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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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Z selects auto-increment for the address. Clears the IRQ signal to the microprocessor. Strobed keyboard, interfae display scan. This unit contains registers to store the keyboard, display modes, and other operations as programmed by the CPU. To determine if a character has been typed, the FIFO status register is checked.

BB works similarly except that they blank turn off half of the output pins. These lines can be programmed as encoded or decoded, using the mode control register. Encoded keyboard with 2-key lockout.

The Shift input line status is stored along with every key code in FIFO in the scanned keyboard mode. It is enabled only when D is low. These are the Return Lines which are connected to one interfzce of keys, while the other terminal of the keys is connected to the decoded scan lines. Allows half-bytes to be blanked.

This unit controls the flow of data through the microprocessor. The display is controlled from an internal 16×8 RAM that stores the coded display information. The 74LS drives 0’s on one line at a time. This mode deals with the input given by the keyboard and this mode is further classified into 3 modes.

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Provides a timing source to the internal speaker and other devices. 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. Encoded keyboard with N-key rollover.

Till it is pulled low with a key closure, it is pulled up internally to keep it high. The data from these lines is synchronized with the scan lines to scan the display and the keyboard.

Intel 8279

DD Function Encoded interafce 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: Interrupts the micro at interrupt vector 8 for a clock tick.

This mode is further classified into two output modes. In the keyboard mode, this line is used as a control input and stored in FIFO on a key closure. Keyboard Interface of The keyboard matrix can be any size from 2×2 to 8×8.

8279 – Programmable Keyboard

Unlike the 82C55, the must be programmed first. Once done, a procedure is needed to read data from the keyboard. Sl outputs are active-high, follow binary bit pattern or In the djsplay scan modethe counter internally decodes the least significant 2 bits and provides a decoded 1 out of 4 scan on SL 0 -SL 3. When it is low, it indicates the transfer of data.

In the Polled modethe CPU periodically reads an internal flag of to check whether any key is pressed or not with key pressure. Selects the number of display positions, type of key scan RL pins incorporate internal pull-ups, no need for external resistor pull-ups. The timing and control unit handles the timings for the operation of the circuit.


In the scanned sensor matrix mode, this unit acts as sensor RAM where its each row is loaded with the status of their corresponding row of sensors 8297 the matrix. MMM sets keyboard mode.

The keyboard first scans the keyboard and identifies if any key has been pressed. Interface of 2 Keyboard type is programmed next. The Keyboard can be interfaced either in the interrupt or the polled mode. The keyboard consists of maximum 64 keys, which are interfaced with the CPU by using the key-codes.

Counter reloaded if G is pulsed again. There are 6 modes of operation for each counter: Each counter has a program control word used to select the way the counter operates.

Microprocessor – Programmable Keyboard

Programs internal clk, sets scan and debounce times. If two bytes are programmed, then diplay first byte LSB stops the count, and the second byte MSB starts the counter with the new count.

Encoded mode and Decoded mode. Usually decoded at port address 40HH and has following functions: The line is pulled down with a key closure. Interface of WWBB The display write inhibit control word inhibits writing to either the leftmost 4 bits of the display disllay W or rightmost 4 bits.

Pinout Definition A0: In the Interrupt modethe processor is requested service only if any key is pressed, otherwise the CPU will continue with its main task. Generates a basic timer interrupt that occurs at approximately