CLOCK_TIMER Device

Note: CLOCK_TIMER is the unified hardware timer device in the clock_time subsystem.

Introduction to the Timer

Hardware timers generally have two modes of operation, timer mode and counter mode. No matter which mode is operated, it works by counting the pulse signal counted by the internal counter module. Here are some important concepts of timers.

Timer mode: Counts the internal pulse. Timers are often used as timing clocks for timing detection, timing response, and timing control.

Counter mode: The counter can count up or down. The maximum count value of a 16-bit counter is 65535, and the maximum value of a 32-bit counter is 4 294 967 295.

**Counting frequency**：Since the input frequency is usually fixed, the time it takes for the counter to reach its desired count number can be calculated from just the given frequency - time = count value / count frequency. For example, if the counting frequency is 1 MHz, the counter counts once every 1 / 1000000 seconds. That is, every 1 microsecond, the counter is incremented by one (or subtracted by one), at this time, the maximum timing capability of the 16-bit counter is 65535 microseconds, or 65.535 milliseconds.

Relationship to clock_time

CLOCK_TIMER devices can be used directly by applications through the RT-Thread device API, and can also act as a clock_time event device. When a clock_timer is registered with rt_clock_timer_register() and selected as the default event, its ISR calls rt_clock_time_event_isr() internally, which drives the high-resolution timer scheduler.

Access Hardware Timer Device

The application accesses the hardware timer device through the I/O device management interface provided by RT-Thread. The related interfaces are as follows:

Function	Description
rt_device_find()	to look up the timer device
rt_device_open()	to open the timer device in read-write mode
rt_device_set_rx_indicate()	to set the timeout callback function
rt_device_control()	to control the timer device, you can set the timing mode (single time /cycle),counting frequency, or stop the timer
rt_device_write()	to set the timeout value of the timer. The timer then starts
rt_device_read()	to get the current value of the timer
rt_device_close()	to turn off the timer device.

Find Timer Device

The application obtains the device handle based on the hardware timer device name, and thus can operate the hardware timer device. The device function is as follows:

rt_device_t rt_device_find(const char* name);

Parameter	Description
name	hardware timer device name
return	——
timer device handle	will return to the corresponding device handle if the corresponding device is found
RT_NULL	No device found

In general, the hardware timer device name registered to the system is timer0, timer1, etc. The usage examples are as follows:

#define CLOCK_TIMER_DEV_NAME   "timer0"     /* timer name */
rt_device_t hw_dev;                     /* timer device handle */
/* find timer device */
hw_dev = rt_device_find(CLOCK_TIMER_DEV_NAME);

Details:

• The device name is assigned by the BSP when calling rt_clock_timer_register().
• If multiple timers are registered, each is exposed as a separate device.

Open Timer Device

With the device handle, the application can open the device. When the device is open, it will detect whether the device has been initialized. If it is not initialized, it will call the initialization interface to initialize the device by default. Open the device with the following function:

rt_err_t rt_device_open(rt_device_t dev, rt_uint16_t oflags);

Parameter	Description
dev	hardware timer device handle
oflags	device open mode, is generally opened in read and write mode, which is to take the value：RT_DEVICE_OFLAG_RDWR
return	——
RT_EOK	device opened successfully
other error code	device fail to open

An example of use is as follows:

#define CLOCK_TIMER_DEV_NAME   "timer0"     /* timer name */
rt_device_t hw_dev;                     /* timer device handle */
/* find timer device */
hw_dev = rt_device_find(CLOCK_TIMER_DEV_NAME);
/* to open the timer device in read-write mode */
rt_device_open(hw_dev, RT_DEVICE_OFLAG_RDWR);

Details:

• Opening the device ensures the driver is initialized and the working frequency is applied.
• Call open before using control/read/write APIs; otherwise behavior depends on the BSP driver and may fail with -RT_ENOSYS or other errors.

Set the Timeout Callback Function

Set the timer timeout callback function with the following function - this is the function that will be called when the timer reaches its set count value:

rt_err_t rt_device_set_rx_indicate(rt_device_t dev, rt_err_t (*rx_ind)(rt_device_t dev, rt_size_t size))

Parameter	Description
dev	device handle
rx_ind	timeout callback function, provided by the caller
return	——
RT_EOK	success

An example of use is as follows：

#define CLOCK_TIMER_DEV_NAME   "timer0"     /* timer name */
rt_device_t hw_dev;                     /* timer device handle */
 
/* timer timeout callback function */
static rt_err_t timeout_cb(rt_device_t dev, rt_size_t size)
{
    rt_kprintf("this is clock_timer timeout callback function!\n");
    rt_kprintf("tick is :%d !\n", rt_tick_get());
 
    return 0;
}
 
static int clock_timer_sample(int argc, char *argv[])
{
    /* find timer device */
    hw_dev = rt_device_find(CLOCK_TIMER_DEV_NAME);
    /* open the device in read and write mode */
    rt_device_open(hw_dev, RT_DEVICE_OFLAG_RDWR);
    /* set the timeout callback function */
    rt_device_set_rx_indicate(hw_dev, timeout_cb);
}

Details:

• The callback runs in the timer interrupt context, so it must be fast and non-blocking.
• size is the size of the timeout data (sizeof(rt_clock_timerval_t)) passed by the driver; most users can ignore it.

Control the Timer Device

By sending control words, the application can configure the hardware timer device with the following function:

rt_err_t rt_device_control(rt_device_t dev, rt_uint8_t cmd, void* arg);

Parameter	Description
dev	device handle
cmd	command control word
arg	controlled parameter
return	——
RT_EOK	function executed successfully
-RT_ENOSYS	execution failed，dev is null
other error code	execution failed

The command control words available for the hardware timer device are as follows：

Control word	Description
CLOCK_TIMER_CTRL_FREQ_SET	set the counting frequency
CLOCK_TIMER_CTRL_STOP	stop the timer
CLOCK_TIMER_CTRL_INFO_GET	get timer feature information
CLOCK_TIMER_CTRL_MODE_SET	set timer mode

Get the timer parameter argument, which is a pointer to the structure struct rt_clock_timer_info, to save the obtained information.

>Setting frequency is valid only when the timer hardware and included driver set the counting frequency. Generally, the default frequency of the driving setting can be used.

When setting the timer mode, the parameter argument can take the following values：

CLOCK_TIMER_MODE_ONESHOT    /* Single timing */
CLOCK_TIMER_MODE_PERIOD     /* Periodic timing */

An example of using the timer count frequency and timing mode is as follows:

#define CLOCK_TIMER_DEV_NAME   "timer0"     /* timer name */
rt_device_t hw_dev;                     /* timer device handle */
rt_clock_timer_mode_t mode;                 /* timer mode */
rt_uint32_t freq = 10000;               /* counting frequency */
 
/* Timer timeout callback function */
static rt_err_t timeout_cb(rt_device_t dev, rt_size_t size)
{
    rt_kprintf("this is clock_timer timeout callback function!\n");
    rt_kprintf("tick is :%d !\n", rt_tick_get());
 
    return 0;
}
 
static int clock_timer_sample(int argc, char *argv[])
{
    /* find timer device */
    hw_dev = rt_device_find(CLOCK_TIMER_DEV_NAME);
    /* open the device in read and write mode */
    rt_device_open(hw_dev, RT_DEVICE_OFLAG_RDWR);
    /* Set the timeout callback function */
    rt_device_set_rx_indicate(hw_dev, timeout_cb);
 
    /* Set the counting frequency (1Mhz or the supported minimum counting frequency by default) */
    rt_device_control(hw_dev, CLOCK_TIMER_CTRL_FREQ_SET, &freq);
    /* Set the mode to periodic timer */
    mode = CLOCK_TIMER_MODE_PERIOD;
    rt_device_control(hw_dev, CLOCK_TIMER_CTRL_MODE_SET, &mode);
}

Details:

• CLOCK_TIMER_CTRL_FREQ_SET:
  • arg points to an integer frequency in Hz.
  • The driver checks against minfreq and maxfreq from rt_clock_timer_info.
  • On success, the new frequency is used for subsequent timeouts.
• CLOCK_TIMER_CTRL_STOP:
  • Stops the current timer cycle. It does not close the device.
• CLOCK_TIMER_CTRL_INFO_GET:
  • arg points to struct rt_clock_timer_info.
  • Returns hardware limits (min/max frequency, max count, count direction).
• CLOCK_TIMER_CTRL_MODE_SET:
  • arg points to rt_clock_timer_mode_t.
  • ONESHOT fires once, PERIOD repeats using the calculated reload.

Set the Timer Timeout Value

The timer timeout value can be set by the following function：

rt_size_t rt_device_write(rt_device_t dev, rt_off_t pos, const void* buffer, rt_size_t size);

Parameter	Description
dev	device handle
pos	write data offset, unused now, can set 0 value
buffer	pointer to the timer timeout structure
size	timeout structure size
return	——
The actual size of the written data
0	fail

The prototype of the timeout structure is shown below :

typedef struct rt_clock_timerval
{
    rt_int32_t sec;      /* second */
    rt_int32_t usec;     /* microsecond */
} rt_clock_timerval_t;

An example of using the timer timeout value is as follows:

#define CLOCK_TIMER_DEV_NAME   "timer0"     /* timer name */
rt_device_t hw_dev;                     /* timer device handle */
rt_clock_timer_mode_t mode;                 /* timer mode */
rt_clock_timerval_t timeout_s;              /* Timer timeout value */
 
/* Timer timeout callback function */
static rt_err_t timeout_cb(rt_device_t dev, rt_size_t size)
{
    rt_kprintf("this is clock_timer timeout callback function!\n");
    rt_kprintf("tick is :%d !\n", rt_tick_get());
 
    return 0;
}
 
static int clock_timer_sample(int argc, char *argv[])
{
    /* find timer device */
    hw_dev = rt_device_find(CLOCK_TIMER_DEV_NAME);
    /* open the device in read-write mode */
    rt_device_open(hw_dev, RT_DEVICE_OFLAG_RDWR);
    /* set the timeout callback function */
    rt_device_set_rx_indicate(hw_dev, timeout_cb);
    /* set the mode as periodic timer */
    mode = CLOCK_TIMER_MODE_PERIOD;
    rt_device_control(hw_dev, CLOCK_TIMER_CTRL_MODE_SET, &mode);
 
    /* Set the timer timeout value to 5s and start the timer */
    timeout_s.sec = 5;      /* second */
    timeout_s.usec = 0;     /* microsecond */
    rt_device_write(hw_dev, 0, &timeout_s, sizeof(timeout_s));
}

Details:

• size must match sizeof(rt_clock_timerval_t); otherwise the write fails.
• In periodic mode, the driver may split a long timeout into multiple cycles based on the hardware maxcnt and current frequency.
• For one-shot mode, if the timeout fits in a single cycle, the timer stops automatically after expiry.

Obtain the Current Value of the Timer

The current value of the timer can be obtained by the following function：

rt_size_t rt_device_read(rt_device_t dev, rt_off_t pos, void* buffer, rt_size_t size);

Parameter	Description
dev	timer device handle
pos	write data offset, unused now , can set 0 value
buffer	output parameter, a pointer point to the timeout structure
size	timeout structure size
return	——
Timeout structure size	success
0	fail

An example of use is shown below：

rt_clock_timerval_t timeout_s;      /* Used to save the time the timer has elapsed */
/* Read the elapsed time of the timer  */
rt_device_read(hw_dev, 0, &timeout_s, sizeof(timeout_s));

Details:

• The returned value represents elapsed time since the timer started.
• In periodic mode, the driver accounts for completed cycles plus the current counter value.
• In one-shot mode, the elapsed time is limited to the active cycle.

Close the Timer Device

The timer device can be closed with the following function:

rt_err_t rt_device_close(rt_device_t dev);

Parameter	Description
dev	timer device handle
return	——
RT_EOK	close device successfully
-RT_ERROR	the device has been completely shut down and cannot be closed repeatedly
other error code	fail to close the device

When a timer device has been used and is not necessary anymore, it should be closed, otherwise the device will remain in an open status.

An example of use is shown below：

#define CLOCK_TIMER_DEV_NAME   "timer0"     /* timer name */
rt_device_t hw_dev;                     /* timer device handle */
/* find timer device */
hw_dev = rt_device_find(CLOCK_TIMER_DEV_NAME);
... ...
rt_device_close(hw_dev);

Details:

• Closing the device may stop the timer and deinitialize hardware resources.
• Reopen the device before using it again.

>Timing errors may occur. Assume that the counter has a maximum value of 0xFFFF, a counting frequency of 1Mhz, and a timing time of 1 second and 1 microsecond. Since the timer can only count up to 65535us at a time, the timing requirement for 1000001us can be completed 20 times at 50000us, and the calculation error will be 1us.

Driver Integration (BSP/Driver Authors)

To provide a hardware timer as a clock_timer device, implement rt_clock_timer_ops, fill rt_clock_timer_info, and register it with rt_clock_timer_register(). The ISR should call rt_clock_timer_isr() to handle overflow, callback dispatch, and (if selected) clock_time event notification.

static const struct rt_clock_timer_ops timer_ops =
{
    .init      = drv_timer_init,
    .start     = drv_timer_start,
    .stop      = drv_timer_stop,
    .count_get = drv_timer_count_get,
    .control   = drv_timer_control,
};
 
static const struct rt_clock_timer_info timer_info =
{
    .maxfreq = 1000000,
    .minfreq = 1000,
    .maxcnt  = 0xFFFFFFFF,
    .cntmode = CLOCK_TIMER_CNTMODE_UP,
};
 
static rt_clock_timer_t hw_timer =
{
    .ops  = &timer_ops,
    .info = &timer_info,
};
 
void drv_timer_register(void)
{
    rt_clock_timer_register(&hw_timer, "timer0", RT_NULL);
}
 
void drv_timer_isr(void)
{
    rt_clock_timer_isr(&hw_timer);
}

If the timer supports one-shot start, it can also serve as the default clock_time event device (used by hrtimer).

Details:

• rt_clock_timer_register():
  • Registers the device into the RT-Thread device framework as a timer class.
  • If this is the first registered timer with a valid start op, it becomes the clock_time event device automatically.
  • The default event is opened and initialized if not already active.
• rt_clock_timer_isr():
  • Maintains internal overflow/cycle accounting.
  • Triggers the user callback via rx_indicate.
  • If the timer is the clock_time event owner, it calls rt_clock_time_event_isr() to drive hrtimer.

Hardware Timer Device Usage Example

The specific use of the hardware timer device can refer to the following sample code. The main steps of the sample code are as follows:

1. First find the device handle based on the timer device name "timer0".
2. Open the device "timer0" in read-write mode.
3. Set the timer timeout callback function.
4. Set the timer mode to periodic timer and set the timeout period to 5 seconds. At this time, the timer starts.
5. Read the timer after 3500ms delay, the read value will be displayed in seconds and microseconds.

 /*
  * Program listing: This is a clock_timer device usage routine
 * The routine exports the clock_timer_sample command to the control terminal
 * Command call format: clock_timer_sample
 * Program function: The hardware timer timeout callback function periodically prints the current tick value, and the difference between the two tick values is converted to the time equivalent to the timing time value.
  */
 
#include <rtthread.h>
#include <rtdevice.h>
 
#define CLOCK_TIMER_DEV_NAME   "timer0"     /* timer name */
 
/* Timer timeout callback function */
static rt_err_t timeout_cb(rt_device_t dev, rt_size_t size)
{
    rt_kprintf("this is clock_timer timeout callback function!\n");
    rt_kprintf("tick is :%d !\n", rt_tick_get());
 
    return 0;
}
 
static int clock_timer_sample(int argc, char *argv[])
{
    rt_err_t ret = RT_EOK;
    rt_clock_timerval_t timeout_s;      /* timer timeout value */
    rt_device_t hw_dev = RT_NULL;   /* timer device value */
    rt_clock_timer_mode_t mode;         /* timer mode */
 
    /* find timer device */
    hw_dev = rt_device_find(CLOCK_TIMER_DEV_NAME);
    if (hw_dev == RT_NULL)
    {
        rt_kprintf("clock_timer sample run failed! can't find %s device!\n", CLOCK_TIMER_DEV_NAME);
        return -RT_ERROR;
    }
 
    /* Open the device in read-write mode */
    ret = rt_device_open(hw_dev, RT_DEVICE_OFLAG_RDWR);
    if (ret != RT_EOK)
    {
        rt_kprintf("open %s device failed!\n", CLOCK_TIMER_DEV_NAME);
        return ret;
    }
 
    /* set timeout callback function */
    rt_device_set_rx_indicate(hw_dev, timeout_cb);
 
    /* Setting mode is periodic timer */
    mode = CLOCK_TIMER_MODE_PERIOD;
    ret = rt_device_control(hw_dev, CLOCK_TIMER_CTRL_MODE_SET, &mode);
    if (ret != RT_EOK)
    {
        rt_kprintf("set mode failed! ret is :%d\n", ret);
        return ret;
    }
 
    /* Set the timer timeout value to 5s and start the timer. */
    timeout_s.sec = 5;      /* second */
    timeout_s.usec = 0;     /* microsecond */
 
    if (rt_device_write(hw_dev, 0, &timeout_s, sizeof(timeout_s)) != sizeof(timeout_s))
    {
        rt_kprintf("set timeout value failed\n");
        return -RT_ERROR;
    }
 
    /* delay 3500ms */
    rt_thread_mdelay(3500);
 
    /* read the current value of timer */
    rt_device_read(hw_dev, 0, &timeout_s, sizeof(timeout_s));
    rt_kprintf("Read: Sec = %d, Usec = %d\n", timeout_s.sec, timeout_s.usec);
 
    return ret;
}
/* Export to the msh command list */
MSH_CMD_EXPORT(clock_timer_sample, clock_timer sample);