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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The scans RL pins synchronously with the scan. This mode is further classified into two output modes. Z selects auto-increment for the address.

Programmable Keyboard/Display Interface –

Consists of bidirectional pins that connect to data bus on micro. Causes DRAM memory system to be refreshed. Controls up interfave a digit numerical display.

If more than 8 characters are entered in the FIFO, then it means more than eight keys are pressed at a time. DD Function Encoded keyboard with 2-key lockout Decoded keyboard with 2-key lockout Encoded keyboard with N-key rollover Decoded keyboard with N-key rollover Encoded sensor matrix Decoded sensor matrix Strobed keyboard, encoded display scan Strobed keyboard, decoded display scan Encoded: Generates a continuous square-wave with G set to 1.


It has two modes i. Scans and encodes up to a key keyboard.

The display is controlled from an internal 16×8 RAM that stores the coded display information. Allows half-bytes to be blanked. This unit contains registers to store the keyboard, display modes, and other operations as programmed by the CPU.

Intel 8279

The 74LS drives 0’s on one line at a time. Interface of 2 Keyboard type is programmed next. Sl outputs are active-high, follow binary bit pattern or Interrupt request, becomes 1 when a key is pressed, data is available.

RL pins incorporate internal pull-ups, no need for external resistor pull-ups. These are the output ports for two 16×4 or one 16×8 internal display refresh registers.

It then sends their relative response of the pressed key to the CPU and vice-a-versa. Decoded keyboard with 2-key lockout. Interface of Code given in text for reading keyboard. Max is 3 MHz. This is when the overrun status is set. Each counter has a program control word used to select the way the counter operates. Selects type of write and the address of the write.

Used internally for timing.

8279 – Programmable Keyboard

Unlike the 82C55, the must be programmed first. Counter reloaded if G is pulsed again.

Minimum count is 1 all modes except 2 and 3 with minimum count of 2. Till it is pulled low with a key closure, it is pulled up internally to keep it high. Interface of Displa The display write inhibit control word inhibits writing to either the leftmost 4 bits of the display left W or rightmost 4 bits.


When it is low, it indicates the transfer of data. 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. Clears the IRQ signal to the microprocessor.

Keyboard Interface of MMM field: Selects the number of display positions, type of key scan The output becomes a logic 0 when the control word is written and remains there until N plus the number of programmed counts.

Keyboard Interface of In the decoded scan modethe counter internally decodes the least significant 2 bits and provides a decoded 1 out of 4 scan on SL 0 -SL 3. This unit controls the flow of data through the microprocessor. Generates a basic timer interrupt that occurs at approximately