Keyboard Interfacing and Display Interface - Computer Science Engineering (CSE) PDF Download

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Keyboard Interfacing

• In most  keyboards, the key switches are connected in a matrix of Rows and Columns.
• Getting meaningful data from a keyboard requires three major tasks:
  • Detect   a  key press
  • Debounce the key press .
  • Encode the keypress (produce a standard code for the pressed key).
• Logic „0‟ is read by the microprocessor when the key is pressed.

Key Debounce:

Whenever a mechanical push-bottom is pressed or released once,the mechanical components of the key do not change the positionsmoothly; rather it generates a transient response. These may be interpreted as the multiple pressures and responded accordingly.

• The rows of the matrix are connected to four output Port lines, &columns are connected to four input Port lines.
• When no keys are pressed, the column lines are held high by the pull-up resistors connected to +5v.
• Pressing a key connects a row & a column.
• To detect if any key is pressed is to output 0‟s to all rows & then check columns to see it a pressed key has connected a low (zero) to a column.
• Once the columns are found to be all high, the program enters another loop, which waits until a low appears on one of the columns i.e indicating a key press.
• A simple 20/10 m sec delay is executed to debounce task.
• After the debounce time, another check is made to see if the key is still pressed. If the columns are now all high, then no key is pressed & the initial detection was caused by a noise pulse.
• To avoid this problem, two schemes are suggested:

1. Use of Bistable multivibrator at the output of the key to debounce it.
2. The microprocess or has to wait for the transient period (at l e ast for 10 ms), so that the transient response settles down and reaches a steady state.

• If any of the columns are low now, then the assumption is made that it was a valid key press.
• The final task is to determine the row & column of the pressed key &convert this information to Hex-code for the pressed key.
• The 4-bit code from I/P port & the 4-bit code from O/P port (row &column) are converted to Hex-code.

Interfacing 4x4 keyboard

Display Interface

Interfacing multiplexed 7-segment display