The 8279 is a programmable keyboard and display interfacing chip that can interface with up to a 64-key matrix keyboard and control up to a 16-digit multiplexed display. It has sections for keyboard scanning, display control, scan generation, and an MPU interface. The keyboard and display sections utilize scan lines to multiplex rows and columns to read key presses and update display segments.

1. Programmable Keyboard/Display Interface THE 8279

2. Block diagram of 8279 based system 8085 8279 16-character Display 16-Key Matrix Keyboard Keyboard Control Decoder SL0 SL1 SL2 SL3 BD A3-A0 B3-B0 RL3-RL0 D7-D0 RD WR IRQ CLK A0

3. Introduction 8279 A programmable keyboard and display interfacing chip Hardware approach to interfacing a matrix keyboard and a multiplexed display Scans and encodes up to 64-key keyboard Controls up to 16-digit numerical display

4. 8279 has 4 main segments Keyboard Scan Display MPU Interface

5. 1. Keyboard Section Consists of 8 lines (RL0-RL7) that can be connected to eight columns of a matrix keyboard Shift and CNTL/STB (Control/Strobe) 8-byte FIFO RAM Keys are automatically debounced

6. Block Diagram 8279 Keyboard Section

7. 2. Display Section Consists of 8 output lines (A0-A3) and (B0-B3) 16 byte display RAM BD line used to blank displays

8. Block Diagram 8279 Display Section

9. 3. Scan Section Consists of Scan Counter Four scan lines (SL0-SL3) Used by keyboard and display section for proper functioning

10. Block Diagram 8279 Scan Section

11. MPU Interface section 8 bidirectional data lines (DB0-DB7) IRQ line RD line WR line CS line RESET line Buffer address line (A0)

12. Block Diagram 8279 MPU Interface Section

13. Pin Description 8279 A0: Selects data (0) or control/status (1) for reads and writes between micro and 8279. Output that blanks the displays. CLK: Used internally for timing. Max is 3 MHz. CN/ST: Control/strobe, connected to the control key on the keyboard. Chip select that enables programming, reading the keyboard, etc .  DB7-DB0: Consists of bi-directional pins that connect to data bus on micro .

14. IRQ: Interrupt request, becomes 1 when a key is pressed, data is available. OUT A3-A0/B3-B0: Outputs that sends data to the most significant/least significant nibble of display. : Connects to micro's IORC or RD signal, reads data/status registers. RESET: Connects to system RESET. RL7-RL0: Return lines are inputs used to sense key depression in the keyboard matrix. Shift: Shift connects to Shift key on keyboard. SL3-SL0: Scan line outputs scan both the keyboard and displays.

15. Scanning keys of a Matrix Keyboard Vcc Row0 Row1 Row2 Row3 C0 C1 C2 C3 R = 4.7K R = 4.7K 0 1 2 3 4 5 6 7 8 9 A B C D E F

16. Connecting a 7-segment display with 8279 8279 4-to-16 decoder SL0 SL1 SL2 SL3 A0 A1 A2 A3 B0 B1 B2 B3 a b c d e f g h O 15 O 0 V CC a b c d e f g h V CC O 1

17. Working with a multiplexed display 8279 4-to-16 decoder SL0 SL1 SL2 SL3 A0 A1 A2 A3 B0 B1 B2 B3 O 15 O 0 T1 O 1 T2 O 2 T3 O 3 T4

18. Connecting a 64-key Keyboard with 8279 8279 4-to-16 decoder SL0 SL1 SL2 SL3 RL0 O 13 O 0 O 7 Multiplexed 7-Segment Display O 8 O 9 O 10 O 11 O 12 O 14 O 15 RL1 RL2 RL3 RL4 RL5 RL6 RL7