Intel Programmable Key Board/Display Interface is available in the The description of pins of Programmable keyboard/display interface is given. The INTEL is specially developed for interfacing keyboard and display devices Programmable scan timing. Block diagram of The four major sections of are keyboard, scan, display and CPU interface. Keyboard section. The INTEL is a Keyboard/Display Controller specially developed for interfacing keyboard Programmable scan timing. Keyboard section: The CPU interface section takes care of data transfer between the and the processor.

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The first 3 bits of sent to control port selects one of 8 control words. To determine if a character has been typed, the FIFO status register is checked.

8279 – Programmable Keyboard

Clears the IRQ signal to the microprocessor. Selects the number of display positions, type of key scan Controls up to a digit numerical display.

DD sets displays mode. In the Polled modethe CPU periodically reads an internal flag of to check whether any key is pressed or not with key pressure. Consists of bidirectional pins that dissplay to data bus on micro. This mode deals with the input given by the keyboard and this mode is further classified into 3 modes.


The keyboard first scans the keyboard and identifies if any key has been pressed. Its data buffer interfaces the external bus of the system with the internal bus of the microprocessor. This unit first scans the key closure row-wise, if found then the keyboard debounce unit debounces the key entry.

Microprocessor – Programmable Keyboard

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. This unit controls the flow of data through the microprocessor. Allows half-bytes to be blanked. Encoded mode and Decoded mode. There are 6 modes of operation for each counter: The 74LS drives 0’s on one line at a time. It then sends their relative response of the pressed key to the CPU and vice-a-versa.

Clears the display or FIFO. Once done, a procedure is needed to read data from the keyboarr.

The timing and control unit handles the timings for the operation of the circuit. When it is low, it indicates the transfer of data.

Intel 8279

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. Scan line outputs scan both the keyboard and displays. The previous example illustrates an encoded keyboard, external decoder used to drive matrix.


The data from these lines is synchronized with the scan lines to scan the display and the keyboard. The keyboard consists of maximum 64 keys, which are interfaced with the CPU by using the key-codes. Keyboard Interface of The keyboard matrix can be any size from 2×2 to 8×8. BB works similarly except that they blank turn off half of the output pins.

Interface of Code given in text for reading keyboard. MMM sets keyboard mode. Interrace Definition A0: Interrupts the micro at interrupt vector 8 for a clock tick.

Unlike the 82C55, the must be programmed first. Minimum count is 1 all modes except 2 and 3 with minimum count of 2.

It has two modes i. The Shift input line status is stored along with every key code in FIFO in the scanned keyboard mode.