Jason Heeris

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8 years, 4 months ago.

Why does the LPC1768 sleep() function prolong timer interrupts?

I have the following program, that just blinks an LED on the application board every 2ms using a busy-loop:

#include "mbed.h"
#include "LPC17xx.h"

#define TICK_TIME_US (2000)

static volatile bool tick_lapsed = false;

void handle_timer()
{
    tick_lapsed = true;
}

int main()
{
    DigitalOut led(p23);
    Ticker ticker;

    ticker.attach_us(handle_timer, TICK_TIME_US);

    do
    {
        tick_lapsed = false;
        while(!tick_lapsed);
        led = !led;
    } while(true);

    return 0;
}

I've checked with a logic analyser, and the pulse width is indeed 2ms (4ms total period).

But if I change the busy loop to a sleep call:

    do
    {
        tick_lapsed = false;

        while(!tick_lapsed)
        {
            sleep();
        }

        led = !led;
    } while(true);

...then the pulse width blows out to 10ms!

I was under the impression that timers should still run during the LPC's sleep mode. How can I configure them to interrupt every 2ms? Even configuring TIMER0 using the registers manually results in nothing below 10ms.

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