Interrupt Delay Using Callback Function in MCC


A timer interrupt is often used to launch an Interrupt Service Routine (ISR) that may toggle a port pin or some other function. Prescalers can be used to extend the time delay although the timer prescale limits may not produce a large enough delay. This is where a tick counter or a counter that increments on every timer overflow can be useful for creating long delays. This can be included in the ISR but within the MPLAB® Code Configurator (MCC) generated driver code for timers is a callback function that handles this tick counter automatically. By setting a callback counter value during the MCC driver setup, longer delays are easily established. This project shows how to control the Flash rate of an LED using a callback function.


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Curiosity Board User Guide/Schematic

 Connection Diagram

The Curiosity board has four LEDs prewired to the I/O pins shown below. This project controls the D7 LED.

Hardware Function Pin Setting
IO_LED_D4 RA5 (2) Output
IO_LED_D5 RA1 (18) Output
IO_LED_D6 RA2 (17) Output
IO_LED_D7 RC5 (5) Output



Create a Project

Create a new project and select the PIC16F1619 along with the Curiosity board and MPLAB XC8.

If this is your first project, please visit the "MPLAB® Xpress: Create a Project" page.


Launch MCC

Open MCC under the Tools > Embedded menu of MPLAB X IDE.



System Setup

From Project Resources, choose System Module to open the System Setup window within MCC.

  1. In the clock settings, make sure you select INTOSC.
  2. Select the system clock FOSC.
  3. Set Internal Clock to 4MHz_HF setting.
  4. Check the PLL Enabled box.
  5. The Curiosity board uses a Programmer/Debugger On-Board (PKOB) and uses a low voltage program method to program the MCU. Therefore, the low voltage programming must be enabled by checking the Low-voltage programming Enable box.


Timer4 Setup

Add the TMR4 peripheral to the project from the Device Resources area of MCC. To do that, scroll down to the timer entry and expand the list by clicking on the arrow. Now, double click on the TMR4 entry to add it to the Project Resources list. Then, click on TMR4 to open the Timer4 configuration setup screen.

  1. Check the Enable Timer box.
  2. Select Clock Source FOSC/4, Postscaler 1:1, and Prescaler 1:64.
  3. Set Timer Period value to 9.088 ms.
  4. Set External Reset Source to T4IN and Control Mode to Roll over pulse.
  5. Set Start/Reset Option to Software Control (this will inhibit hardware reset of timer).
  6. Check the Enable Timer Interrupt box.

To extend the Timer4 based delay, software overflows will use a callback function. The overflow of TMR4 will trigger an interrupt and the MCC generated TMR4 driver code will count up to the Callback Function Rate (CFR) value of events, before calling a predefined interrupt function that will then implement the ISR in the main.c file (that will be created in step 7).


To configure the callback value, enter "11" in the CFR field. This will give us about a 100 ms rate (11 × 9.088 ms = 99.968 ms).


Pin Module Setup

Click on Pin Module in the Project Resources area. This will open up the Pin Module and the Pin Management Grid. Click on the RC5 Output row blue lock to turn it green.


Note: The T4IN pin will automatically be selected and show a green lock from the TMR4 setup.

Rename the I/O pin

In the Pin Module, rename the pin RC5 to "IO_LED_D7". Only the Output box should be checked.



Generate Driver Code

Click on Generate under Project Resources to have the MCC create the drivers and a base main.c file for the project.



The main.c file needs to be updated to handle the callback function using the function pointers generated by the MCC drivers. The function pointers allow main.c to handle all operations without having to modify the driver code.

The first modification is to add void myTimer4ISR(void); right after the #include mcc_generated_files/mcc.h.

#include "mcc_generated_files/mcc.h"

void myTimer4ISR(void);

This defines the name of the ISR that will be created to control the LED.

Now, the Timer4 driver default Interrupt Handler has to be connected to the ISR defined in main.c.. This is handled by adding the line TMR4_SetInterruptHandler (myTimer4ISR);, right after the SYSTEM_Initialize function.

// initialize the device

    TMR4_SetInterruptHandler (myTimer4ISR);  //Define interrupt Handler

Remove the double backslash comment markers from Global and Peripheral Interrupt Enable functions:
// INTERRUPT_GlobalInterruptEnable();
// INTERRUPT_PeripheralInterruptEnable();
This action will include a call to the predefined macros and enable interrupts.

// Enable the Global Interrupts

// Enable the Peripheral Interrupts

Finally, define what the ISR does when the number of callbacks is complete and the interrupt is processed. For this, a macro that was created in the MCC generated pin_manager.h file is used. The macro selected will toggle the LED from off to on and on to off on each ISR call. Add the myTimer4ISR ISR after the while(1) loop. This will complete the modifications to main.c required for this project.

    while (1)
        // Add your application code

void myTimer4ISR(void){
    IO_LED_D7_Toggle(); //Control LED
 End of File


Build Project

Click on Build Project icon to compile the code and you will see a "BUILD SUCCESSFUL" message in the Output window of MPLAB X within several seconds of processing time.

BUILD SUCCESSFUL (total time: 8s)


Make sure your Curiosity board is connected to the USB port. Then, click on the Make and Program Device icon. This will build the project again and launch the programmer built into the Curiosity board. In the Output window, you should see a series of messages and when successful, it will end with a "Programming and Verify Successful" message.


Output Window:

Connecting to MPLAB Starter Kit on Board...

Currently loaded firmware on Starter Kit on Board
Firmware Suite Version.....01.41.07
Firmware type..............Enhanced Midrange

Target detected
Device ID Revision = 2004

The following memory area(s) will be programmed:
program memory: start address = 0x0, end address = 0xbf
configuration memory

Programming/Verify complete

If it's the first time the programmer is connected to the board, the programming tool may need to download the proper operating firmware for the exact device. You may see a series of messages if this occurs. This should only happen once.

Downloading Firmware…
Downloading bootloader
Bootloader download complete
Programming download…
Downloading RS…
RS download complete
Programming download…
Downloading AP…
AP download complete
Programming download…
Firmware Suite Version…..01.34.11
Firmware type…………..Enhanced Midrange


The D7 LED will begin to blink on the Curiosity board. This shows that the callback function is working and controlling the blink rate of the LED. To prove that to yourself, go back to the TMR4 setup window in MCC and change the callback to a larger value of 55, Generate the drivers again, then Make and Program the device. This will flash the LED at the slower rate of a half-second on/off.


Using the callback function generated in the TMR4 driver code simplifies creating interrupt delays. It reduces the ISR in your main code to just the action you want to implement when the time delay (or the number of interrupts) has occurred.


Try different callback functions and also modify the ISR to do other things, such as possibly controlling more than one LED. These modifications will help you better understand how useful callback functions can be when using interrupts.

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