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: Scan line outputs scan both the keyboard and displays. Usually decoded at port address interfade and has following functions: This unit controls the flow of data through the microprocessor.

This mode deals with display-related operations.

Microprocessor – Programmable Keyboard

DD sets displays mode. Six Digit Display Interface of MMM sets keyboard mode.

If two bytes are programmed, then the first byte LSB stops the count, and the second byte MSB starts the counter with the new count. Strobed keyboard, decoded display scan. Minimum count intreface 1 all modes except 2 and 3 with minimum count of 2. Programs internal clk, sets scan and debounce times. The timing and control unit handles the timings for the operation of the circuit. The keyboard consists of maximum keyyboard keys, which are interfaced with the CPU by using the key-codes.


These lines are set to 0 when any key is pressed. Max is 3 MHz. Generates a basic timer interrupt that occurs at approximately Clears the display or FIFO. Keyboard Interface of Dlsplay three bits given below select one of 8 control registers opcode.

Z selects auto-increment for the address. This unit contains registers to store the keyboard, display modes, and other operations as programmed by the CPU.

Interrupt request, becomes 1 when a key is pressed, data is available. Selects type of write and the address of the write. Pins SL2-SL0 sequentially scan each column through a counting operation. These are the scan lines used to scan the keyboard matrix and display the digits. Interrupts the micro at interrupt vector 8 for a clock tick.

Generates a continuous square-wave with G set to 1. To determine if a character has been typed, the FIFO status register is checked. The output becomes a logic 0 when the control word is written and remains there until N plus the number of programmed counts.

Intel 8279

Once done, a procedure is needed to read data from the keyboard. Selects the number of display positions, type of keybboard scan Selects type of display read and address of the read. SL outputs are active-low only one low at any time.


These are the Return Lines which are connected to one terminal of keys, while the other terminal of the keys is connected to the decoded scan lines.

Allows half-bytes to be blanked. When it is low, it indicates the transfer of data. It has an internal pull up.

Intel – Wikipedia

In the oeyboard scan modethe counter internally decodes the least significant 2 bits and provides a decoded 1 out of 4 scan on SL 0 -SL 3. It has two modes i. Encoded keyboard with 2-key lockout.

The keyboard first scans the keyboard and identifies if any key has been pressed.

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: The line is pulled down keyboarx a key closure.