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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In the Interrupt mode intrrface, the processor is requested service only if any key is pressed, otherwise the CPU will continue with its main task. The 74LS drives 0’s on keyboatd line at a time. Pins SL2-SL0 sequentially scan each column through a counting operation. This is when the overrun status is set. DD field selects either: Consists of bidirectional pins that connect to data bus on micro.

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. The line is pulled down with a key closure. Selects the number of display positions, type of key scan The Keyboard can be interfaced either in the interrupt or the polled mode. Sl outputs are active-high, follow binary bit pattern or This mode deals with the input given by the keyboard and this mode is further classified into 3 modes.

If two bytes are programmed, then the first byte LSB stops the keyboarv, and the second byte MSB starts the counter with the new count. 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. 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.


Iterface lines are set to 0 when any key is pressed. Selects type of display read and address of the read. Strobed keyboard, decoded display scan.

Z selects auto-increment for the address. This mode deals with display-related operations.

Interrupts the micro disply interrupt vector 8 for a clock tick. Interface of WWBB The display write inhibit control word inhibits writing to either the leftmost 4 bits of the display left W or rightmost 4 bits. Keyboard has a built-in FIFO 8 character buffer.

Intel 8279

Controls up to a digit numerical display. Chip select that enables programming, reading the keyboard, etc. These are the scan lines used to scan the keyboard matrix and display the digits. Clears the IRQ signal dusplay the microprocessor. Interrupt request, becomes 1 when a key is pressed, data is available.

Intel – Wikipedia

Pinout Definition A0: Allows half-bytes to be blanked. Keyboard Interface of First three bits given below select one of 8 control registers opcode.

Causes DRAM memory system to be refreshed. Strobed keyboard, encoded display interfsce. Scans and encodes up to a key keyboard. The first 3 bits of sent to control port selects one of 8 control words.

Keyboard Interface of The keyboard matrix can be any size from 2×2 to 8×8. It has two modes i. In the keyboard mode, this line is used as a control input and stored in FIFO on a key closure. An events counter enabled with G. Keyboard Interface of The Shift input line status is stored along with every key code in FIFO in the scanned keyboard mode. These lines can be programmed as encoded dlsplay decoded, using the mode control register.


BB works similarly except that they blank turn off half of the output pins. Encoded keyboard with inrerface lockout. Encoded mode and Decoded mode.

Till it is pulled low with a key closure, it is pulled up internally to keep it high. The output becomes a logic 0 when the keyboarv word is written and remains there until N plus the number of programmed counts. Its data buffer interfaces the external bus of the system with the internal bus of the microprocessor.

There are 6 modes of operation for each counter: In the Polled modethe CPU periodically reads an internal flag of to check whether any key is pressed or not with key pressure. Each counter has a program control word used to select the way the counter operates.

MMM sets keyboard mode. The timing and control unit handles the timings for the operation of the circuit. Unlike interfsce 82C55, keynoard must be programmed first. Scan line outputs scan both the keyboard and displays. Once done, a procedure is needed to read data from the keyboard. Six Digit Display Interface of