The keypad discussed here is available from the Electronics Resource Room in CEB 355. Its model number is 83-BB1-002.

The layout of the keyboard is shown below. The y-signals from the keyboard are connected to pull-up resistors so that they are high when no key is pressed. The x-signals are connected to pull-down resistors so that they are low when no key is pressed.

Debouncing immediately after a key is pressed or released

Debouncing occurs right after a key is pressed or released. Each time the X and Y inputs change because of a keypress or keyrelease, we wait 1 ms before sampling the X and Y values. Hopefully, any oscillations would have died out during this time. A counter is used to count from 0 up to 25175 each time the clock signal goes high, since this corresponds to 1 ms when the internal 25.174 kHz is used.

Description of pins
 

Signal	Description	Input/Output
Col_input	These are the pins connected between the keyboard and the Altera board. They correspond to X1 to X4 and Y1 to Y4.	Input, 8 bits
KeyData	Binary representation of which key is pressed (0 to F)	Output, 4 bits
KeyAvailable	When high, indicates that the value of KeyData is valid	Output, 1 bit

 

Here is a hard copy of the VHDL file and the scf file :  keydecoder.tar.gz

Last update : October 28, 1998

Send any questions or comments to Su-tarn Lim : sulim@ee.ualberta.ca

Group : Sydney Tang, Su-Tarn Lim, Chen-song Qin
Acknowledgements: Edgar Wong for his debouncing ideas