Switch Debouncing - Revision history

Hwang: /* Software Debouncing */

2006-08-14T18:49:36Z

Software Debouncing

Hwang at 18:44, 14 August 2006

2006-08-14T18:44:26Z

Hwang: /* Example Circuit */

2006-08-11T20:10:17Z

Example Circuit

Hwang: /* Example Circuit */

2006-08-11T19:58:32Z

Example Circuit

Hwang at 19:58, 11 August 2006

2006-08-11T19:58:01Z

Hwang at 22:44, 29 June 2006

2006-06-29T22:44:08Z

Hwang at 21:18, 29 June 2006

2006-06-29T21:18:50Z

New page

When a mechanical switch is flipped/pressed, the metal contacts inside may not open or close cleanly. In the microseconds before the switch achieves a good solid connection, the switch's contacts may "bounce" against each other, turning the switch on and off in rapid succession. The result of closing a pushbutton switch is shown below on an oscilloscope:

[[Image:bouncing_switch.jpg]]

Even though the bounce only lasted milliseconds, a microprocessor is fast enough to tell the difference, and it will think that the switch was pressed twice when it was actually only pressed once.

We can remedy this problem by '''debouncing''' the switch. There are many ways to do this with both hardware and software.

← Older revision		Revision as of 18:49, 14 August 2006
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	Remeber that the 7414 Schmitt trigger will invert the signal.		Remeber that the 7414 Schmitt trigger will invert the signal.

	==Software Debouncing==

← Older revision		Revision as of 18:44, 14 August 2006
Line 25:		Line 25:

	Remeber that the 7414 Schmitt trigger will invert the signal.		Remeber that the 7414 Schmitt trigger will invert the signal.

			==Software Debouncing==

← Older revision		Revision as of 20:10, 11 August 2006
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	[[Image:debounce_with_schmitt.jpg]]		[[Image:debounce_with_schmitt.jpg]]

			Remeber that the 7414 Schmitt trigger will invert the signal.

@@ Line 11: / Line 11: @@
 ==Example Circuit==
 Below is a simple circuit for debouncing a switch for digital logic.  When the switch is closed, the voltage is pulled down through the switch, and when the switch is open, the output voltage is pulled high through the resistor.  The resistor also prevents a short circuit when the switch is closed. The capacitor "absorbs" the bounces when the switch changes states.  Keep in mind that this circuit will cause a short delay in switching because the capacitor takes time to charge through the resistor.  The delay can be adjusted by varying the values of the capacitor and resistor, but if the resistor is too small, it may not be sufficient to absorb the bounces.
+[[Image:debounce_schematic.png]]
 There is one problem: the capacitor exhibits a curve when charging, and parts of this curve will fall in the region where a logic chip is undecided whether to treat it as a high or low.  To fix this, a [[Schmitt_Trigger |Schmitt trigger]] is added to the output.

@@ Line 1: / Line 1: @@
+__TOC__
+==Introduction==
 When a mechanical switch is flipped/pressed, the metal contacts inside may not open or close cleanly.  In the microseconds before the switch achieves a good solid connection, the switch's contacts may "bounce" against each other, turning the switch on and off in rapid succession.  The result of closing a pushbutton switch is shown below on an oscilloscope:
@@ Line 6: / Line 8: @@
 We can remedy this problem by '''debouncing''' the switch.  There are many ways to do this with both hardware and software, such as using a flip-flop to "latch" the signal, using a capacitor to "absorb" the bounces, or programming the software to ignore the bounces.
+==Example Circuit==
+Below is a simple circuit for debouncing a switch for digital logic.  When the switch is closed, the voltage is pulled down through the switch, and when the switch is open, the output voltage is pulled high through the resistor.  The resistor also prevents a short circuit when the switch is closed. The capacitor "absorbs" the bounces when the switch changes states.  Keep in mind that this circuit will cause a short delay in switching because the capacitor takes time to charge through the resistor.  The delay can be adjusted by varying the values of the capacitor and resistor, but if the resistor is too small, it may not be sufficient to absorb the bounces.
+There is one problem: the capacitor exhibits a curve when charging, and parts of this curve will fall in the region where a logic chip is undecided whether to treat it as a high or low.  To fix this, a [[Schmitt_Trigger |Schmitt trigger]] is added to the output.
+Without Schmitt trigger:
+[[Image:debounce_without_schmitt.jpg]]
+With Schmitt trigger:
+[[Image:debounce_with_schmitt.jpg]]

← Older revision		Revision as of 22:44, 29 June 2006
Line 5:		Line 5:
	Even though the bounce only lasted milliseconds, a microprocessor is fast enough to tell the difference, and it will think that the switch was pressed twice when it was actually only pressed once.		Even though the bounce only lasted milliseconds, a microprocessor is fast enough to tell the difference, and it will think that the switch was pressed twice when it was actually only pressed once.

	We can remedy this problem by '''debouncing''' the switch. There are many ways to do this with both hardware and software.		We can remedy this problem by '''debouncing''' the switch. There are many ways to do this with both hardware and software, such as using a flip-flop to "latch" the signal, using a capacitor to "absorb" the bounces, or programming the software to ignore the bounces.