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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8279 – Programmable Keyboard
Each counter has a program control word used to select the way the counter operates. These are the scan lines used to scan the keyboard matrix and display the digits. Used for controlling real-time events such as real-time clock, events counter, and motor speed and direction control.
Interface of 2 Keyboard type is programmed next. Return lines are inputs used to sense key depression in the keyboard matrix. Encoded keyboard with N-key rollover. Scans and encodes up to a key keyboqrd. Z selects auto-increment for the address. DD sets displays mode. DD Function 00 8-digit display with left entry 01 digit display with left entry 10 keyboarx display with right entry 11 digit display with right entry.
There are 6 modes of operation for each counter: Till it is pulled low with a key closure, it is pulled keyboar internally to keep it high. Keyboard Interface of MMM field: Selects type of write and the address of the write. In the keyboard mode, this line is used as a control input and stored in FIFO on a key closure.
The line is pulled down with a key closure. In interace Interrupt modethe processor is requested service only if any key is pressed, otherwise the CPU will continue with its main task. It is enabled only when D is low. This unit contains registers to store the keyboard, display modes, and other operations as programmed by the CPU. Counter reloaded if G is pulsed again.
Programmable Keyboard/Display Interface –
These lines can be programmed as encoded or decoded, using the mode control register. The keyboard first scans the keyboard and identifies if any key has been pressed.
Clears the display or FIFO. Unlike the 82C55, the must be programmed first.
Intel – Wikipedia
Strobed keyboard, decoded display scan. The previous example illustrates an encoded keyboard, external decoder used to drive matrix.
The Keyboard can be interfaced either in the interrupt or the polled mode. This mode is further classified into two output modes. This unit controls the flow of data through the microprocessor. In 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.
Pinout Definition A0: The address keyboare select one of dislay four internal registers with the as follows: Sl outputs are active-high, follow binary bit pattern or Minimum count is 1 all modes except 2 and 3 with minimum count of 2. Usually decoded at port address 40HH and has following functions: In the scanned sensor matrix mode, this unit acts as sensor RAM where its each row is loaded with the status of their corresponding row of sensors into the matrix.
If more than 8 characters are entered in the FIFO, then it means more than eight keys are pressed at a time. Selects type of display read and address of the read.