2. CONTENTS:
What is Programmable keyboard / Display Controller (8279)
Features of 8279
Pin Diagram of 8279
Modes of operation of 8279
3. WHAT IS 8279 ?
Intel’s 8279 is a general purpose Keyboard Display controller that simultaneously
drives the display of a system and interfaces a Keyboard with the CPU.
The Keyboard Display interface scans the Keyboard to identify if any key has been
pressed and sends the code of the pressed key to the CPU.
It also transmits the data received from the CPU, to the display device.
4. There are two approaches to interface a matrix keyboard and a multiplexed display
to the microprocessor namely,
1) Hardware Approach
2) Software Approach
The disadvantage of software approach is that the microprocessor is occupied for a
considerable amount of time in checking the keyboard and refreshing the display.
Intel 8279 follows hardware approach and it has the advantage of relieving the
processor from the above mentioned two tasks. But it is expensive.
5. FEATURES OF 8279:
• Intel 8279 is a 40 pin LSI device used to interface the keyboard and the
multiplexed display sections to the microprocessor 8085 or 8086.
• It supports keyboard of size up to 64 keys Matrix with 2-key lockout and n-key
rollover options.
• It supports display interface of up to 16 digits with many options.
• It has a built in hardware for key debouncing* operation.
• It provides 8 byte FIFO RAM to store keywords.
6. • It provides multiplexed display interface with blanking and inhibit options.
• It provides a 16 byte RAM corresponding to the 16 digit display, allowing to interface 16
digits.
• It supports simultaneous keyboard and display operations.
• It is fully programmable, providing three modes of operation.The three modes of operation
are Scanned Keyboard mode, Scanned Sensor Matrix mode, Strobed Input mode.
• It provides two output modes.They are Left entry(similar to type writer) and Right
entry(Similar to calculator)
• It automatically controls display refreshing and multiplexing.
8. These pins are divided in four functional groups :
1) CPU interface
2) Key data
3) Display data
4) Scan
9. CPU INTERFACE PINS:
It consists of 8-bit data bus, RD, WR, A0, CS, RESET, CLK and IRQ lines.
1) DB0-DB7 : Bi-directional data bus :
All data, commands and status information between the CPU and the 8279 are
transmitted on these bi-directional 8-bit data bus.
2) RD : Read:
It is an active low signal. It is enabled by the microprocessor. When RD signal is
low CPU reads the contents of selected register (display RAM, status register or FIFO
RAM) from 8279; depending on the type of command and the status of the A0 signal.
10. 3)WR : Write:
It is an active low signal. It is enabled by the microprocessor. When WR signal is
low, CPU loads the data or the command into selected register (control register or display
register) depending on the status of A0 signal.
4)A0 : Address line:
It is the buffer address line used to select the control port (A0=1) or the data
port(A0=0).It is enabled by the microprocessor.When A0 is high, signals are interpreted as
a command or status. When A0 is low, signals are interpreted as a data.
5)CS : Chip select :
It is an active low signal.It is enabled by the microprocessor. When low, enables the
communication between CPU and 8279.
11. 6)RESET :
A high signal on this pin resets 8279. After being reset 8279 is configured in the
following modes: (i) 16 x 8 bit character display-left entry. (ii)Two key lockout.
7)CLK :
This signal is usually provided by an external circuit and used to generate internal
timings.The clock frequency must be greater than 200KHz to provide correct delays.
8)IRQ : Interrupt Request :
It is an interrupt request signal provided by the 8279 to the microprocessor
indicating the request for the next data transfer.
It goes low with each FIFO/sensor RAM read and returns high if there is still
information in the RAM.
12. KEYBOARD DATA PINS:
This group consists of return, SHIFT and CNTL/STB lines.
1)RL0-RL7 : Return lines :
These input lines are used to interface matrix keyboard. These lines have active
internal pullups which keep their status high.
When the key from the matrix keyboard is pressed corresponding return line goes low.
In the strobed input mode these lines are used as 8 input lines.
2)SHIFT :
It is a special key input line. Its status is stored along with the key position on the
key closure in the scanned keyboard modes. It has an active internal pullup to keep it
high until, a switch closure pulls it low.
13. 3)CNTL/STB : (Control/strobe)
For scanned keyboard mode this line is used as a control input. Like SHIFT
key, its status is stored along with the key position on the key closure. It also
has an active internal pullup to keep it high until a switch closure pulls it low.
In the strobed input mode this line is used as a strobe input. When activated,
loads the status of keyboard into the FIFO RAM.
14. DISPLAY DATA PINS:
This group consists of OUT A3-A0, OUT B3-B0 and BD lines.
1)OUT A3-A0 and OUT B3-B0 :
These two four bit output ports; which can be considered as an one 8-bit port.
These are used for sending data to display drivers from display RAM and connected
to the segment inputs of 7 segment display or row inputs of dot matrix displays.
These lines are synchronized to the scan lines (SL0-SL3) for multiplexed digit
display. In other words we can say that when the data on the scan lines is 0000, ports
will have data from register 0 of display RAM and when the data on the scan lines is
1111, ports will have data from register 15 of display RAM.
The two 4-bit ports can be blanked independently.
2)BD : Blank Display :
This is an active low output used to blank the display during digit switching or by a
display blanking command.
15. SCAN DATA:
The scan lines SL0-SL3 scan the keyboard matrix and the display digits.
These lines can be programmed as encoder or decoder using mode control
registers.
16. MODES OF OPERATION OF 8279:
1) INPUT MODES
2) DISPLAY MODES
1)INPUT MODES:
8279 provides 3 basic input modes for the keyboard interface section.
(i)Scanned keyboard mode,
(ii)Scanned sensor matrix mode,
(iii)Strobed input mode.
(i)SCANNED KEYBOARD MODE:
In this mode,the keyboard can be scanned in two ways:Encoded scan and
decoded scan.The scanned keyboard mode allows key depressions in 2-key lockout or N-
key rollover.
17. (ii)SCANNED SENSOR MATRIX MODE:
In this mode,FIFO buffer is converted into 8 x 8 bit map.Each bit represents
the state of the switch at the same location in the keyboard matrix.
(iii)STROBED INPUT MODE:
In this mode,data is entered into the FIFO RAM from the return lines at the
rising edge of the CNTL/STB Signal.
18. 2) DISPLAY MODES:
(i) LEFT ENTRY:
In the left entry mode,the 8279 displays characters from left to right in
the multiplexed displays.
Eg:Typewriters
(ii) RIGHT ENTRY:
In the left entry mode,the 8279 displays characters from right to left in
the multiplexed displays.
Eg:Calculators.
