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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Scans and encodes up to a key keyboard. Consists of bidirectional pins that connect to data bus on micro. Decoded keyboard with N-key rollover. RL pins incorporate internal pull-ups, no need for external resistor pull-ups.

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.

These are the output ports for two 16×4 or one 16×8 internal display refresh registers. Generates a basic timer interrupt that occurs at approximately If more than 8 characters are entered in the FIFO, then it means more than eight keys are pressed diaplay a time. Used internally for timing. Interface of 2 Keyboard type is programmed next.

It is enabled only when D is low. If two bytes are programmed, then the first byte LSB stops the count, intertace the second byte MSB starts the counter with the new count. Displsy are 6 modes of operation for each counter: This is when the overrun status is set. Decoded keyboard with 2-key lockout.


Controls up to a digit numerical display. Unlike the 82C55, the must be programmed first. SL outputs are active-low only one low at any time.

Used for controlling real-time events such as real-time clock, events counter, and motor speed and direction control. This interfce controls the flow of data through the microprocessor. Keyboard Interface of These lines can be programmed as encoded or decoded, using the mode control register.

It has an internal pull up. This unit first scans the key closure row-wise, if found then the keyboard debounce unit debounces the key entry.

8279 – Programmable Keyboard

Scan line outputs scan both the keyboard and displays. Selects type of display read and address of the read.

Provides a timing source to the internal speaker and other devices. The data from these lines is synchronized with the scan lines to scan the display and the keyboard. The output becomes a logic 0 when the control word is written and remains there until N plus the number of programmed counts. Z selects auto-increment for the address. To determine if a character has been typed, the FIFO status register is checked. Encoded mode and Decoded mode. In the Polled modethe CPU periodically reads an internal flag of to check whether any key is pressed or not with key pressure.

Selects type of FIFO read and address of the read. Encoded keyboard with N-key rollover. DD sets displays mode. Output that blanks the displays.


DD field selects either: This mode deals with the input keyboaard by the keyboard and this mode is further classified into 3 modes. The 74LS drives 0’s on one line at a time.

Programmable Keyboard/Display Interface –

Interrupt request, becomes 1 when a key is pressed, data is available. Max is 3 MHz.

Keyboard Interface of Keyboarx field: 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. MMM sets keyboard mode.

Microprocessor – Programmable Keyboard

Till it is pulled low with a key closure, it is pulled up internally to keep it high. Programs internal clk, sets scan and debounce times. The keyboard consists of maximum 64 keys, which are interfaced with inteeface CPU by using the key-codes.

DD Function 00 8-digit display with left keygoard 01 digit display with left entry 10 8-digit display with right entry 11 digit display with right entry. These lines are set to 0 when any key is pressed. Six Digit Display Interface of When it is low, it indicates the transfer of data.