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29.2 SWITCH BOUNCING
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Output Double Throw, Single Pole Switch
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FIGURE 29-5 Two Schmitt triggered inverters along with a couple of other components can be used to make an effective button debounce circuit.
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When the common contact is not touching either connector (and is disconnected from Vcc and ground), the flip-flop circuitry remembers the last common contact position. In the circuit given in Fig. 29-5, note that the inverters are Schmitt-triggered CMOS gates the Schmitt trigger helps ensure that the flip-flop only changes state at a definite voltage level. The CMOS gates along with the 10k current limiting resistor ensures that there is not a large current flow or backdriving of an inverter s output. There are other hardware debounce circuits that you can use in your application as well. One of the most popular is using a 555 set up as a monostable multivibrator with enough of a delay to ensure that any bounces are masked by the 555 s output. The circuit shown in Fig. 29-5 is very efficient, both in terms of operation as well as cost; you will be hard pressed to find a circuit that is as simple and effective as this one.
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29.2.2 SOFTWARE DEBOUNCE
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Debouncing button inputs in software can be tricky; the need for performing other operations in an application means that the debounce code must be integrated with the complete application. This requirement adds a level of complexity to the code that can be difficult to implement for many new programmers. While debouncing button inputs in software is more cost efficient than adding hardware, you may find that the amount of work to get debounce code working in a complete robot application to be much more effort than adding a couple of CMOS Schmitt trigger inverters and a resistor. The rule of thumb for debouncing a button input is that the line is considered to be debounced and the input stable when it has not changed state for at least 20 ms. The code for debouncing a single button in a stand-alone application is actually quite easy:
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Parts List for CMOS Logic Debounce Circuit Hex Schmitt trigger input inverter chip 10k resistor Double throw, single pole switch
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THE SENSE OF TOUCH
Debounce: ' Return when the Button Input is Debounced i = 0 while (i < 20ms) ' Wait for the Button to be Pressed for 20 ms if (Button = Pressed) then i = i + 1 ' Increment Pressed Counter else i = 0 ' Button has been released, reset the counter endif endwhile return ' Return to caller
This code increments a counter each time in a loop that the button is down. If the button is not down then the counter is reset. The counter value is continually checked to see if it has incremented to a value equal to or greater than 20 ms and when it is, the execution of the program leaves the while loop and returns to the subroutine s caller. The problem with debugging a button in software comes when there are additional functions built into the robot. When this happens the time the main robot loop code executes must be known (and becomes the incrementing value for the button counter) and the main loop above becomes distributed. If the robot execution loop was assumed to execute in 1 ms, then the basic code could look like:
ButtonUp = 0 ButtonDown = 0 ' Initialize Button Variables
while (1) ' Loop Forever if (Button = Pressed) then ' Button State Counter Update ButtonUp = 0 ' Button not Up ButtonDown = ButtonDown + 1 else ButtonUp = ButtonUp + 1 ButtonDown = 0 endif if (ButtonUp > 20) then ' Limit Counter Values ButtonUp = 20 endif if (ButtonDown > 20) then ButtonDown = 20 endif ' Execute Robot Functions Dlay(1ms - ExecutionTime) endwhile ' ' Find Robot Outputs to Inputs Ensure Loop Executes in 1 ms
Note that the button state variables are limited to a maximum value of 20. This is done to ensure that the variables do not overflow and become zero, which indicates the buttons are not pressed. While the code in the listing seems complex, you should remember that multiple buttons could be added to the robot application by simply copying this code for each additional button (new counter variables will be required as well). You will discover that the execution time penalty of the button debounce software is negligible, and using this code, you should be able to add a number of buttons to your robot application relatively easily.
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