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Switch Bounce Button Input
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1) Ch1
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5 Volt
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100 us
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Figure 17.4 When a switch makes or breaks a connection, the contacts bounce against each other, possibly being interpreted as multiple button presses.
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have any problems with wiring the display to a PIC microcontroller. In Chap. 21 I show how 7- and 16-segment LEDs can be used to display letters and numbers.
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Switch Bounce
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When a button is opened or closed, we perceive that as a clean operation that really looks like a step function. In reality, the contacts of a switch bounce when they make contact, resulting in the jagged signal shown in Fig. 17.4. When this signal is passed to a PIC microcontroller, the microcontroller can recognize this as multiple button presses, which will cause the application software to act as if multiple, very fast button presses have taken place. To avoid this problem so that the switch press is treated like an idealized press, the step function I mentioned earlier and show in Fig. 17.5 is used. The signal will have to be debounced. There are two common methods used for debouncing button inputs.
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Figure 17.5 The ideal waveform to come out of a switch is a clean transition like this one.
PIC MCU INPUT AND OUTPUT DEVICE INTERFACING
Button Polls
Switch Debounced
1) Ch1
5 Volt
100 us
Polling the line continuously to wait for the bouncing to stop.
The rst is to poll the switch line at short intervals until the switch line stays at the same level for an extended period of time. A button is normally considered debounced if it does not change state for 20 ms or more. By polling the line every 5 ms, this debouncing can be conceptualized quite easily, as shown in Fig. 17.6. The advantage of this method is that it can be done in an interrupt handler, and the line can be scanned periodically with a ag set if the line is high and another ag if the line is low. For the indeterminate stage, neither bit would be set. This method is good for debouncing keyboard inputs. The second method is to poll the line continually and wait for 20 ms to go by without the line changing state. The algorithm I use for this function is
do; while (Button == High); // Poll Until Button is Pressed for (Dlay = 0; (Dlay < 20 ms) and (Button == Low); Dlay++); until (Dlay >= 20 ms);
This code will wait for the button to be pressed and then poll it continuously until either 20 ms has passed or the switch has gone high again. If the switch goes high, the process is repeated until it is held low for 20 ms. This method is well suited to applications that don t have interrupts and only have one button input and no need for processing while polling the button. As restrictive as it sounds, there are many applications that t these criteria. This method also can be used with interrupt inputs along with TMR0 in the PIC microcontroller, which eliminates the restrictions I just detailed. The interrupt handler behaves
SWITCH BOUNCE
like the pseudocode below when one of the port changes on interrupt bits is used for the button input:
interrupt ButtonDebounce() { // Set Flags According to the // Debounced State of the Button
if (T0IF == 1) { // TMR0 Over ow, Button Debounced T0IF = 0; T0IE = 0; // Reset and Turn off TMR0 Interrupts if (Button == High) { Pressed = 0; NotPressed = 1; // Set the State of the Button } else { Pressed = 1; NotPressed = 0; } // } else { // Port Change Interrupt Pressed = 0; NotPressed = 0; // Nothing True RBIF = 0; // Reset the Interrupt TMR0 = 20msDlay; // Reset Timer 0 for 20 msecs T0IF = 0; T0IE = 1; // Enable the Timer Interrupt } // } // End ButtonDebounce
This code waits for the input pin to change state and then resets the two ags indicating the button state and starts TRM0 to request an interrupt after 20 ms. After a portchange interrupt, notice that I reset the button state ags to indicate to the mainline that the button is in a transition state and is not yet debounced. If TMR0 over ows, then the button is polled for its state, and the appropriate button state ags are set and reset. The mainline code should poll the Pressed and NotPressed ags when it is waiting for a speci c state. In Chap. 20 I will show how this method using TMR0 and interrupts can be implemented with or without interrupts. If you don t want to use the software approaches, you can use a capacitor to lter the bouncing signal and pass it into a Schmidt trigger input. Schmidt trigger inputs have different thresholds depending on whether the signal is rising or falling. For rising edges, the trigger point is higher than for falling edges. Schmidt trigger inputs have the symbol put in the buffer shown in the circuit presented in Fig. 17.7.
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