04.8279-Key Board Display

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8279 KEYBOARD/DISPLAYINTERFACE


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nter ac ng ey oar sp ay to t e croprocessor

Using Intel 8279


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Keypad De-Bouncing

wait

until any

key

pressed

key bounce, tBOUNCE

typically, tBOUNCE < 10 ms

wait

debouncing

period

key bounce, tBOUNCE

decode_key;

action dependent

on the key

wait

debouncing

period

wait until all

keys released

wait

until any

key

pressed

NONE NONENONE NONE


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Description

Hardware approach to interfacing a matrix keyboard and amultiplexed display.

Disadvantage of the software approach is that the microprocessor isoccupied for a considerable amount of time in checking the keyboardand refreshing the display.

40 pin device.

has two major segments : Keyboard & Display.Keyboard is connected to a 64-contact key matrix.

Keyboard entries are debounced and stored in the internal FIFOmemory; an interrupt signal is generated with each entry.

Display segment can provide a 16-character scanned displayinterface and has 168 RAM, which can be used to read/write

information for display purposes.

Display can be set up in either right-entry (Calculator) (or) left-entry(Typewriter) mode.


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Data Buffers

168

Display

RAM

Display

Registers

Display

Address

Registers

Control

& Timing

Registers

Scan

Counter

Keyboard

debounce and

Control

88

FIFO/Sensor

RAM

I/O ControlFIFO/Sensor

RAM Status

Block Diagram of 8279

CLK RESET

IRQ

Return

CNTL/STBSHIFT

RL0-RL7SL0-SL3

OUT A0-A3 OUT B0-B3

A0CSWRRD

BD

Timing &

Control

Registers

Internal Data Bus (8)


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Major Sections of 8279

1) CPU Interface Section

2) Keyboard Section

3) Scan Section

4) Display Section


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CPU Interface Section

has eight (8) bi-directional data lines (DB0

-DB7

),

one interrupt request line (IRQ) and six more lines

(CLK, RESET, RD,WR,CS and A0).

If A0=1(high), signals are interpreted as control

(or) status words.

If A0=0(low), signals are interpreted as data

The IRQ line goes high whenever data entries are

stored in the FIFO RAM and this line is used to

interrupt the MPU to indicate the availability of

data.


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Continued.

A0 RDbar WRbar Operation

0 1 0 MPU writes the data in8279

0 0 1MPU reads the data from

8279

1 1 0MPU writes the control

word to 8279

1 0 1MPU reads the status

word from 8279


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Keyboard Section

Includes 88 FIFO (First In First Out) RAM

consisting of eight (8) registers that can store

eight (8) keyboard entries.

Each is then read in the order of entries.

The status logic (FIFO/Sensor RAM Status)

keeps track of the number of entries and

provides an IRQ (Interrupt Request) signal

when the FIFO is not empty.

Has eight return lines (RL7-RL0), connected to

the eight columns of a keyboard.


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Continued

has two additional lines : SHIFT & CNTL/STB.

status of the SHIFT & CNTL (control) key canbe stored along with the key closure.

The keys are automatically debounced , and the

keyboard can be operated in either of the twomodes,

(i) Two-key lockout : if two keys are pressed

simultaneously, only the first key is recognized.(ii)N-key roll over: simultaneously pressed keys

are stored in the internal buffer and no key is

recognized until only one key remains pressed.


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Scan Section

has a scan counter & four scan lines(SL3-SL0).

Scan lines (4) can be decoded by using

a 4-to-16 decoder to generate 16 lines

for scanning.

These lines can be connected to therows of a matrix keyboard.


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Display Section

has eight (8) output lines divided into

two groups A0-A3 & B0-B3.

can be used either as a group of 8 lines

(or) as two groups of four. display can be blanked by using

Includes 168 display RAM.

MPU can read (or) write into any of

these registers.

BD


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Operating Modes of 8279


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(1) Keyboard/Display Mode set

Code 0 0 0 D D K K K

D D Operation

0 0 Eight 8-bit character display-Left entry

0 1 Sixteen 8-bit character display-Left entry

1 0 Eight 8-bit character display-Right entry

1 1 Sixteen 8-bit character display-Right entry

K K K Operation

0 0 0 Encoded Scan Keyboard-2 key lockout

0 0 1 Decoded Scan Keyboard-2 key lockout

0 1 0 Encoded Scan Keyboard-N- key rollover

0 1 1 Decoded Scan Keyboard-N- key rollover

1 0 0 Encoded Scan Sensor Matrix

1 0 1 Decoded Scan Sensor Matrix

1 1 0 Strobed Input, Encoded Display Scan

1 1 1 Strobed Input, Decoded Display Scan


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(2) Program Clock

Code0 0 1 P P P P P

All timing & multiplexing signals for

8279 are generated by an internal pre-scalar.

Pre-scalar divides the external clock by

a programmable integer.Bits PPPPP determine the value ofthis integer which ranges from 2 to 31.


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(3) Read FIFO/Sensor RAM

Code 0 1 0 AI X A A A

CPU sets up the 8279 for a read of the FIFO/SensorRAM by first writing this command.

In the scan keyboard mode, the Auto Increment flag

(AI) and the RAM address bits (AAA) are irrelevant.(i.e) 8279 will automatically drive the data bus foreach subsequent read in the same sequence in whichthe data first entered the FIFO RAM.

In the sensor matrix mode, the RAM address bitsAAA select one of the 8 rows of the Sensor RAM andif AI=1, each successive read will be from thesubsequent row of the sensor RAM.


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(4) Read Display RAM

Code 0 1 1 AI A A A A

CPU sets up the 8279 for a read of the

Display RAM by first writing this command.

The address bits AAAA select one of the16-rows of the Display RAM.

If AI=1, this (current) row address is

incremented after each write command to the

display RAM.


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(5) Write Display RAM

Code 1 0 0 AI A A A A

CPU sets up the 8279 for a write to the

Display RAM by first writing this command.

The address bits AAAA select one of the16-rows of the Display RAM.

If AI=1, this (current) row address is

incremented after each read command to the

display RAM.

(6) Di l i I hibi /Bl ki


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(6) Display write Inhibit/Blanking

Code 1 0 1 X IW IW BL BL

IW bits can be used to mask nibble A &

nibble B in application requiring separate 4-bit display ports.

By setting IW flag (IW=1), for one of the

ports, the port becomes masked.

The BL flags are available for each nibble.

BAA B

(7) Cl


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(7) Clear

Code 1 1 0 CD2 CD1 CD0 CF CA

CD2-CD0 bits available in this command are used to clear all rows

of the display RAM to a selectable blanking code as follows:

If CF=1, FIFO status is cleared & the interrupt output line isreset.

CA =1, clear all bits. (has the combined effect of CD bits and

CF).

CD2 CD1 CD0 Function

1 0 X All zeros (for common cathode displays)

1 1 0 AB=(20)H=(0010 0000)B (for alphanumeric displays)

1 1 1 All ones (for common anode displays)

CD2 must be 1 to enable clear display mode

(8) E d I /E d


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(8) End Interrupt/Error mode set

Code 1 1 1 E X X X X

In sensor matrix mode, this command

lowers the IRQ (Interrupt Request) lineand enables further writing into RAM.

In N-key roll over mode, if E=1, the

chip will operate in the special error

mode.


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1.Write a command word to read third location with auto-

increment of the sensor RAM in sensor matrix mode

Code0 1 0 AI X A A A

1 100 1

Command Word = 53H


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2.Write a command word to write fifth location without

auto-increment of the display RAM

Code1 0 0 AI A A A A

0 110 0

Command Word = 85H


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3.Write a command word to read fourth location with auto-

increment of the display RAM

Code0 1 1 AI A A A A

1 010 0

Command Word = 74H


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4.Write a command word to inhibit nibble A & to blank

nibble B of the display

Code1 0 1 X

IW

(A)

IW

(B)

BL

(A)

BL

(B)

0 101 0

Command Word = A9H


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5.Write a command word to clear IRQ line in sensor matrix

mode

Code1 1 1 E X X X X

0 000 0

Command Word = E0H

Status of E-bit can be 0 or 1.

Command Word = F0H

1

6 Give the command word to set keyboard/display for the


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6.Give the command word to set keyboard/display for the

following configuration:

(i) Encoded scan keyboard N Key rollover

(ii) Sixteen 8-bit character display Left entry

Code0 0 0 D D K K K

0 001 1

Command Word = 0AH


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7.Write a command word to set blanking code for common

anode display & to clear the FIFO RAM Status

Code1 1 0 CD2 CD1 CD0 CF CA

1 011 1

Command Word = DEH

Blanking code for common anode display is all

ones (CD1 & CD0).

8 Find the program clock word if external clock frequency is


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8.Find theprogram clock word if external clock frequency is

2 MHz.

Code 0 0 1 P P P P P1 010 0

Command Word =(0011 0100)2=34H

To give proper scan & key debounce times, theinternal frequency should be 100 KHz

Pre-scalar value = External Clock / Internal Frequency

Pre-scalar value = (2x106) / (100x103)= (20)D = (10100)H = (PPPPP)


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04.8279-Key Board Display