uboot中可能會有需求提供rtc的支援目的達到uboot啟動也可以進行牆上時間的顯示和後面判斷。大部分rtc支援的一個必要條件就是已經有i2c的支援。因為很多的rtc是i2c介面控制的，uboot中i2c的移植在前一篇中已經提過。這裡就只做rtc的移植。這裡選擇了常用的一款rtc晶片pcf8653來分析。
第一步 開啟支援的選項，
uboot/include/configs/mpc5200-common.h這個配置檔案中開啟了8653這個晶片的支援。
/*
* RTC configuration
*/

define CONFIG_RTC_PCF8563

define CONFIG_SYS_I2C_RTC_ADDR 0x51

這裡定義了8653的開啟配置和他的i2c的device id
第二步：uboot/drivers/rtc/Makefile
COBJS-$(CONFIG_RTC_PCF8563) += pcf8563.o
確定有這麼一條，如果移植裡面不存在的你就要自己新增了。
第三步 實現pcf8563.c 這個檔案，這個就相當於新建立一個命令。但是這個命令還是uboot做了統一的，這樣就可以方便使用者了。
/*
* (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
*/

/*
* Date & Time support for Philips PCF8563 RTC
*/

/* #define DEBUG */

include

include

include

include

if defined(CONFIG_CMD_DATE)

static uchar rtc_read (uchar reg);
static void rtc_write (uchar reg, uchar val);

/* ————————————————————————- */

int rtc_get (struct rtc_time *tmp)
{
int rel = 0;
uchar sec, min, hour, mday, wday, mon_cent, year;

sec = rtc_read (0x02)-0x80;
min = rtc_read (0x03);
hour    = rtc_read (0x04);
mday    = rtc_read (0x05);
wday    = rtc_read (0x06);
mon_cent= rtc_read (0x07);
year    = rtc_read (0x08);

debug ( "Get RTC year: %02x mon/cent: %02x mday: %02x wday: %02x "
    "hr: %02x min: %02x sec: %02x\n",
    year, mon_cent, mday, wday,
    hour, min, sec );
debug( "Alarms: wday: %02x day: %02x hour: %02x min: %02x\n",
    rtc_read (0x0C),
    rtc_read (0x0B),
    rtc_read (0x0A),
    rtc_read (0x09) );

/*
if (sec & 0x80) {
puts (“xu Warning: RTC Low Voltage - date/time not reliable\n”);
rel = -1;
}
*/
tmp->tm_sec = bcd2bin (sec & 0x7F);
tmp->tm_min = bcd2bin (min & 0x7F);
tmp->tm_hour = bcd2bin (hour & 0x3F);
tmp->tm_mday = bcd2bin (mday & 0x3F);
tmp->tm_mon = bcd2bin (mon_cent & 0x1F);
tmp->tm_year = bcd2bin (year) + ((mon_cent & 0x80) ? 2000 : 1900);
tmp->tm_wday = bcd2bin (wday & 0x07);
tmp->tm_yday = 0;
tmp->tm_isdst= 0;

debug( "Get DATE: %4d-%02d-%02d (wday=%d)  TIME: %2d:%02d:%02d\n",
    tmp->tm_year, tmp->tm_mon, tmp->tm_mday, tmp->tm_wday,
    tmp->tm_hour, tmp->tm_min, tmp->tm_sec);

return rel;

}

int rtc_set (struct rtc_time *tmp)
{
uchar century;

debug ( "Set DATE: %4d-%02d-%02d (wday=%d)  TIME: %2d:%02d:%02d\n",
    tmp->tm_year, tmp->tm_mon, tmp->tm_mday, tmp->tm_wday,
    tmp->tm_hour, tmp->tm_min, tmp->tm_sec);

rtc_write (0x08, bin2bcd(tmp->tm_year % 100));

century = (tmp->tm_year >= 2000) ? 0x80 : 0;
rtc_write (0x07, bin2bcd(tmp->tm_mon) | century);

rtc_write (0x06, bin2bcd(tmp->tm_wday));
rtc_write (0x05, bin2bcd(tmp->tm_mday));
rtc_write (0x04, bin2bcd(tmp->tm_hour));
rtc_write (0x03, bin2bcd(tmp->tm_min ));
rtc_write (0x02, bin2bcd(tmp->tm_sec ));

return 0;

}

void rtc_reset (void)
{

/* clear all control & status registers */
rtc_write (0x00, 0x00);
rtc_write (0x01, 0x00);
rtc_write (0x0D, 0x00);

/* clear Voltage Low bit */
rtc_write (0x02, rtc_read (0x02) & 0x7F);

/* reset all alarms */
rtc_write (0x09, 0x00);
rtc_write (0x0A, 0x00);
rtc_write (0x0B, 0x00);
rtc_write (0x0C, 0x00);

}

/* ————————————————————————- */

static uchar rtc_read (uchar reg)
{
return (i2c_reg_read (CONFIG_SYS_I2C_RTC_ADDR, reg));
}

static void rtc_write (uchar reg, uchar val)
{
i2c_reg_write (CONFIG_SYS_I2C_RTC_ADDR, reg, val);
}

endif

這倆可以看到
開啟CONFIG_CMD_DATE 這個配置
在uboot/include/rtc.h中也是同樣對rtc定義了幾個標準函式：
int rtc_get (struct rtc_time *);
int rtc_set (struct rtc_time *);
void rtc_reset (void);

void GregorianDay (struct rtc_time *);
void to_tm (int, struct rtc_time *);
unsigned long mktime (unsigned int, unsigned int, unsigned int,
unsigned int, unsigned int, unsigned int);
我們上面的8563檔案中也是對應的實現這幾個函式。按照標準的引數配置迴應標準的資料或者配置。
這裡還要關心一個date.c的檔案 這個檔案實現了date命令unsigned long mktime (unsigned int, unsigned int, unsigned int,
unsigned int, unsigned int, unsigned int);