Pixhawk原生固件PX4之日期时间的确定

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偶然注意到PX4日志中老是出现类似于2000.01.01 00:00:00这种日期(有兴趣的可以搜索一下千年虫问题)，于是结合代码进行了一波分析。

最后定位到gmtime_r(FAR const time_t *timer, FAR struct tm *result)这个函数以及struct tm这个结构体。

/* * time.h * /...struct tm{  int tm_sec;     /* second (0-61, allows for leap seconds) */  int tm_min;     /* minute (0-59) */  int tm_hour;    /* hour (0-23) */  int tm_mday;    /* day of the month (1-31) */  int tm_mon;     /* month (0-11) */  int tm_year;    /* years since 1900 */  int tm_wday;    /* day of the week (0-6) */                         /*not supported by NuttX*/  int tm_yday;    /* day of the year (0-365) */                       /*not supported by NuttX*/  int tm_isdst;   /* non-0 if daylight savings time is in effect */   /*not supported by NuttX*/};

PX4中所有输出的函数都是由tm这个结构体给出的。

这里需要注意到的是tm_year是从1900.01.01 00:00:00年开始计的。称为系统时间System Time。

而gmtime_r()是根据从1970.01.01 00:00:00后的秒数换算过来的。称为格林威治时间/Unix time。

这实在是一件有意思的事情啊。

下面给出gmtime_r()函数的换算过程吧，有趣

/**************************************************************************** * libc/time/lib_gmtimer.c *  * Function:  gmtime_r * * Description: *  Time conversion (based on the POSIX API) * ****************************************************************************/FAR struct tm *gmtime_r(FAR const time_t *timer, FAR struct tm *result){  time_t epoch;  time_t jdn;  int    year;  int    month;  int    day;  int    hour;  int    min;  int    sec;  /* Get the seconds since the EPOCH */  /* 纪元开始 1970.01.01 00:00:00*/  epoch = *timer;  sdbg("timer=%d\n", (int)epoch);  /* Convert to days, hours, minutes, and seconds since the EPOCH */  jdn    = epoch / SEC_PER_DAY; // 天数  epoch -= SEC_PER_DAY * jdn; // 减去 天数 * 一天的秒数  hour   = epoch / SEC_PER_HOUR; // 小时数  epoch -= SEC_PER_HOUR * hour; // 减去 小时数 * 一小时的秒数  min    = epoch / SEC_PER_MIN; // 分钟数  epoch -= SEC_PER_MIN * min; // 减去 分钟数 * 一分钟的秒数  sec    = epoch; // 最终秒数  sdbg("hour=%d min=%d sec=%d\n",       (int)hour, (int)min, (int)sec);  /* Convert the days since the EPOCH to calendar day */  clock_utc2calendar(jdn, &year, &month, &day); // 确定日期，自1970.1.1之后的  sdbg("jdn=%d year=%d month=%d day=%d\n",       (int)jdn, (int)year, (int)month, (int)day);  /* Then return the struct tm contents */  result->tm_year = (int)year - 1900; /* Relative to 1900 */  result->tm_mon  = (int)month - 1;   /* zero-based */  result->tm_mday = (int)day;         /* one-based */  result->tm_hour = (int)hour;  result->tm_min  = (int)min;  result->tm_sec  = (int)sec;  return result;}

上面调用了clock_utc2calendar函数，将UTC确定的天数转换成年月日。

#ifdef CONFIG_GREGORIAN_TIME // 公历时间/* Only handles dates since Jan 1, 1970 *//* 处理1970.1.1之后的日期  */static void clock_utc2calendar(time_t days, int *year, int *month, int *day){  int  value;  int  min;  int  max;  int  tmp;  bool leapyear;  /* There is one leap year every four years, so we can get close with the   * following:   */  value   = days  / (4*365 + 1);  /* Number of 4-years periods since the epoch 多少个四年 */  days   -= value * (4*365 + 1);  /* Remaining days 剩余天数 */  value <<= 2;                    /* Years since the epoch 左移两位，乘以4，得到从EPOCH后的年数 */  /* Then we will brute force the next 0-3 years */ // 简单匹配接下来的0到3年 // 将剩下的天数分配到接下来的3年  for (;;)    {      /* Is this year a leap year (we'll need this later too) */      leapyear = clock_isleapyear(value + 1970);      /* Get the number of days in the year */      tmp = (leapyear ? 366 : 365);      /* Do we have that many days? */      if (days >= tmp)        {           /* Yes.. bump up the year */           value++;           days -= tmp;        }      else        {           /* Nope... then go handle months */           break;        }    }  /* At this point, value has the year and days has number days into this year */  *year = 1970 + value;  /* Handle the month (zero based) */  min = 0;  max = 11;  do    {      /* Get the midpoint */      value = (min + max) >> 1; // value = (min + max)/2      /* Get the number of days that occurred before the beginning of the month       * following the midpoint.      */      tmp = clock_daysbeforemonth(value + 1, leapyear); // (value + 1)月开始前过了多少天了      /* Does the number of days before this month that equal or exceed the     * number of days we have remaining?     */      if (tmp > days)        {          /* Yes.. then the month we want is somewhere from 'min' and to the       * midpoint, 'value'.  Could it be the midpoint?       */          tmp = clock_daysbeforemonth(value, leapyear);          if (tmp > days)            {              /* No... The one we want is somewhere between min and value-1 */              max = value - 1;            }          else            {              /* Yes.. 'value' contains the month that we want */              break;            }        }      else        {           /* No... The one we want is somwhere between value+1 and max */           min = value + 1;        }      /* If we break out of the loop because min == max, then we want value       * to be equal to min == max.       */      value = min;    }  while (min < max);  /* The selected month number is in value. Subtract the number of days in the   * selected month   */  days -= clock_daysbeforemonth(value, leapyear);  /* At this point, value has the month into this year (zero based) and days has   * number of days into this month (zero based)   */  *month = value + 1; /* 1-based */  *day   = days + 1;  /* 1-based */}#endif /* CONFIG_GREGORIAN_TIME */

然后返回到gmtime_r()函数可以确定，struct tm结构体的赋值源头所在。

闲人看来是历史遗留问题，懒得改了。

最后的结论是，在使用tm的结构体时，要把年数加1900，月数加1，才能得到正确的UTC时间。

关于北京东八区的问题这里不再论述，之前在log日志正确显示的博文中已经有介绍，笔者觉得结合这篇博客看才是真。

然后回到2000.01.01这个日期上来，没有接收GPS的授时信息的情况下，默认时间会是它。而不是1970.01.01！显然这30年的差距得有代码站出来认了。

关于这个，系统默认时间。

一个猜测是CLOCK_REALTIME与CLOCK_ACTIVETIME两个clk_id的选取

/* CLOCK_REALTIME refers to the standard time source.  For most implementations, * the standard time source is the system timer interrupt.  However, if the * platform supports an RTC, then the standard time source will be the RTC * for the clock_gettime() and clock_settime() interfaces (the system timer * is still the time source for all of the interfaces). */#define CLOCK_REALTIME     0/* If an RTC is supported, then the non-standard CLOCK_ACTIVETIME is also * supported to manage time based on the system timer interrupt separately from * the RTC.  This may be necessary, for example, in certain cases where the * system timer interrupt has been stopped in low power modes. * * CLOCK_ACTIVETIME is only recognized by clock_gettime() and clock_settime(). */#ifdef CONFIG_RTC#  define CLOCK_ACTIVETIME 1#else#  define CLOCK_ACTIVETIME CLOCK_REALTIME#endif

因为系统在确定当前时间时调用了这个函数

/* * sdlog2.c */bool get_log_time_tt(struct tm *tt, bool boot_time) {    struct timespec ts;    px4_clock_gettime(CLOCK_REALTIME, &ts); // 请注意，时间由我定    /* use RTC time for log file naming, e.g. /fs/microsd/2014-01-19/19_37_52.px4log */    time_t utc_time_sec = 0;    if (_gpstime_only && has_gps_3d_fix) {        utc_time_sec = gps_time_sec;    } else {        utc_time_sec = ts.tv_sec + (ts.tv_nsec / 1e9);    }    if (utc_time_sec > PX4_EPOCH_SECS) {        /* strip the time elapsed since boot */        if (boot_time) {            utc_time_sec -= hrt_absolute_time() / 1e6;        }        /* apply utc offset (min, not hour) */        utc_time_sec += _utc_offset*60;        struct tm *ttp = gmtime_r(&utc_time_sec, tt);        return (ttp != NULL);    } else {        return false;    }}

px4_clock_gettime()的时间由clk_id决定，最后链接到clock_gettime()

/* * px4_time.h */#elif defined(__PX4_LINUX) || defined(__PX4_NUTTX) || defined(__PX4_DARWIN)#define px4_clock_gettime clock_gettime#define px4_clock_settime clock_settime

这里提一点：目前还是没有能够找到一个合适的编辑器查看PX4代码，时常会跳转错误。特别是涉及到NuttX后，非常多相同名称的函数，针对posix、qurt、NuttX的，还望细心。

/************************************************************************ * Name: clock_gettime * * Description: *   Clock Functions based on POSIX APIs * ************************************************************************/int clock_gettime(clockid_t clock_id, struct timespec *tp){#ifdef CONFIG_SYSTEM_TIME64  uint64_t msecs;  uint64_t secs;  uint64_t nsecs;#else  uint32_t msecs;  uint32_t secs;  uint32_t nsecs;#endif  int ret = OK;  sdbg("clock_id=%d\n", clock_id);  DEBUGASSERT(tp != NULL);  /* CLOCK_REALTIME - POSIX demands this to be present. This is the wall   * time clock.   */#ifdef CONFIG_RTC  if (clock_id == CLOCK_REALTIME || clock_id == CLOCK_ACTIVETIME)#else  if (clock_id == CLOCK_REALTIME)#endif    {      /* Do we have a high-resolution RTC that can provie us with the time? */#ifdef CONFIG_RTC_HIRES      if (g_rtc_enabled && clock_id != CLOCK_ACTIVETIME)        {          /* Yes.. Get the hi-resolution time from the RTC *//////////// 使用RTC //////          ret = up_rtc_gettime(tp);        }      else#endif        {          /* Get the elapsed time since power up (in milliseconds) biased           * as appropriate.           */        /* 获取从系统上电后的时间 不上电不走数 */          msecs = MSEC_PER_TICK * (g_system_timer - g_tickbias);          sdbg("msecs = %d g_tickbias=%d\n",               (int)msecs, (int)g_tickbias);          /* Get the elapsed time in seconds and nanoseconds. */          secs  = msecs / MSEC_PER_SEC;          nsecs = (msecs - (secs * MSEC_PER_SEC)) * NSEC_PER_MSEC;          sdbg("secs = %d + %d nsecs = %d + %d\n",               (int)msecs, (int)g_basetime.tv_sec,               (int)nsecs, (int)g_basetime.tv_nsec);          /* Add the base time to this. */          secs  += (uint32_t)g_basetime.tv_sec;          nsecs += (uint32_t)g_basetime.tv_nsec;          /* Handle carry to seconds. */          if (nsecs > NSEC_PER_SEC)            {              uint32_t dwCarrySecs = nsecs / NSEC_PER_SEC;              secs  += dwCarrySecs;              nsecs -= (dwCarrySecs * NSEC_PER_SEC);            }          /* And return the result to the caller. */          tp->tv_sec  = (time_t)secs;          tp->tv_nsec = (long)nsecs;        }      sdbg("Returning tp=(%d,%d)\n", (int)tp->tv_sec, (int)tp->tv_nsec);    }  else    {      sdbg("Returning ERROR\n");      errno = EINVAL;      ret = ERROR;    }  return ret;}

其中RTC的几个宏定义如下

/* * rtc.h *//**************************************************************************** * Pre-processor Definitions ****************************************************************************//* Configuration ************************************************************//* CONFIG_RTC - Enables general support for a hardware RTC.  Specific *   architectures may require other specific settings. * * CONFIG_RTC_DATETIME - There are two general types of RTC:  (1) A simple *   battery backed counter that keeps the time when power is down, and (2) *   A full date / time RTC the provides the date and time information, often *   in BCD format.  If CONFIG_RTC_DATETIME is selected, it specifies this *   second kind of RTC. In this case, the RTC is used to "seed" the normal *   NuttX timer and the NuttX system timer provides for higher resoution *   time. * * CONFIG_RTC_HIRES - If CONFIG_RTC_DATETIME not selected, then the simple, *   battery backed counter is used.  There are two different implementations *   of such simple counters based on the time resolution of the counter: *   The typical RTC keeps time to resolution of 1 second, usually *   supporting a 32-bit time_t value.  In this case, the RTC is used to *   "seed" the normal NuttX timer and the NuttX timer provides for higher *   resoution time. * *   If CONFIG_RTC_HIRES is enabled in the NuttX configuration, then the *   RTC provides higher resolution time and completely replaces the system *   timer for purpose of date and time. * * CONFIG_RTC_FREQUENCY - If CONFIG_RTC_HIRES is defined, then the frequency *   of the high resolution RTC must be provided.  If CONFIG_RTC_HIRES is *   not defined, CONFIG_RTC_FREQUENCY is assumed to be one. * * CONFIG_RTC_ALARM - Enable if the RTC hardware supports setting of an *   alarm.  A callback function will be executed when the alarm goes off */

有CONFIG_RTC_HIRES的说明可以看出，定义了此项后，有飞控板上的纽扣电池支持断电情况下的计时器工作，并且精度也是高高的。

那么问题来了。
经过实际测试，编写时间获取程序，发现，时间是在系统上电时才增加。

int fantasy_test_main(int argc, char *argv[]){    PX4_INFO("Hello Sky!");    time_t timeSec = time(NULL);//1970.01.01    struct tm tt;    struct tm *timeinfo = gmtime_r(&timeSec, &tt);    PX4_INFO("The time is %d-%d-%d  %d:%d:%d  \n",\        timeinfo->tm_year+1900,\        timeinfo->tm_mon+1,\        timeinfo->tm_mday,\        timeinfo->tm_hour,\        timeinfo->tm_min,\        timeinfo->tm_sec);    return 0;}

时间一直是从2000.01.01 00：00：00开始的，并且只有USB上电时间才增加，难道电池没电?

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而GPS给出的UTC秒数，由下列代码可以看出

## vehicle_gps_position.msg#uint64 time_utc_usec        # Timestamp (microseconds, UTC), this is the timestamp which comes from the gps module. It might be unavailable right after cold start, indicated by a value of 0 

最后的总结，确定当前系统时间时，先获取已经经过的秒数，再调用struct tm *timeinfo = gmtime_r(&timeSec, &tt);就能获取tm的时间了。

先接受一次GPS卫星授时吧，需要3d fix。

关于RTC时间，再看。

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