uboot一般不会要求开启rtc，只是还是支持rtc以备特殊需求的。

底层驱动移植前面两篇已经介绍。这里介绍顶层的调用过程。顶层在uboot/common/cmd_date.c

/*
* (C) Copyright 2001
* Wolfgang Denk, DENX Software Engineering, wd@denx.de.
*
* See file CREDITS for list of people who contributed to this
* project.
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License as
* published by the Free Software Foundation; either version 2 of
* the License, or (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place, Suite 330, Boston,
* MA 02111-1307 USA
*/

/*
* RTC, Date & Time support: get and set date & time
*/

include

include

include

include

ifdef CONFIG_RELOC_FIXUP_WORKS

define RELOC(a) a

else

define RELOC(a) ((typeof(a))((unsigned long)(a) + gd->reloc_off))

endif

int mk_date (char , struct rtc_time );

int do_date (cmd_tbl_t *cmdtp, int flag, int argc, char *argv[])
{
struct rtc_time tm;
int rcode = 0;
int old_bus;

/* switch to correct I2C bus */
old_bus = I2C_GET_BUS();
I2C_SET_BUS(CONFIG_SYS_RTC_BUS_NUM);

switch (argc) {
case 2:         /* set date & time */
    if (strcmp(argv[1],"reset") == 0) {
        puts ("Reset RTC...\n");
        rtc_reset ();
    } else {
        /* initialize tm with current time */
        rcode = rtc_get (&tm);

        if(!rcode) {
            /* insert new date & time */
            if (mk_date (argv[1], &tm) != 0) {
                puts ("## Bad date format\n");
                break;
            }
            /* and write to RTC */
            rcode = rtc_set (&tm);
            if(rcode)
                puts("## Set date failed\n");
        } else {
            puts("## Get date failed\n");
        }
    }
    /* FALL TROUGH */
case 1:         /* get date & time */
    rcode = rtc_get (&tm);

    if (rcode) {
        puts("## Get date failed\n");
        break;
    }

    printf ("Date: %4d-%02d-%02d (%sday)    Time: %2d:%02d:%02d\n",
        tm.tm_year, tm.tm_mon, tm.tm_mday,
        (tm.tm_wday<0 || tm.tm_wday>6) ?
            "unknown " : RELOC(weekdays[tm.tm_wday]),
        tm.tm_hour, tm.tm_min, tm.tm_sec);

    break;
default:
    cmd_usage(cmdtp);
    rcode = 1;
}

/* switch back to original I2C bus */
I2C_SET_BUS(old_bus);

return rcode;

}

/*
* simple conversion of two-digit string with error checking
*/
static int cnvrt2 (char *str, int *valp)
{
int val;

if ((*str < ‘0‘) || (*str > ‘9‘))
    return (-1);

val = *str - ‘0‘;

++str;

if ((*str < ‘0‘) || (*str > ‘9‘))
    return (-1);

*valp = 10 * val + (*str - ‘0‘);

return (0);

}

/*
* Convert date string: MMDDhhmm[[CC]YY][.ss]
*
* Some basic checking for valid values is done, but this will not catch
* all possible error conditions.
*/
int mk_date (char *datestr, struct rtc_time *tmp)
{
int len, val;
char *ptr;

ptr = strchr (datestr,‘.‘);
len = strlen (datestr);

/* Set seconds */
if (ptr) {
    int sec;

    *ptr++ = ‘\0‘;
    if ((len - (ptr - datestr)) != 2)
        return (-1);

    len = strlen (datestr);

    if (cnvrt2 (ptr, &sec))
        return (-1);

    tmp->tm_sec = sec;
} else {
    tmp->tm_sec = 0;
}

if (len == 12) {        /* MMDDhhmmCCYY */
    int year, century;

    if (cnvrt2 (datestr+ 8, &century) ||
        cnvrt2 (datestr+10, &year) ) {
        return (-1);
    }
    tmp->tm_year = 100 * century + year;
} else if (len == 10) {     /* MMDDhhmmYY   */
    int year, century;

    century = tmp->tm_year / 100;
    if (cnvrt2 (datestr+ 8, &year))
        return (-1);
    tmp->tm_year = 100 * century + year;
}

switch (len) {
case 8:         /* MMDDhhmm */
    /* fall thru */
case 10:        /* MMDDhhmmYY   */
    /* fall thru */
case 12:        /* MMDDhhmmCCYY */
    if (cnvrt2 (datestr+0, &val) ||
        val > 12) {
        break;
    }
    tmp->tm_mon  = val;
    if (cnvrt2 (datestr+2, &val) ||
        val > ((tmp->tm_mon==2) ? 29 : 31)) {
        break;
    }
    tmp->tm_mday = val;

    if (cnvrt2 (datestr+4, &val) ||
        val > 23) {
        break;
    }
    tmp->tm_hour = val;

    if (cnvrt2 (datestr+6, &val) ||
        val > 59) {
        break;
    }
    tmp->tm_min  = val;

    /* calculate day of week */
    GregorianDay (tmp);

    return (0);
default:
    break;
}

return (-1);

}

/*****************************************/

U_BOOT_CMD(
date, 2, 1, do_date,
“get/set/reset date & time”,
“[MMDDhhmm[[CC]YY][.ss]]\ndate reset\n”
” - without arguments: print date & time\n”
” - with numeric argument: set the system date & time\n”
” - with ‘reset’ argument: reset the RTC”
);
这里反向分析：
第一步是注冊 date命令
U_BOOT_CMD(
date, 2, 1, do_date,
“get/set/reset date & time”,
“[MMDDhhmm[[CC]YY][.ss]]\ndate reset\n”
” - without arguments: print date & time\n”
” - with numeric argument: set the system date & time\n”
” - with ‘reset’ argument: reset the RTC”
);
这个命令就是rtc的控制命令了 调用的函数是 do_date
int do_date (cmd_tbl_t *cmdtp, int flag, int argc, char *argv[])
能够看到这命令是调用了rtc里面注冊的rtc_reset、rtc_get、rtc_set
分为case1 ：获取当年的时间
case2 设置时间；
时间转换函数mk_date 能够将我们输入的字符串转化成为时间的年月日 供rtc_set 配置下去去