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Schmidt Trigger Buffer
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Debounced Button Input
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Figure 17.7 Bouncing into the PIC microcontroller can be eliminated by adding a lter to the button input line.
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PIC MCU INPUT AND OUTPUT DEVICE INTERFACING
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This method is fairly reliable but requires a Schmidt trigger gate in your circuit. There may be a Schmidt trigger input available in your PIC microcontroller, but check the datasheet to nd out which states and peripheral hardware functions can take advantage of it. One comment I want to make about debouncing switches is to choose buttons with a positive click when they are pressed and released. These have reduced bouncing and are a lot easier to work with than other switches that don t have this feature. I have used a number of switches over the years that don t have this click, and they can be a real problem in circuits with intermittent connections and unexpected bouncing that occurs while the button is pressed and held down.
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Switch matrix keyboards and keypads are really just an extension to the simple buttons of the preceding section with many of the same concerns and issues to watch out for. The big advantage that a matrix keyboard gives you is the ability to handle a large number of pins for a relatively small number of PIC microcontroller pins. The PIC microcontroller is well designed for simply implementing switch matrix keypads, which, like liquid-crystal displays (LCDs) explained in the next section, can add a lot to your application with a very small investment in hardware and software. A switch matrix is simply a two-dimensional matrix of wires with switches at each vertex. The switch is used to interconnect rows and columns in the matrix, as can be seen in Fig. 17.8. This diagram may not look like the simple button, but it will become more familiar when I add pull-ups on the rows and switchable ground connections on the columns, as I show in Fig. 17.9. In this case, by connecting one of the columns to ground, if a switch is closed, the pull-down on the row will connect the line to ground. When the row is polled by an I/O pin, a 0, or low voltage, will be returned instead of a 1 (which is what will be returned if the switch in the row that is connected to the ground is open). As a rule of thumb, the number of PIC microcontroller I/O pins required to implement a switch matrix keypad is twice the square root of the number of keys rounded up to the nearest whole number. For example, if you want to implement a 29-button keypad
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Figure 17.8 Multiple buttons can be wired together in a matrix to minimize the number of I/O pins required.
MATRIX KEYPADS
Row 0
Row 1
Column 0 Control Column 1 Control
Figure 17.9 To read the state of the buttons, the columns are brought to ground, and the pulled-up rows are polled.
in your application, you would nd the square root of 29 (which is 5.4) and round it up to the nearest whole number (6) and double it to discover that you should plan for 12 PIC I/O pins for the keypad. Again, this is a basic rule of thumb; I m sure that you re thinking that the optimal number of PIC I/O pins would be 11. The error comes in because 29 is less than halfway between 25 and 36 (two evenly square rooted numbers); if it were 31, you would need 12 I/O pins to support the keypad. As I said earlier, the PIC microcontroller is well suited to implementing switch matrix keyboards with PORTB s internal pull-ups and the ability of the I/O ports to simulate the open-drain pull-downs of the columns, as shown in Fig. 17.10. Normally, the pins
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