The INTEL 8279 is specially developed for
interfacing keyboard and display devices to 8085/8086/8088 microprocessor based
system. The important features of 8279 are,
1.
Simultaneous
keyboard and display operations.
2.
Scanned
keyboard mode.
3.
Scanned
sensor mode.
4.
8-character
keyboard FIFO.
5.
1
6-character display.
6.
Right or
left entry 1 6-byte display RAM.
7.
Programmable
scan timing.
Keyboard section:
·
The
keyboard section consists of eight return lines RL0 - RL7 that can be used to
form the columns of a keyboard matrix.
·
It has
two additional input: shift and control/strobe. The keys are automatically
debounced.
·
The two
operating modes of keyboard section are 2-key lockout and N-key rollover.
·
In the
2-key lockout mode, if two keys are pressed simultaneously, only the first key
is recognized.
·
In the
N-key rollover mode simultaneous keys are recognized and their codes are stored
in FIFO.
·
The
keyboard sections also have an 8 x 8 FIFO (First In First Out) RAM.
·
The FIFO
can store eight key codes in the scan keyboard mode. The status of the shift
key and control key are also stored along with key code. The 8279 generate an
interrupt signal when there is an eFIFO.
Block diagram of 8279:
·
The
functional block diagram of 8279 is shown.
·
In
sensor matrix mode the condition (i.e., open/close status) of 64 switches is
stored in FIFO RAM. If the condition of any of the switches changes then the
8279 asserts IRQ as high to interrupt the processor.
Display section:
·
The
display section has eight output lines divided into two groups A0-A3 and B0-B3.
·
The
output lines can be used either as a single group of eight lines or as two
groups of four lines, in conjunction with the scan lines for a multiplexed
display.
·
The
output lines are connected to the anodes through driver transistor in case of
common cathode 7-segment LEDs.
·
The
cathodes are connected to scan lines through driver transistors.
·
The
display can be blanked by BD (low) line.
·
The
display section consists of 16 x 8 display RAM. The CPU can read from or write
into any location of the display RAM.
Scan section:
·
The scan
section has a scan counter and four scan lines, SL0 to SL3.
·
In decoded
scan mode, the output of scan lines will be similar to a 2-to-4 decoder.
·
In
encoded scan mode, the output of scan lines will be binary count, and so an
external decoder should be used to convert the binary count to decoded output.
·
The scan
lines are common for keyboard and display.
·
The scan
lines are used to form the rows of a matrix keyboard and also connected to
digit drivers of a multiplexed display, to turn ON/OFF.
CPU interface section:
·
The CPU
interface section takes care of data transfer between 8279 and the processor.
·
This
section has eight bidirectional data lines DB0 to DB7 for data transfer between
8279 and CPU.
·
It
requires two internal address A =0 for selecting data buffer and A = 1 for
selecting control register of8279.
·
The
control signals WR (low), RD (low), CS (low) and A0 are used for read/write to
8279.
·
It has
an interrupt request line IRQ, for interrupt driven data transfer with
processor.
·
The 8279
require an internal clock frequency of 100 kHz. This can be obtained by dividing
the input clock by an internal prescaler.
·
The
RESET signal sets the 8279 in 16-character display with two -key lockout
keyboard modes.
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