I/O interfacing circuits –Hand shaking,serial and parallel interfacing – Address decoding Interfacing chips Programmable peripheral interfacing. In this presentation we get to know about keyboard Features, Cpu interface pins, Key board Data, Display data, Timing and control. Intel Programmable Key Board/Display Interface is available in the The description of pins of Programmable keyboard/display interface is given.

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Pinout Definition A0: Strobed keyboard, encoded display scan. RL pins incorporate internal pull-ups, no need for external resistor pull-ups. Interrupt request, becomes 1 when a key is pressed, data is available. The display is controlled from an internal 16×8 RAM that stores the coded display information. DD field selects either: The address inputs select one of the four internal registers with the as follows: If more than 8 characters are entered in the FIFO, then it means more than eight keys are pressed at a time.

Encoded keyboard with 2-key lockout. Keyboard Interface of Interface of WWBB The display write inhibit control word inhibits writing to either the leftmost 4 bits of the display left W or rightmost 4 bits. In the Polled modethe CPU periodically reads an internal flag of to check whether any key is pressed or not with key pressure.


This unit controls the flow of data through the microprocessor. Selects type of display read and address of the read. The Shift input line status is stored along with every key code in FIFO in the scanned keyboard mode.

Used for controlling real-time events such as real-time clock, events counter, and motor speed and direction control. DD sets displays mode.

Unlike the 82C55, the must be programmed first. Till it is pulled low with a key closure, it is pulled up internally to keep it high.

Causes DRAM memory interfsce to be refreshed. DD Function 00 8-digit display with left entry 01 digit display with left entry 10 8-digit display with right entry 11 digit display with right entry.

SL outputs are active-low only one low at any time. Strobed keyboard, decoded display scan.

Microprocessor – Programmable Keyboard

Generates a basic timer interrupt that occurs at approximately Chip select that enables programming, interfac the keyboard, etc.

Provides a timing source to the internal speaker and other devices.

These are the output ports for two 16×4 or one 16×8 internal display refresh registers. In the keyboard mode, this line is used as a control input and stored in FIFO on a key closure. Encoded mode and Decoded mode. The output becomes a logic 0 when the control word is written and remains there until N plus the number of programmed counts.


Decoded keyboard with 2-key lockout. Sl outputs are active-high, follow binary bit pattern or Disllay half-bytes to be blanked. Max is 3 MHz. The timing and control unit handles the timings for the operation of the circuit.

Keyboard Interface of First three bits given below select one of 8 control registers opcode. Counter reloaded if G is pulsed again.

Intel 8279

Keyboard Interface of The keyboard matrix can be any size from 2×2 to 8×8. Selects type of FIFO read and address of the read. The scans RL pins synchronously with the scan.

Selects type of write and the address of the write. These lines can be programmed as encoded or decoded, using the mode control register.