3 months, 1 week ago.

Waiting microsecond in RTOS

Hi!

I am developing a device which has to send a package (using CAN-bUS) every 2.5ms, 7.5ms, and 10ms. If I send the pack in advance or late I lose synchrony and my device blocks. I am using 3 threads (could be 2 ;-)) with osPriorityRealtime and a delay inside a forever-one loop. My problem is that I only can reach delays in milliseconds. I mean, I need a delay of 2.5ms but I only can get 2 or 3ms. I had used wait_us, wait, Thread::wait...but it doesn't work

    while(1){
        wait_us(2500);
        //wait(0.0025);
            can.send_1();
        }
    } 

could anybody help me?

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1 Answer

3 months, 1 week ago.

Hello Bruno,

Below is a test program to measure delays created using wait_us() but without sending any CAN message. The results on LPC1768 seem OK (see below). Maybe it can help:

#include "mbed.h"

Timer   timer1, timer2, timer3;
int     delay1[20];
int     delay2[10];
int     delay3[5];
int     i1 = 0;
int     i2 = 0;
int     i3 = 0;

void task1()
{
    timer1.start();
    while (1) {
        timer1.reset();
        wait_us(2500);
        if (i1 < 20)
            delay1[i1++] = timer1.read_us();
        else
            break;
    }
    timer1.stop();
}

void task2()
{
    timer2.start();
    while (1) {
        timer2.reset();
        wait_us(7500);
        if (i2 < 6)
            delay2[i2++] = timer2.read_us();
        else
            break;
    }
    timer2.stop();
}

int main()
{
    Thread* thread1 = new Thread(osPriorityRealtime, OS_STACK_SIZE);
    Thread* thread2 = new Thread(osPriorityRealtime, OS_STACK_SIZE);

    printf("Starting..\r\n");
    thread1->start(task1);
    thread2->start(task2);
    timer3.start();
    while (1) {
        timer3.reset();
        wait_us(10000);
        if (i3 < 5)
            delay3[i3++] = timer3.read_us();
        else
            break;
    }
    timer3.stop();
    printf("-------------\r\n");
    for (int i = 0; i < i1; i++)
        printf("delay1[%d]=%d\r\n", i, delay1[i]);
    printf("-------------\r\n");
    for (int i = 0; i < i2; i++)
        printf("delay2[%d]=%d\r\n", i, delay2[i]);
    printf("-------------\r\n");
    for (int i = 0; i < i3; i++)
        printf("delay3[%d]=%d\r\n", i, delay3[i]);
}

Results on LPC1768:

Starting..
-------------
delay1[0]=2511
delay1[1]=2511
delay1[2]=2511
delay1[3]=2509
delay1[4]=2512
delay1[5]=2509
delay1[6]=2508
delay1[7]=2509
delay1[8]=2511
delay1[9]=2508
delay1[10]=2511
delay1[11]=2512
delay1[12]=2512
delay1[13]=2508
delay1[14]=2511
delay1[15]=2512
delay1[16]=2511
delay1[17]=2509
delay1[18]=2511
delay1[19]=2508
-------------
delay2[0]=7509
delay2[1]=7510
delay2[2]=7511
delay2[3]=7510
delay2[4]=7511
delay2[5]=7510
-------------
delay3[0]=10012
delay3[1]=10010
delay3[2]=10009
delay3[3]=10010
delay3[4]=10010


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