5 硬件定时器计数存在误差的问题

发布于 2022-04-15 08:55:54 浏览：994 订阅该版

环境:RTT中配置了硬件时钟，计数频率是1MHZ,向上计数，最大计数为0xFFFF

测试步骤:

1.定时器触发模式为周期触发，定时器时间为100US,当定时器超时，进入回调中，读取计数器的值

/* 设置模式为周期性定时器 */
    mode = HWTIMER_MODE_PERIOD;
    ret = rt_device_control(hw_dev, HWTIMER_CTRL_MODE_SET, &mode);
    if (ret != RT_EOK)
    {
        rt_kprintf("set mode failed! ret is :%d\n", ret);
        return ret;
    }
B.

/* 定时器超时回调函数 */
static rt_err_t timeout_cb(rt_device_t dev, rt_size_t size)
{
    rt_hwtimerval_t timeout_s;      /* 定时器超时值 */

/* 读取定时器当前值 */
    rt_device_read(hw_dev, 0, &timeout_s, sizeof(timeout_s));
    rt_kprintf("Read: Usec = %d\n", timeout_s.usec);
    return 0;
}

打印结果为:

Read: Usec = 536656
Read: Usec = 536719
Read: Usec = 536881
Read: Usec = 536942
2.

定时器触发模式为周期触发，定时器时间为100US,当定时器超时，进入回调中，读取计数器的值并重新设置定时器的超时时间为100US

/* 定时器超时回调函数 */
static rt_err_t timeout_cb(rt_device_t dev, rt_size_t size)
{
    rt_hwtimerval_t timeout_s;      /* 定时器超时值 */

/* 读取定时器当前值 */
    rt_device_read(hw_dev, 0, &timeout_s, sizeof(timeout_s));
    rt_kprintf("Read: Usec = %d\n", timeout_s.usec);

/* 设置定时器超时值为100us并启动定时器 */
    timeout_s.sec = 0;      /* 秒 */
    timeout_s.usec = 100;     /* 微秒 */
    rt_device_write(hw_dev, 0, &timeout_s, sizeof(timeout_s));
    return 0;
}

打印结果为:

Read: Usec = 103
Read: Usec = 103
Read: Usec = 103
Read: Usec = 103

定时器触发模式单次触发，定时器时间为100US,当定时器超时，进入回调中，读取计数器的值并重新设置定时器的超时时间为100US

/* 设置模式为单次定时器 */
    mode = HWTIMER_MODE_ONESHOT;
    ret = rt_device_control(hw_dev, HWTIMER_CTRL_MODE_SET, &mode);
    if (ret != RT_EOK)
    {
        rt_kprintf("set mode failed! ret is :%d\n", ret);
        return ret;
    }
B.

/* 定时器超时回调函数 */
static rt_err_t timeout_cb(rt_device_t dev, rt_size_t size)
{
    rt_hwtimerval_t timeout_s;      /* 定时器超时值 */

/* 读取定时器当前值 */
    rt_device_read(hw_dev, 0, &timeout_s, sizeof(timeout_s));
    rt_kprintf("Read: Usec = %d\n", timeout_s.usec);

打印结果为:

Read: Usec = 100
Read: Usec = 100
Read: Usec = 100
Read: Usec = 100

问:以上三次的测试步骤，导致的结果不同的原因是？