8279 PROGRAMMABLE KEYBOARD DISPLAY INTERFACE PDF

Draw the pin diagram of The pin diagram of is shown below: Fig. Draw the functional block diagram of and elaborate on the different blocks. The functional block diagram of is shown below: The different functional blocks of are a a CPU interface, b a set of scan lines, c input lines for key data and d output lines for display data. This line is normally connected to one of the hardware interrupt Fig. The scan lines SL0—3 along with the eight return lines RL0—7 can be used for construction of a keyboard matrix.

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Draw the pin diagram of The pin diagram of is shown below: Fig. Draw the functional block diagram of and elaborate on the different blocks. The functional block diagram of is shown below: The different functional blocks of are a a CPU interface, b a set of scan lines, c input lines for key data and d output lines for display data.

This line is normally connected to one of the hardware interrupt Fig. The scan lines SL0—3 along with the eight return lines RL0—7 can be used for construction of a keyboard matrix. The display output is available through A0—3 and B0—3 which can be used together as an 8-bit port. BD output signal is used for display blanking purposes. What are the two most important functions performed by ? The two most important functions performed by are as follows: a It scans the keyboard, then detects the key press and transmits to the CPU information which corresponds to the particular key pressed.

What are the various input modes in which operate? How many character definitions are possible using ? A maximum of character definitions are possible using When the CPU is actually involved for the scan and display functions to be realised? For the above two functions to be realised, CPU involvement is required only when data is actually transmitted to or received from the CPU. What are the modes in which the four scan lines can operate? The four scan lines SL0 — SL3 can be operated in two modes—encoded and decoded mode.

Discuss the encoded and decoded mode. Encoded mode: Here 16 lines are generated using the 4 scan lines and a 4 to 16 external decoder, although the manufacturers recommend not to use the SL3 line. Thus eight decoded scan lines are possible with SL0 — SL2 lines and a 3 to 8 decoder. Thus it leads to 64 different character definitions. Describe the Scanned Keyboard Mode. Both encoded and decoded scan versions are applicable in this case. This mode can be divided into two ways.

D5 — D3 bits correspond to the position of the row on which the key is pressed while D2 — D0 correspond to the position of the column on which the key is pressed. Several possibilities do arise which need to be addressed separately. In this case two possibilities occur: i If the first key i. Another possibility is pressing of two keys within one debounce cycle the time required for eliminating contact bounce effect is known as contact debounce time. In this case, no key is recognised.

When one key is released, the other key that remains pressed is recognised as a single valid key depression. N-Key Rollover: In this case, the debounce circuit waits for two scans after the first key press.

It then checks whether key is still in the pressed condition or not. If the answer is yes, then the data corresponding to the key press is taken into RAM of No limit is there to the number of key presses. For simultaneous key presses, data are entered according to the order of key press. If within a single debounce cycle, two keys are found pressed, the error flag is set and data entry into the RAM is prohibited.

Describe the Scanned Sensor Matrix Mode. Apart from switches, other logic circuit output lines can be connected to the RL lines. Describe the Strobed Input Mode. In this mode, data are placed on the return lines RLs. The source of data may be an encoded keyboard or a switch matrix. State the options available in the display mode.

The available options are: z Display format—either left entry also known as typewriter mode , or right entry also known as calculator mode. In the left entry or typewriter mode, the first entry goes to address 0, the second entry to address 1 and so on.

The first entry goes to the left most display position. The second entry to the just right of the earlier one. Thus the 16th entry goes to 15th address position.

In this mode, data can be entered at any arbitrary RAM address position. Assuming a 8-position display, if a command is inserted after the 2nd entry, then the next data will be displayed at 7th position. The explanation is like this: The most significant three bits represent the code for WRITE display, the next bit, i.

This is shown in Fig. In the right entry calculator mode, the characters are entered from the right most position. As characters are entered one after another, the present data occupies the right most position, just the earlier one occupies the left of the right most position etc. This is explained in Fig. In this mode, no correspondence exists between RAM address and the display position.

What are the different types of software operations possible with

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8279 - Programmable Keyboard

The advantage of is that it is able to drive the signals for both the keyboard and display and hence it is possible for the microprocessor to concentrate on its processing tasks without wasting time. The has two major sections. One is the Keyboard section and the other is the Display section. The chip provides four scan lines and eight return lines for interfacing keyboards and a set of eight output lines for interfacing a display.

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Programmable Keyboard-Display Interface - 8279

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.

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