Debouncing a Push button on the PIC18F16Q41

In this example, a push button will be debounced using the Configurable Logic Cell (CLC) and Timer 2 (TMR2) peripherals on the PIC18F16Q41. When this occurs, the UART peripheral will print a message to a serial terminal.

Related Documentation

• PIC18F16Q41 Datasheet

• PIC18F16Q41 Device Page

• PIC18F16Q41 Curiosity Nano Users Guide

Software Used

• MPLAB® X IDE v6.20.0

• MPLAB Code Configurator (MCC) v4.1.0

  • Melody Library v1.37.22

• MPLAB XC8 Compiler v3.0.0

• MPLAB Data Visualizer Plugin or other serial terminal

Hardware Used

• PIC18F16Q41 Curiosity Nano

Setup

Step #1: Creating the Project

• On the tool bar, click on New Project
• Microchip Embedded; Standalone Project
• Enter the Device
  • For this Project: PIC18F16Q41
• Enter a name for this project, such as uart-button
  • Name: “uart-button”
  • Note: The project name cannot have any empty spaces

When a push button is pressed, the internal contacts in the switch will bounce momentarily, causing the digital value read to vary wildly. This can create false detections of button press and release. To remove the contact bounce from the digital input, a CLC and TMR2 are used to create a debouncer.

Step #2: MPLAB Code Configurator (MCC)

• Set Configuration Bits
  • Disable "External Oscillator Mode Selection"
  • Set "Power-up Default Value for COSC" to "HFINTOSC with HFFRQ = 64MHz and CDIV = 1:1"
  • Ensure that "WDT Operating Mode" is set to "WDT Disabled; SWDTEN is ignored"

• Modify the Clock Control
  • Set “Clock Source” to High Frequency Internal Oscillator (HFINTOSC)
  • Set “HF Internal Clock” to 64_MHz
  • Set “Clock Divider” to 1

Step #3: Adding Timer, CLC and UART Peripheral

• In Device Resources:
  • Drivers → Timer → TMR2
  • Drivers → CLC → CLC1
  • Drivers → UART → UART1

Once the peripherals are added, modify the peripherals:

• TMR2
  • Hardware Settings
    • Control Mode: Monostable
    • Start/Reset Option: Starts on rising/falling edge on TMR2_ers
  • Timer Clock
    • Clock Source: LFINTOSC
    • Prescaler: 1:16
    • Potscaler: 1:1
  • Timer Period(s)
    • Time Period (s): 0.00012

• CLC1
  • Mode: "JK flip-flop with R"
  • Set TMR2 and CLC1_OUT to the top two logic gates respectively in the CLC1 configuration
    • Invert the OR gates that are connected to the J and K inputs

• UART1
  • Software Settings:
    • Enable "Redirect STDIO to UART" in order to use the function (printf) for sending messages.
  • Hardware Settings:
    • Enable UART box should be checked
    • Enable transmit and Receive should be checked
    • Set the Baud Rate to 19200
    • Everything else can be left as default settings

Step #4: Configure the Pins

• TX1 is connected to pin RB7
• RX1 is connected to pin RB5
• Connect TMR2 input to RC0
• Connect CLC1 output to RC1

Step #5: Generate the project

• Click the generate button in MCC to create the appropriate header and source files for this configuration

Step #6: Modifying main.c

• Upon the generation being completed, the new MCC generated header and source files will be in the project window. Select the main.c file and you will see an empty while(1) loop where you can add your application code.
• The function CLC1_OutputStatusGet() is used to poll the output of the CLC directly from the hardware register.
• Select on the source files and open the “main.c” file
  • Before the while loop, define a boolean named pressed and initialize it to false.

  bool pressed = false;
    while (1)
   {
       if ((!pressed) && (CLC1_OutputStatusGet())) {
               printf("The button was pressed!\n\r");
               pressed = true;
       }
       else if ((pressed) && (!CLC1_OutputStatusGet()))
       {
           pressed = false;
       }

   }

• Make and Program the Device

Step #7: Terminal Emulator

• For this project, the terminal emulator program that is being used is TeraTerm
• Open up the serial terminal on the host computer and select the COM port associated with the Curiosity Nano.

• Configure the serial terminal to communicate at 19200 baud, no parity, and 1 stop bit.
  • Setup → Serial Port → Speed: 19200 → New Setting

• If everything is setup correctly, then the serial terminal should start displaying the printf statement when the push button is activated.

The figure below shows an example of the button being debounced. The yellow signal is the push button after being released and the blue signal shows the output of the CLC.

Summary

This application demonstrates how to debounce a push button.