一、functions for handling system time

clock_time_t clock_time(void);//return the current system time in clock ticks
unsigned long clock_seconds(void);//return the system time in seconds
void clock_set_seconds(unsigned long ec);//set the value of the platform seconds

这些函数都是platform dependent的，我们是在stm8中实现的。

#if USE_RTC_CLK
#if USE_LSE  // 32768Hz
#define CLOCK_CONF_SECOND 1024
#else        // 38000Hz
#define CLOCK_CONF_SECOND 1000
#endif
#else
#define CLOCK_CONF_SECOND 1024
#endif
typedef unsigned long clock_time_t;

/**
 * A second, measured in system clock time.
 *
 * \hideinitializer
 */
#ifdef CLOCK_CONF_SECOND
#define CLOCK_SECOND CLOCK_CONF_SECOND
#else
#define CLOCK_SECOND (clock_time_t)32
#endif

其中我们的clock_time_t是unsigned long型的，在stm8中unsigned long是32bit，最大数值是4294967295。

The number of clock ticks per second is specified with the constant CLOCK_SECOND.

CLOCK_SECOND 按1024来算，clock_time函数wrap around的时间是：

4294967295/1024/(60*60*24*365) = 0.133年

clock_seconds函数wrap aound的时间是：

4294967295/(60*60*24*365) = 136.2年

The system time starts from zero when the Contiki system starts.

二、functions for blocking the CPU

/**
 * Wait for a given number of ticks.
 * \param t   How many ticks.
 *
 */
void clock_wait(clock_time_t t);

/**
 * Delay a given number of microseconds.
 * \param dt   How many microseconds to delay.
 *
 * \note Interrupts could increase the delay by a variable amount.
 */
void clock_delay_usec(uint16_t dt);

These functions are normally only used in low-level drivers where it sometimes is necessary to wait a short time without giving up the control over the CPU.

The function clock_init() is called by the system during the boot-up procedure to initialize the clock module.

main函数：

int
main(void)
{
  reset_sr = RST->SR;
  RST->SR = 0xFF;

  clock_init();

  leds_init();
  leds_on(LEDS_GREEN);
  HALT_LED_ON();

  rs232_init(RS232_PORT_0, USART_BAUD_9600, USART_PARITY_NONE);

  node_id_restore();
  node_init();

  process_init();

  process_start(&etimer_process, NULL);

  ctimer_init();

略……

  return 0;
}

三、Porting the Clock Module

The clock module is platform dependent and is implemented in the file clock.c. Since the clock module handles the system time, the clock module implementation usually also handles the notifications to the etimer library when it is time to check for expired event timers.

具体相关代码如下：

if(etimer_pending() && etimer_next_expiration_time() <= current_clock) {
    etimer_request_poll();
  }