The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Switches and Keypads
• A single capacitor adds debouncing. A capacitor connected at KBM (keybounce mask)
sets the debounce period. With a 1-microfarad capacitor, the 74C922 ignores
keypresses shorter than 10 milliseconds. Only when a keypress lasts longer than this
does the chip latch the data and bring DA high. In a similar way, after the key opens, the
debouncing must time out before a new key press is detected. The debounce period
varies directly with capacitor size, with larger values increasing the time. By
experimenting with different values, you can adjust the timing until all key bounces are
ignored, yet no noticeable delay is required between key presses.
Adding a Keypad
Figure 7-9 shows the 74C922 connected to the 8052-BASIC system. The 74C922 interfaces
the keypad to a 72LS244 buffer, as described in Chapter 6. The 74C922 is shown addressed
at E000h, but you can use any available chip-select. Pin 13 of the 74C922 is tied low to
permanently enable the data outputs. Keypresses are read by reading the buffer at E000h.
The keypad’s X and Y lines connect to their corresponding pins on the 74C922. Data outputs
A-D connect to D0-D3 on the 8052’s data bus. DA, which signals when a key has been pressed,
connects to INT1 on the 8052-BASIC. If you want to use polling to detect keypresses, you
can use any input port pin instead of INT1.
Figure 7-9. Circuits for adding a matrix-encoded keypad to an
8052-BASIC system.
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