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AN2805

Robust Debouncing with Core-Independent Peripherals

Introduction

Author: Qubo Hu, Microchip Technology Inc.

Typical button debouncing approaches are often not sufficiently robust, and with slow setup and

response, which may lead to ghost or non-detected stimuli. This Tips and Tricks document presents two

more robust and quick debouncing solutions, which equally applies to rotary encoders, buttons, switches,

keypads, knobs, and the like. By using an internal timer to provide customized slower clock and

Configurable Logic Cells (CLCs), it is possible to filter out various kinds of button generated noise. One

solution requires two CLCs per button, which on PIC18F Q10 family devices with eight CLCs can support

the debouncing of four buttons. Another solution requires three CLCs per button, which is more robust

and addresses a timing corner case in the 2-CLCs solution. The debouncing is performed in hardware

peripherals with no CPU execution required, after a system initialization which contains the timer and

CLCs configuration. Both solutions require two CLC clock cycles as this gives a very fast debouncing.

An example creation in MPLAB and MCC configuration is described in this document. The example code

for replicating the results is also available from MPLAB Xpress code example: https://

mplabxpress.microchip.com/mplabcloud/example/details/799.

Table of Contents

Introduction......................................................................................................................1

1. Background............................................................................................................... 3

1.1. Existing Solutions.........................................................................................................................3

2. Robust Debouncing with CLCs..................................................................................5

2.1. 2-CLCs Debouncing Solution....................................................................................................... 5

2.2. 3-CLCs Debouncing Solution....................................................................................................... 7

2.3. Further Discussions......................................................................................................................8

3. Project Creation and Configuration......................................................................... 10

3.1. Project Overview........................................................................................................................ 10

3.2. Hardware Prerequisites..............................................................................................................10

3.3. Software Prerequisites............................................................................................................... 10

3.4. Hardware Setup..........................................................................................................................11

3.5. 2-CLCs Solution Configuration in MPLAB..................................................................................12

3.6. Generate Code and Program Device.........................................................................................18

3.7. 3-CLCs Solution Configuration in MPLAB..................................................................................18

4. Revision History.......................................................................................................21

The Microchip Website..................................................................................................22

Product Change Notification Service.............................................................................22

Customer Support......................................................................................................... 22

Microchip Devices Code Protection Feature................................................................. 22

Legal Notice...................................................................................................................23

Trademarks................................................................................................................... 23

Quality Management System........................................................................................ 24

Worldwide Sales and Service........................................................................................25

AN2805

1. Background

When physically rotating an encoder or pressing a button, two pieces of metal come into contact with

each other. The two pieces of metal can make and break contact intermittently before they are in full

contact. When the input signal crosses the digitization threshold, it could result in multiple pulses of

varying duration in the digitized signal, as illustrated in . Because of the high sample rate of theFigure 1-1

input signal line for the microcontroller to process, multiple pulses on the input signal line can result in

one press/release to be registered as multiple triggers. Ideally, short period pulses of the digitized input

signal are just noise, and these noisy pulses should be filtered out. Only a stable pulse with a long

enough period, which means the input signal is stable for a certain time, should trigger debounce. This

method is called “debouncing”.

Figure 1-1 also illustrates a number of scenarios associated with signal debouncing. In this figure:

• Scenario 1 refers to the case when the encoder is rotated to ON

• Scenario 2 refers to the case when the encoder is rotated to OFF

• Scenario 3 refers to the case when the encoder is mechanically loose or halfway rotated, making the

input signal very unstable

Debouncing is a type of low-pass filter. The debounce time is the delay between the occurrence of the

trigger and the registration of the trigger. This debounce time influences the accuracy of the timestamps.

For scenarios 1 and 2, the edge of a trigger indicating a stable pulse should be registered after the

debounce time, named debounce signal. For scenario 3, many short period pulses can occur in a very

short period on the digitized signal. The user should, in advance, define if these short period pulses are

noisy pulses to be ignored, or if it could be considered as a valid and stable pulse in a certain condition.

In this document, a rotary encoder has been used as an example for demonstration purposes. However,

the same technique applies to also buttons, switches, keypads, and knobs.

Figure 1-1. Debouncing Illustration

1.1 Existing Solutions

For debouncing of slow signals, the simplest hardware solution is to debounce with an external resistive-

capacitive (RC) filter component to filter out quick pulse changes in order to keep clean edges for the

AN2805

Background

Product specificaties

Merk:	Microchip
Categorie:	Niet gecategoriseerd
Model:	PIC24FJ32GU202

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