November 25, 2021

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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Intel 8279

keyboarx Output that blanks the displays. The 74LS drives 0’s on one line at a time. Keyboard has a built-in FIFO 8 character buffer. Consists of bidirectional pins that connect to data bus on micro.

Microprocessor – Programmable Keyboard

This mode is further classified into two output modes. Shift connects to Shift key on keyboard. Strobed keyboard, encoded display scan.

Max is 3 Sisplay. Encoded mode and Decoded mode. This mode deals with the input given by the keyboard and this mode is further classified into 3 modes. Strobed keyboard, decoded display scan.

DD field selects either: Causes DRAM memory system to be refreshed. Keyboard Interface of The keyboard matrix can be any size from 2×2 to 8×8. Interfafe for controlling real-time events such as real-time clock, events counter, and motor speed and direction control. 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. Encoded keyboard with 2-key lockout. Decoded keyboard with N-key rollover. This unit first interfcae the key closure row-wise, if found then the keyboard debounce unit debounces the key entry.


Clears the display or FIFO.

Programs internal clk, sets scan and debounce times. Each counter has a program control word used to select the way the counter operates. Till 88279 is pulled low with a key closure, it is pulled up internally to keep it high. This mode deals with display-related operations. Keyboard Interface of First three bits given below select one of 8 control registers opcode.

The timing and control unit handles the timings for the operation of the circuit. MMM sets keyboard unterface. Usually decoded at port address 40HH and has following functions: 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. Generates a continuous square-wave with G set to 1.

Intel – Wikipedia

The Keyboard can be interfaced either in the interrupt or the polled mode. These lines are set to 0 when any key is pressed. In the encoded mode, the counter provides the binary count that is to be externally decoded to provide the scan lines for the keyboard and display. Counter reloaded if G is pulsed again.


Generates a basic timer interrupt that occurs at approximately Selects type of write and the address of the write. Interrupt request, becomes 1 disppay a key is pressed, data is available. Chip select that enables programming, reading the keyboard, etc.

Scan line outputs scan both the keyboard and displays. Sl outputs are active-high, follow binary bit pattern or If more than 8 characters are entered in the FIFO, then it means more than eight keys are pressed at a time. Minimum count is 1 all modes except 2 and 3 with minimum count of 2.

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