How to Write an ISR for Timer 3 in C for Microcontrollers

[Lomanu4]

In this article, we'll explore how to write an Interrupt Service Routine (ISR) for Timer 3 in C, utilizing an INV instruction to toggle an LED connected to Port A, RA5. This guide is aimed at developers working with embedded systems, particularly for microcontrollers. Let’s dive into setting up Timer 3 and crafting an effective ISR.

Understanding Timer Setup


The Timer 3 setup in this example utilizes an internal frequency of 40 MHz. To understand how we arrive at our settings, we must consider the pre-scaler set at 1:256. This configuration allows for more manageable timing intervals by dividing our clock signal, enabling precise control over our LED’s toggling rate.

The formula to calculate the timer frequency is as follows:

• Timer Frequency = Clock Frequency / Prescale Factor

With a clock frequency of 40 MHz and a pre-scaler of 256, the Timer 3 frequency will be:

Timer Frequency = 40,000,000 Hz / 256 = 156250 Hz


This results in a time period for Timer 3 of approximately 6.4 microseconds.

Writing the Interrupt Service Routine (ISR)


To create the ISR for Timer 3, you need to consider how to properly toggle the LED. The INV instruction will flip the state of the RA5 pin, leading to the LED turning on or off alternately. Here’s how we can set that up.

Step 1: Setting up Timer 3


Before implementing the ISR, ensure Timer 3 is configured correctly within your microcontroller environment. Here's an example of how to initialize Timer 3:

void Timer3_Init(void) {
// Configure Timer 3
T3CONbits.TCKPS = 0b11; // Set Timer 3 prescaler to 1:256
TMR3 = 0; // Clear Timer 3 register
PR3 = 156; // Set Timer 3 period register for 1ms delay
T3CONbits.TON = 1; // Turn Timer 3 on
IEC0bits.T3IE = 1; // Enable Timer 3 interrupts
IPC1bits.T3IP = 1; // Set Timer 3 interrupt priority to 1
}

Step 2: Implementing the ISR


Next, we will write the ISR itself. The following code snippet demonstrates how to configure the Timer3Interrupt function to toggle the RA5 pin that controls the LED.

void __attribute__((interrupt, no_auto_psv)) _T3Interrupt(void) {
// Clear the interrupt flag
IFS0bits.T3IF = 0; // Reset Timer 3 interrupt flag

// Toggle RA5 state
LATA ^= (1 << 5); // Using INV or toggle operation on RA5
}

Complete Example


Here's a complete example that incorporates both the initialization and the ISR for Timer 3.

#include <xc.h> // Include your microcontroller's specific header file

void Timer3_Init(void) {
T3CONbits.TCKPS = 0b11; // Set Timer 3 prescaler to 1:256
TMR3 = 0; // Clear Timer 3 register
PR3 = 156; // Set Timer 3 period register for 1ms delay
T3CONbits.TON = 1; // Turn Timer 3 on
IEC0bits.T3IE = 1; // Enable Timer 3 interrupts
IPC1bits.T3IP = 1; // Set Timer 3 interrupt priority to 1
}

void __attribute__((interrupt, no_auto_psv)) _T3Interrupt(void) {
IFS0bits.T3IF = 0; // Clear the interrupt flag
LATA ^= (1 << 5); // Toggle RA5 to turn the LED on/off
}

int main(void) {
// Set up RA5 as output
TRISA &= ~(1 << 5); // Set RA5 as output
LATA = 0; // Initially turn off the LED

Timer3_Init(); // Initialize Timer 3

while (1) {
// Your main code here
}
return 0;
}

Frequently Asked Questions (FAQ)

What is an ISR?


An Interrupt Service Routine (ISR) is a special function in embedded programming that is executed in response to an interrupt.

How does a Timer work in microcontrollers?


Timers in microcontrollers measure time intervals, generate precise delays, and can trigger events, such as toggling an LED based on the timer status.

Why is RA5 used for the LED?


RA5 is typically chosen because it is readily accessible and can be configured as an output pin for driving an LED on many microcontroller boards.

Conclusion


In this article, we outlined how to write an ISR for Timer 3 that effectively toggles an LED connected to RA5. Through proper configuration of both Timer 3 and our ISR, you can achieve dynamic LED control in your embedded applications. Remember to adjust the timer settings according to your project requirements, and happy coding!

Пожалуйста Авторизируйтесь или Зарегистрируйтесь для просмотра скрытого текста.