8279 Programmable Keyboard/display interface

Short Description
This is a hardware approach to interface a matrix keyboard and a multiplexed display.
The display can be set as a right entry or a left entry.

8279 Block Diagram


KEYBOARD SECTION


This sectio has 8 lines. RL0 – RL7. Plus 2 additional lines. Shift and CNTL/STB. The keys are automatically debounced and keyboard can operate in two modes:
>>two key lockout mode or
>>N-key rollover.
In two key lockout mode if 2 keys are pressed simultaneously only firstkey is recongnized.
In N key rollover mode, simultaneous keys are recongnized and stored in internal buffer: it can also
be set up so that no key is recongnized until only one key is remained pressed.


This has a FIFO RAM.
The status logic keeps track of number of entries and provides IRQ(interrupt request) signal when
FIFO is empty.


DISPLAY SECTION


This section has 8 output lines divided into 2 groups of 4. A0 – A3 and B0 – B3.
These lines can be used in both ways 8 lines or 2 sets of 4 lines.
The display can be blanked using BD line. The section has 16×8 display RAM.


SCAN SECTION


This section has scan counter and 4 scan lines. SL0 – SL3. These 4 scan lines can be 
decoded using a 4 – 16 decoder to generate 16 lines for scanning.
These 16 lines can be connected to rows of a matrix keyboard and digit driveers of multiplexed
display.


MPU INBTERFACE SECTION


This section has 8 bi directional lines. DB0 – DB7. 1 interrupt request line(IRQ). 
6 lines for interfacing including buffer address lines A0.


When A0 is high – signals are interpreted as control word or status.
When A0 is low – signal is interpreted as data.
IRQ goes high whenever data is ready to be loaded into MPU.


QUESTIONS FOR ABOVE CONTEXT
Q) List major components of 8279 and explain their function
Q) Explain block diagram of 8279.


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