Applications

Applications

In this section we will discuss many useful applications with different modes of keyboard and display interfacing. In addition to this we are going to see the software requirement to control the interfacing circuits. All these applications are illustrated using different examples.

Lab Experiment 12.9.1 : Hardware and software for 8x8 keyboard interface using 8279.

Statement : Interface an 8 × 8 matrix keyboard to 8085 through 8279 in 2-key lockout mode and write an assembly language program to read keycode of the pressed key. The external clock frequency is 2 MHz. Use I/O mapped I/O technique.

Solution : The 8×8 matrix keyboard can be interfaced to 8085 through 8279 in two ways.

1. Without interrupt signal

2. With interrupt signal (Interrupt driven Input)

We will see both the ways one by one.

1. Without interrupt signal

Hardware : Fig. 12.9.1 shows the interfacing of 8 × 8 matrix keyboard.

I/O Map :

Software :

Step 1 : Find keyboard/display command word. To interface 8x8 matrix keyboard we need 8 scan lines and 8 return lines. To get 8 scan lines. We have to select encoded scan keyboard mode. Therefore, the keyboard/display command word for above keyboard configuration is given as follows :

Note 000 → Encoded scan keyboard - 2 key lockout

X →  don't care

Step 2 : Find program clock command word

External clock frequency is 2 MHz

Prescaler value = 2 MHz / 100kHz

= 20 = (10100)₂

Therefore, the program clock command word is as given below :

Step 3 : Find Read FIFO/sensor RAM command word we want to read first entry from the FIFO RAM. Therefore command word is as given below.

Program :

MVI A, 00H

OUT 81H ; Initialize keyboard/display

; in encoded scan keyboard-2 keylockout mode

MVI A 34H

OUT 81H ; Initialize prescaler count

BACK : IN 81H ; Read FIFO status word

ANI 07H ; Mask bit B₃ to B₇

JZ BACK ; if 0, key is not pressed wait for

; key press otherwise read FIFO

; RAM

MVI A, 40H ; Initialize 8279 in read

OUT 81H ; FIFO RAM mode

IN 80H ; Read FIFO RAM (keycode)

HLT ; Stop program execution

2. With interrupt signal

Hardware : Fig. 12.9.2 shows the interfacing of 8 × 8 matrix keyboard in interrupt driven keyboard mode.

In the interrupt driven mode interrupt line from 8279 is connected to the one of the interrupt input of 8085 except INTR. Here, INT line from 8279 is connected to the interrupt RST 7.5 of 8085. Other signal connections are same as in the non interrupt mode.

Software : All the command words required to initialize 8279 are same as in the non interrupt mode.

Flowchart :

Main Program :

MVI A, 00H

OUT 81H ; Initialize keyboard/display in encoded

; scan keyboard 2 key lockout mode

MVI A, 34H        

OUT 81H ; Initialize prescaler count

MVI A, 0BH ; Load mask pattern to enable RST 7.5 and mask other interrupts

SIM  

EI ; Enable interrupt

HERE : JMP HERE ; Wait for the interrupt

Interrupt Subroutine : 

MVI A, 40H         ; Initialize 8279 in read FIFO

OUT 81H ; RAM mode

IN 80H ; Read FIFO RAM (keycode)

EI ; Enable interrupt

RET ; Return to main program

In the interrupt driven keyboard, when key is pressed, key code is loaded into FIFO RAM and interrupt is generated. This interrupt signal is used to tell CPU that there is a keycode in the FIFO RAM. CPU then initiates read command with in the interrupt service routine to read keycode from the FIFO RAM.

 

Lab Experiment 12.9.2 : Hardware and software to interface 8×4 matrix keyboard using 8279

Statement : Interface an 8 x 4 matrix keyboard to 8085 through 8279.

Solution : Fig. 12.9.5 shows interfacing of an 8 x 4 matrix keyboard to 8085 through 8279.

As keyboard is having 8 rows and 4 columns, only 4 scan lines are required and we can avoid external decoder to generate scan lines by selecting decoded scan keyboard mode.

Main Program :

MVI A, 00H

OUT 81H ; Initialize keyboard/display in encoded

; scan keyboard 2 key lockout mode

MVI A, 34H

OUT 81H ; Initialize prescaler count

MVI A, 0BH ; Load mask pattern to enable RST 7.5

; and mask other interrupts

SIM

EI ; Enable Interrupt

HERE : JMP HERE ; Wait for the interrupt

Interrupt Subroutine :

MVI A, 40H ; Initialize 8279 in read FIFO

OUT 81H ; RAM mode

IN 80H ; Read FIFO RAM (keycode)

EI ; Enable Interrupt

RET  ; Return to main program

 

Lab Experiment 12.9.3 : Hardware and software to interface eight 7-segment digits using 8279.

Statement : Interface 8/7-segment digits (common cathode) to 8085 through 8279 and write an 8085 assembly language program to display 1 to 8 on the eight seven segment digits. External clock frequency is 3 MHz.

Solution : Fig. 12.9.6 shows the interfacing of eight 7-segment digits to 8085 through 8279.

As shown in the Fig. 12.9.6, eight display lines (B₀-B₃ and A₀-A₃) are buffered with the help of transistor and used to drive display digits. These buffered lines are connected in parallel to all display digits. S₀, S₁ and S₂ lines are decoded and decoded lines are used for selection of one of the eight digits.

Software : To display 1 to 8 numbers on the eight 7-segment digits we have to load 7-segment codes for 1 to 8 numbers in the corresponding display locations.

Step 1 : Find keyboard/display command word. To interface 8 digit 7 segment display we need 8/8-bit character display mode with left entry. For selection of 8 digits we need encoded scan mode. Therefore, the keyboard/display command word is as given below.

Step 2 : Find program clock command word. External clock frequency is 3 MHz.

Prescaler value = 3 MHz / 100 MHz = 30 = (11110)₂

Therefore, the program clock command word is as given below

Step 3 : Find write display RAM command word. We want to write first eight locations with autoincrement mode by command word given below.

Program :

LXI H, 6200H ; Initialize lookup table

; pointer

MVI C, 80H ; Initialize counter

MVI A, 00H ; Initialize keyboard/display

OUT 81H ; Mode

MVI A, 3EH ; Initialize prescaler count

OUT 81H

MVI A, 90H ; Initialize 8279 in write

; Display

OUT 81H ; RAM mode

BACK : MOV A,M ; Get the 7-segment code

OUT 80H ; Write 7-segment code in

; display RAM

INXH ; Increment lookup table

; pointer

DCR C ; Decrement counter

JNZ BACK ; if count = 0 stop otherwise

; go to back

HLT ; Stop program execution 

Lookup table:

 

Lab Experiment 12.9.4 : Write an assembly language program to roll message 'HELL0123'.

Statement : Using hardware same as in the example 3, write an assembly language program to roll message 'HELLO123' from right to left.

Solution : To roll above message we have to load 7-segment codes for characters within the message and it is necessary to configure 8279 in right entry mode.

Step 1 : Find keyboard/display command word

Note DD = 10 = 8 8-bit character display right entry.

Program clock command word and write display RAM command word are same as in the previous example.

Clear command word

Program :

LXI H, 6200H ; Initialize lookup table pointer

MVI C,08H ; Initialize counter

MVI A.10H          ; Initialize keyboard/display in right entry mode.

OUT 81H ; Mode

MVI A,34H          ; Initialize prescaler count

OUT 81H

MVI A, D0H ; Clear display

OUT 81H

MVI A, 90H ; Initialize 8279 in write display

OUT 81H   ; RAM mode

BACK : MOV A,M ; Get the 7-segment code

OUT 80H ; Write 7 segment code in display RAM

INXH          ; Increment lookup table pointer

DCR C ; Decrement counter

JNZ BACK ; If count-0 stop ; otherwise

; goto BACK

HLT ; Stop program execution

Lookup table :

 

Lab Experiment 12.9.5 : Interface 4X4 matrix keyboard and 4 digit 7-segment display using 8279

Statement : Interface 4×4 matrix keyboard and 4 digit 7-segment display and write an assembly language program to read keycode of the pressed key and display same key on the 7 segment display.

Solution : Fig. 12.9.9 shows interfacing diagram. Here, only 4 scan lines are sufficient to scan matrix keyboard and to select display digits. Hence decoded mode is used.

Software :

Step 1 : Find keyboard / display command word.

DD = 00 8/8-bit character display left entry

KKK = 001-Decoded scan keyboard-2 key lockout

Step 2 : Find program clock command word.

External clock frequency is 2.5 MHz

ஃ Prescaler value = 2.5 MHz / 100kHz = 25 (11001)₂

Therefore, the program clock command word is as given below

Step 3 : Find Read FIFO RAM command word. We want to read first entry from the FIFO RAM. Therefore command word is as given below.

Step 4 : Find Write FIFO RAM command word.

We have to display at a time only single key number. Here mode is not required.

Interrupt Service Routine :

Program :

MVI A, 00H ; Initialize keyboard / display in

OUT 81H ; encoded scan keyboard 2-key

; lockout mode.

MVI A, 39H ; Initialize prescaler count

OUT 81H

MVI A, 0BH ; load mask pattern to enable

RST 7.5

SIM ; and mask other interrupts

EI ; Enable interrupt

HERE : JMP HERE ; Wait for the interrupt

Interrupt Service routine :

MVI A, 40H ; Initialize 8279 in read FIFO RAM mode

OUT 81H

IN 80H ; Get dat from RAM

MVI H, 62H ; Initialize memory pointer to point

MOV L,A ; 7-Segment code

MVI A, 80H ; Initialize 8279 in write display RAM

; mode

OUT 81H

MOV A,M ; Get the 7-segment code

OUT 80H ; Write 7-segment code in display RAM

EI ; Enable interrupt

RET ; Return to main program

Review Questions

1. Write a program using RST 5.5 interrupt to get an input from keyboard and display it on the display system.

AU : May-05, Marks 6

2. Using peripheral mapped I/O, design a seven segment LED output port with device address of F2h using necessary control ICs. Draw the schematic and write 8085 ALP for displaying digit 8.

AU : May-07, Marks 8

3. How is keyboard interfaced with microprocessor ?

AU : Dec.-17 Marks 2