Configuring and playing with Timer of LPC1768

After saying Hello to LPC1768 and blinking an LED using delay through GPIO, it is time to use the timer for blinking the LED. First let us just get the timer running and then with the help of interrupts we shall blink the LED. For all the register description we require LPC17xx manual.



For using any of the LPC1768 peripherals, the general steps to be followed are
1. Power Up the peripheral to be used.
2. Set the Clock Rate for the peripheral
3. Connect necessary pins using Pin Connect Block.
4. Initialize the registers of the peripheral.

Go to page 490 in the manual and follow the links provided there
1. Powering the Timer0 (Table 46, page 63)

LPC_SC->PCONP |= 1 << 1; //Power up Timer 0

2. After enabling it you need to give the clock for Timer0 (Table 40, page 56. For Timer2/3 refer Table 41, page 57) Select clock as per your requirements.

LPC_SC->PCLKSEL0 |= 1 << 3; // Clock for timer = CCLK/2

3. Configuring the interrupt
LPC1768 contains 4 match registers for each timer. These match registers can be used to reset the timer, generate interrupt, stop the timer, to generate timing signal on an external pin. Now we shall use Timer0 Match0 register to generate the interrupt.

Give a suitable value to Match Register 0

LPC_TIM0->MR0 = 1 << 20; // Give a value suitable for the LED blinking frequency based on the clock frequency

Configure it as to interrupt when the timer count matches the Match0 (Table 429, page 496)

LPC_TIM0->MCR |= 1 << 0; // Interrupt on Match0 compare

Reset the timer (Table 427, page 494)

LPC_TIM0->TCR |= 1 << 1; // Reset Timer0

Enable Timer interrupts

NVIC_EnableIRQ(TIMER0_IRQn); // Enable timer interrupt

And finally start the timer (Table 427, page 494)

LPC_TIM0->TCR |= 1 << 0; // Start timer

4. Now configure the pin to which LED is connected on the GPIO port to output mode

    LPC_SC->PCONP |= ( 1 << 15 ); // power up GPIO

    LPC_GPIO1->FIODIR |= 1 << 29; // puts P1.29 into output mode

5. Write the interrupt routine
Give the interrupt vector as TIMER0_IRQHandler and it is very important to check the source of the interrupt inside an interrupt routine. It can be done by checking the IR register of the TIMER0. Here the source of the interrupt can be identified by looking for a bit that is high in IR. The next thing that we should do is to clear this flag. Otherwise another interrupt will be generated as soon as this interrupt is serviced as the interrupt flag will be high. (Table 426, page 493)

void TIMER0_IRQHandler (void)

{

    if((LPC_TIM0->IR & 0x01) == 0x01) // if MR0 interrupt

    {

        LPC_TIM0->IR |= 1 << 0; // Clear MR0 interrupt flag

        LPC_GPIO1->FIOPIN ^= 1 << 29; // Toggle the LED

    }

}

The compiled Eclipse code of the LED blinking with the help of Timer is Here.