Programmable Keyboard/Display Interface – The scans RL pins synchronously with the scan. Clears the IRQ signal to the microprocessor. Sep 20, – Programmable Keyboard/Display InterfaceIIE – SAP. The Intel® is a general purpose programmable keyboard and display 1/0 interface device designed for use with Intel® microprocessors. The keyboard.

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It consists of four main sections. The data buffers are 8-bit bi-directional buffers that connect the internal data bus to the external data bus.

Programmable Keyboard/Display Interface –

When A 0 is logic 0 data is transferred and when A 0 is logic 1 command word or status word is transferred. RD and WR determine -the direction of data flow through the data buffers. The control and timing registers store the keyboard and display modes and other operating conditions programmed by the CPU.

The command is latched 82779 the rising edge of WR.

The timing control consists of the basic timing counter chain. The first counter is divided by N prescaler that can be programmed to give an internal frequency of kHz.

Intel 8279

The other counters divide down the basic internal frequencyto provide the proper keyscan, row scan, keyboard matrix scan, and display scan times. The internal frequency of KHz gives the internal timings as shown in the table The scan section has a scan counter which has two modes: Encoded mode and decoded mode.


In the encoded mode, the scan counter provides a binary count microprocsssor to on the four scan lines SC 3 — SC 0 with active high outputs. This binary count must be externally decoded to provide 16 scan lines. Display can use all 16 scan lines to interface 16 digit 7-segment display, but keyboard can use only 8 scan lines out of 16 scan lines.

In the decoded mode, the internal decoder decodes the least significant 2 bits of binary count and provides four possible combinations on the scan lines SC 3 — SC 0: Thus the output of decoded scan is active low. These four active low output lines can be used directly to interface 4 digit 7 segment display, 8 x 4 matrix keyboard, eliminating the external decoder.

There functions depend on selected keyboard mode out of three keyboard input modes: Keyboard and debounce control is enabled micrprocessor when scanned keyboard mode is selected. In the scanned keyboard mode, return lines are scanned, looking for key closures in that row.

If the debounce circuit detects a close switch, it waits about 10 msec to check if the switch remains closed. This is mmicroprocessor dual function 8 x 8 RAM. In scanned keyboard and strobed input modes, it is a FIFO.

Each new entry is written into successive RAM positions and then read in order of entry. In sensor matrix mode, the memory is referred to as sensor RAM. Each row of the sensor RAM is loaded with the status of the corresponding row of sensor in the sensor matrix. The display section consists of display RAM, display address registers and display registers. It is 16 x 8 RAM, which stores the display codes for 16 digits.


It can be accessed directly by CPU. In decoded mode, uses only first four locations of display RAM. In encoded mode, Block Diagram of uses first eight locations for microprocesso digit display and all 16 lorntions for 16 digits display. The display address registers hold mkcroprocessor address of the microprocesslr currently being written or read by the CPU and scan count value.

If set in auto increment miccroprocessoraddress in the address register is incremented for each read or write. Display registers are two 4-bit registers A and B.

Block Diagram of | CPU interface and control section | Display section

They hold the bit pattern of character to be displayed. The contents of display registers A and B can be blanked and inhibited individually. Your email address will not be published.

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