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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Scan line outputs scan both the keyboard and displays. When it is low, it indicates the interfacw of data. Interface of 2 Keyboard type is programmed next.

Used internally for timing.

Shift connects to Shift key on keyboard. Sl outputs are active-high, follow binary bit pattern or The scans RL pins synchronously with the scan. Strobed keyboard, decoded display scan.

Keyboard Interface of MMM field: 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. Chip select that enables programming, reading the keyboard, etc. 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.

Pinout Definition A0: Six Digit Display Interface of Interrupts the micro at interrupt vector 8 for a clock tick. In the Polled modethe CPU periodically reads an internal flag of to check whether any key is pressed or not with key pressure. This is when the overrun status is keybboard. The Shift input line status is stored along with every interfzce code in FIFO in the scanned keyboard mode.


An events counter enabled with G. Decoded keyboard with 2-key lockout. Unlike the 82C55, the must be programmed first. BB works similarly except that they blank turn off half of the output pins. Provides a timing source to the internal speaker and other devices.

Intel 8279

Used for controlling real-time events such as real-time clock, events counter, and motor speed and direction control. The 74LS drives 0’s on one line at a time. Z selects auto-increment for the address. The Keyboard can be interfaced either in the interrupt or the polled mode.

Clears the display or FIFO.

Intel – Wikipedia

Selects type of FIFO read and address of the read. Encoded keyboard with Inteerface rollover. Keyboard has a built-in FIFO 8 character buffer. MMM sets keyboard mode. Scans and encodes up to a key keyboard. These lines are set to 0 when any key is pressed. Output that blanks the displays. Generates a continuous square-wave with G set to 1.

Each counter has a program control word used to select the way the counter operates. Clears the IRQ signal to the microprocessor. In the Interrupt modethe processor is requested service only if any key is pressed, otherwise the CPU will continue with its main task.

It has two modes i. It then sends their relative response of the pressed key to the CPU and vice-a-versa. These lines intercace be programmed as encoded or decoded, using the mode control register.


Programmable Keyboard/Display Interface –

Programs internal clk, sets scan and debounce times. The display is controlled from an internal 16×8 RAM that stores the coded display information. Its data buffer interfaces the external bus of the system with the internal bus of the microprocessor. 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. Causes DRAM memory system to be refreshed. DD field selects either: Pins SL2-SL0 sequentially scan each column through a counting operation.

Minimum count is 1 all modes except 2 and 3 with minimum count of 2.

8279 – Programmable Keyboard

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 first interfxce bits of sent to control port selects one of 8 control words. The line is pulled down with a key closure. Decoded keyboard with N-key rollover. Selects type of write and the address of the write.

Consists of bidirectional pins that connect to data bus on micro.