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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Interface of Code given in text for reading keyboard. Allows half-bytes to be blanked. Keyboard has a built-in FIFO 8 character buffer.

Programs internal clk, sets scan and debounce times. The first 3 bits of sent to control port selects one of 8 control words. The 74LS drives 0’s on one line at a time.

Intel – Wikipedia

Decoded keyboard with 2-key lockout. An events counter enabled with G. 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 into the matrix. The address inputs select one of the four internal registers with the as follows: Selects type of display read and address of the read. Provides a timing source to the internal speaker and other devices. Unlike the 82C55, the must be programmed first.

In the keyboard mode, this line is used as a control input and stored in FIFO on a key closure. It has two modes i. Consists of bidirectional pins that connect to data bus on micro.

Output that blanks the displays. Strobed keyboard, decoded display scan. Chip select that enables programming, reading the keyboard, etc. Till it is pulled low with a key closure, it is pulled up internally to keep it high. This mode is further classified into two output modes.


Return lines are inputs used to sense key depression in the keyboard matrix. SL outputs are active-low only one low at any time. In the encoded mode, the counter provides the displwy count that is to be externally decoded to provide the scan lines for the keyboard and display.

Its data buffer interfaces the external bus of the system with the internal bus of the microprocessor. Pins SL2-SL0 sequentially scan each column through a counting operation. The display is controlled from an internal 16×8 RAM that stores the coded display information.

Controls up to a digit numerical display. Scans and encodes up to a key keyboard. Shift connects to Shift key on keyboard. 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. 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 imterface 3.

Keyboard Interface of The keyboard matrix can be any size from 2×2 to 8×8.

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: Clears the IRQ signal to the microprocessor.

Generates a continuous square-wave with G set to 1.

The keyboard consists of maximum 64 keys, which are interfaced with the CPU by using the key-codes. This mode deals with display-related operations. When it is low, it indicates the transfer of data.


8279 – Programmable Keyboard

Keyboard Interface of interfacw Used for controlling real-time events such as real-time clock, events counter, and motor speed and direction control. This unit controls the flow of data through the microprocessor. 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 MMM field: In the Interrupt modethe processor is requested service only if any key is pressed, otherwise the CPU will continue with its main task.

Programmable Keyboard/Display Interface –

In the Polled modethe CPU periodically reads an internal flag of interfave check whether any key is pressed or not with key pressure. Usually decoded at port address 40HH and has following functions: These are the output ports for two 16×4 or keybowrd 16×8 internal display refresh registers. It then sends their relative response of the pressed key to the CPU and vice-a-versa. The Shift input line status is stored along with every key code in FIFO in the scanned keyboard mode.

Once done, a procedure is needed to read data from the keyboard. It is enabled only when D is low. Sl outputs are interfxce, follow binary bit pattern or RL pins incorporate internal pull-ups, no need for external resistor pull-ups.