1. Keyboard/Display Controller (8279)

2. Matrix Keyboard Multiplexed Display
Interfacing of a single key switch with 8085 is complex for interfacing more number of switches.
Connected in matrix form
Rows of keyboard matrix – 4 output port lines
Column lines – 4 input port lines
Multiplexed Display
Interfacing more number of seven segment display

3. Functional Block Diagram

4. Features of IC
Keyboard of size up to 64-key matrix with 2-key lockout and n-key rollover options
Simultaneous keyboard and display operation
8 character FIFO memory
16 digits of display

5. 3 Sections Display Section with its own display RAM
Keyboard scan section with FIFO registers
Control Logic with signals for interfacing with the processor

6. Signal description 8279 pins

7. Pin diagram and Signal description of 8279

8. Control Signals CS: Chip Select A0: Select data or control register
A0 = Logic 1 content of data bus is command / status
A0 = Logic 0 content of data bus is data for IC
OUT A0-A3 and OUT B0-B3: Data output lines for the display units.
BD: Blank all displays
IRQ: Interrupt request sent to processor
CNTL/STB: Control or Strobe signal.
Shift: Input to 8279 from shift key of keyboard
SL0-SL3: 4 Scan lines
2 modes : Encoded mode & Decoded Mode

9. Control Word selection using MSB 3 bits
FUNCTION
Mode set 1
Clock
Read FIFO
Read Display
Write Display
Display write inhibit
Clear
End Interrupt

10. Keyboard/Display Mode Set
DD is the display mode and KKK is the keyboard mode

11. Keyboard/Display Mode Set

12. Keyboard/Display Mode Set

13. Programmable Clock
PPPPP is a 5-bit binary constant. The input frequency is divided by a decimal constant ranging from 3 to 31, decided by the bits of an internal prescalar PPPPP

14. Read FIFO/Sensor RAM X – Don’t Care AI – Auto Increment flag
AAA – Address pointer to 8-bit FIFO RAM

15. Read Display RAM
AI is auto increment flag and AAAA, the 4-bit address, points to the 16-byte display RAM that is to be read. If AI=1, the address will be automatically, incremented after each read or write to the display RAM.

16. Write Display RAM

17. Clear Display RAM CD – Clear display RAM
CF – Clear the keyboard FIFO RAM
CA – Clear both display Ram & FIFO RAM

18. End Interrupt/Error Mode Set
For N-key rollover mode, if the E bit is programmed to be ‘1’,
the 8279 operates in special error mode

19. Keyboard status Word format
S/E O U F N
N – Number of keys pressed
F – Indicate FIFO is full
U – Underrun error bit ( read attempt from empty FIFO)
O – Overrun error bit ( entry into full FIFO)
S/E – Multiple key pressed
D – Unavailability of display

20. Keyboard Code Word format
CTRL
SHIFT S2 S1 S0 R2 R1 R0
R0,R1,R2 – Encoded column position
S0,S1,S2 – Encoded Row position

21. Interfacing Keyboard/Display to the Microprocessor Using Intel 8279