Right or left entry byte display RAM. Programmable scan timing. The functional block diagram of is shown. The four major sections of are keyboard, scan, display and CPU interface.
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The keyboard first scans the keyboard and identifies if any key has been pressed. It then sends their relative response of the pressed key to the CPU and vice-a-versa. The Keyboard can be interfaced either in the interrupt or the polled mode. In the Interrupt mode, the processor is requested service only if any key is pressed, otherwise the CPU will continue with its main task.
In the Polled mode, the CPU periodically reads an internal flag of to check whether any key is pressed or not with key pressure. How Does Keyboard Work? The keyboard consists of maximum 64 keys, which are interfaced with the CPU by using the key-codes. If more than 8 characters are entered in the FIFO, then it means more than eight keys are pressed at a time. This is when the overrun status is set. It is enabled only when D is low. Its data buffer interfaces the external bus of the system with the internal bus of the microprocessor.
Control and Timing Register and Timing Control This unit contains registers to store the keyboard, display modes, and other operations as programmed by the CPU. The timing and control unit handles the timings for the operation of the circuit. Scan Counter It has two modes i. Encoded mode and Decoded mode. 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.
In the decoded scan mode, the counter internally decodes the least significant 2 bits and provides a decoded 1 out of 4 scan on SL0-SL3. Return Buffers, Keyboard Debounce, and Control This unit first scans the key closure row-wise, if found then the keyboard debounce unit debounces the key entry.
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. CLK The clock input is used to generate internal timings required by the microprocessor. When it is low, it indicates the transfer of data. Vss, Vcc These are the ground and power supply lines of the microprocessor. These lines can be programmed as encoded or decoded, using the mode control register. These lines are set to 0 when any key is pressed.
It has an internal pull up. The line is pulled down with a key closure. BD It stands for blank display. It is used to blank the display during digit switching. The data from these lines is synchronized with the scan lines to scan the display and the keyboard.
Input Mode This mode deals with the input given by the keyboard and this mode is further classified into 3 modes.
Output Mode This mode deals with display-related operations. This mode is further classified into two output modes.
The Keyboard Display interface scans the Keyboard to identify if any key has been pressed and sends the code of the pressed key to the CPU. It also transmits the data received from the CPU, to the display device. Both of these functions are performed by the controller in repetitive fashion without involving the CPU. The Keyboard is interfaced either in the interrupt or the polled mode. In the interrupt mode, the processor is requested service only if any key is pressed, otherwise the CPU can proceed with its main task. In the polled mode, the CPU periodically reads an internal flag of to check for a key pressure. The Keyboard section can interface an array of a maximum of 64 keys with the CPU.
8279 - Programmable Keyboard
Right or left entry byte display RAM. Programmable scan timing. Used for Interaction between keyboard and different microprocessor. Keyboard section: The keyboard section consists of eight return lines RL0 - RL7 that can be used to form the columns of a keyboard matrix. The keys are automatically debounced. The two operating modes of keyboard section are 2-key lockout and N-key rollover. In the 2-key lockout mode, if two keys are pressed simultaneously, only the first key is recognized.