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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Microprocessor – Programmable Keyboard

Interrupt request, becomes 1 when a key is pressed, data is available. This is when the overrun status is set. This mode deals with display-related operations. Programs internal clk, sets scan and debounce times. Encoded keyboard with N-key rollover. The previous example illustrates an encoded keyboard, external decoder used to drive matrix.

It has an internal pull up. DD Function Encoded keyboard with 2-key lockout Decoded keyboard with 2-key lockout Encoded keyboard with N-key rollover Decoded keyboard with N-key rollover Encoded sensor matrix Decoded sensor matrix Strobed keyboard, encoded display scan Strobed keyboard, decoded display scan Encoded: Causes DRAM memory system to be refreshed.

It has two modes i. Strobed keyboard, encoded display scan. This unit contains registers to store the keyboard, display modes, and other operations as programmed by the CPU. This unit first scans the key closure row-wise, if found then the keyboard debounce unit debounces the key entry.

Decoded keyboard with N-key rollover. Scans and encodes up to a key keyboard. Once done, a procedure is needed to read data from the keyboard.


It then sends their relative response of the pressed key to the CPU and vice-a-versa. Interface of 2 Keyboard type is programmed next.

Intel 8279

These are the scan lines used to scan the keyboard matrix and display the digits. Selects type of FIFO read and address of the read. Interrupts the micro at interrupt vector 8 for a clock tick.

Interface of Code given in text for reading keyboard. This mode deals with the input given by the keyboard and this mode is further classified into 3 modes. Keyboard Interface of First three bits given below select one of 8 control registers opcode.

8279 – Programmable Keyboard

The address inputs select one of the four internal registers with the as follows: Used for controlling real-time events such as real-time clock, events counter, and motor speed and direction control. Z selects auto-increment for inteerface address.

RL pins incorporate internal pull-ups, no need for external resistor pull-ups.

In the Polled modethe CPU periodically reads an internal flag of to check whether any key is pressed or dislay with key pressure. Selects type of display read and address of the read. When it is low, it indicates the transfer of data. Scan line outputs scan both the keyboard and displays. The line is pulled down with a key closure.

Consists of bidirectional pins displzy connect to data bus on micro. There are 6 modes of operation for each counter: The scans RL pins synchronously with the scan.

If two bytes are programmed, then the first byte LSB stops the count, and the second byte MSB starts interfacd counter with the new count. Intterface the decoded scan modethe counter internally decodes the least significant 2 bits and provides a decoded 1 out of 4 scan on SL 0 -SL 3.

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Each counter has a program control word used to select the way the counter operates. Decoded keyboard with 2-key lockout. Till it is pulled low with a key closure, it is pulled up internally to keep it high. Encoded keyboard with 2-key lockout. The Shift input line status is stored along with every key code in FIFO in the scanned keyboard mode. These lines are set to 0 when any key is pressed.

To determine if a character has been typed, the FIFO status register is checked. SL outputs are active-low only one low at any time.

Intel – Wikipedia

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. Max is 3 MHz. This unit 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. DD field selects either: In the Interrupt modethe processor is requested service only if any key is pressed, otherwise the CPU will continue with its main task.

Chip select that enables programming, reading the keyboard, etc. Sl outputs are active-high, follow binary bit pattern or

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