Intel’s is a general purpose Keyboard Display controller that simultaneously drives the display of a system and interfaces a Keyboard with the CPU. The Keyboard Display interface scans the Keyboard to identify if any key has been In the Polled mode, the CPU periodically reads an internal flag of to . KEYBOARD/DISPLAY CONTROLLER – INTEL Features of The important features of are, Simultaneous keyboard and display operations.

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The display section consists of 16 x 8 display RAM. The two operating modes of keyboard section are 2-key lockout and N-key rollover. An events counter enabled with G. This mode deals with the input given by the keyboard and this mode is further classified into 3 modes.

8279 – Programmable Keyboard

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. Consists of bidirectional keybowrd that connect to data bus on micro. BB works similarly except that they blank turn off half of the output pins. Allows half-bytes to be blanked. This can be obtained by dividing the input clock by an internal prescaler. Interface of 2 Keyboard type is programmed next. It has two modes i.


The output lines can be used either as a single group of eight lines or as two groups of four lines, in conjunction with the scan lines for a multiplexed display.

It has two additional input: DD field selects either: Encoded keyboard with 2-key lockout. If more than 8 characters are entered in the FIFO, then it means more than eight keys are pressed at a time.

Intel 8279

Max is 3 MHz. Z selects auto-increment for the address. Pins SL2-SL0 sequentially scan each column through a counting operation.

DD sets displays mode.

Microprocessor – Programmable Keyboard

The cathodes are connected to scan lines through driver transistors. In encoded scan mode, the output of scan lines will be binary count, and so an external decoder should be used to convert dislpay binary count to decoded output.

In the 2-key lockout mode, if two keys are pressed simultaneously, only the first key is recognized. It dsplay sends their relative response of the pressed key to the CPU and vice-a-versa.


Chip select that enables programming, reading the keyboard, etc. Keyboard Interface of First three bits given below select one of 8 control registers opcode.

Clears the IRQ signal to the microprocessor. Used internally for timing. MMM sets keyboard mode. Features of The important features of are, Simultaneous keyboard and display operations. Unlike the 82C55, the must be programmed first. RL pins incorporate internal pull-ups, no need for external resistor pull-ups. When it is low, it indicates the transfer of data. The timing and control unit handles dieplay timings for the operation of the circuit. Counter reloaded if G is pulsed again.

Selects the number of display positions, type of key scan Keyboard Interface of