 1 : 5 × 10 dots           F = 0 : 5×7 dots

BF = 1 Busy in internal operation

BF = 0 Can accept command or instruction

To display  a message on LCD module, it is necessary to initialize it by writing series of command codes in the command register in a proper sequence. The initialization includes command codes for clearing the display, returning the cursor home, and shifting cursor automatically after writing a character. After initialisation we can write data to either DD RAM or CG RAM. Both RAMs are read/write RAMs and have unique addresses to access each location. To write data in any RAM we have to set the address for that RAM by issuing proper command. Then we can write data into it by activating write cycle. To activate write cycle we have to make  signal low, RS signal high, send data on port A and apply high to low pulse of at least 450 ns of duration on E pin of the module. The DD RAM stores the characters in their ASCII code whereas CG RAM stores the character in its internally generated character code. Let us see the 8051 assembly language program to display 'WELCOME' message on the LCD module. Before sending command or data it is necessary to check busy flag, i.e. whether LCD is reading or not.

MAIN Routine :

MOV 81H,#30H ; Initialise stack pointer

MOV A,#3CH ; [Send command code to set font = 5×10 dots,

LCALL COMMAND ; DL = 8-bits and N = 2 lines],

MOV A,#0EH ; [Send command code to set display

LCALL COMMAND ; and cursor ON]

MOV A,#01H ; [Send command code to

LCALL COMMAND ; clear LCD]

MOV A,#86H ; [Send command to set DD RAM

LCALL COMMAND ; address to the seventh location]

MOV A,#'W'       

LCALL DISPLAY ; Display letter W

MOV A,#'E'        

LCALL DISPLAY ; Display letter E

MOV A,#'L'

LCALL DISPLAY ; Display letter L

MOV A,#'C’

LCALL DISPLAY ; Display letter C

MOV A,#'O'

LCALL DISPLAY ; Display letter O

MOV A,#’M'

LCALL DISPLAY ; Display letter M

MOV A,#’E'

LCALL DISPLAY ; Display letter E

SJMP          HERE:HERE ; Loop here after displaying message

COMMAND Routine :

LCALL READY ; Check whether LCD is ready ?

MOV P1, A          ; Issue command code

CLR P3.2 ; Make RS = 0 to issue command

CLR P3.3 ; Make  = 0 to enable writing

SETB P3.4 ;         Make E = 1

CLR P3.4 ; Make E = 0

RET ; Return

READY Routine :

READ : CLR P3.4         ; Disable display

CLR P3.2 ; Make RS = 0 to access command register

MOV P1,#0FFH ; Configure P1 as an input port

SETB P3.3 ; Make  = 1 to enable reading

READ : SETB P3.4 ; Make E = 1

JB P1.7,READ ; Check DB7 bit. If it is 1, LCD is busy hence check if until it is 0

CLR P3.4 ; Make E = 0 to disable display

RET ; Return

DISPLAY Routine :

LCALL READY ; Check whether LCD is ready ?

MOV P1, A          ; Issue data

SETB P3.2 ; Make RS = 1 to issue data

CLR P3.3    ; Make  = 0 to enable writing

SETB P3.4 ; Make E = 1

CLR P3.4    ; Make E = 0

RET ;          Return

Review Questions

1. Interface a 20 × 2 LCD with 8051 microcontroller and write assembly language program to display the following message in it at the middle.

HELLO ! ALL

ARE WELCOME

2. Explain the LCD display interfacing with microcontroller 8051.

3. Explain the interfacing of keyboard/display with 8051 microcontroller.

4. How to interface a 7 segment display using 8051 microcontroller.

5. Draw the circuit diagram to interface an LCD display with 8051 microcontroller and explain how to display a character using LCD display.

6. Explain the interfacing of four digit 7 segment display to 8051 and its program. AU : Dec.-16, Marks 16

7. Illustrate the keyboard and display interface with 8051 and write the program to get the input 45H from the external keyboard and display it on the external display device.

8. Design a system using 8085 or 8051 to blink four LEDs. AU : May-11, 13, 16, 17, Dec.-12, 17, 19