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10. RTC

10.1. Introduction​

ITX-3568Q development BOARD uses HYM8563 as RTC(Real Time Clock), HYM8563 is a low power CMOS real-time Clock/calendar chip, it provides a programmable Clock output, an interrupt Output and a power down detector, all addresses and data are passed serially through the I2C bus interface.The maximum bus speed is 400Kbits/s, after each read and write data, the embedded word address register will automatically increment

  • Timing can be based on 32.768kHz crystals in seconds, minutes, hours, weeks, days, months and years
  • Wide working voltage range :1.0~5.5V
  • Low resting current: Typical 0.25μA(VDD =3.0V, TA =25°C)
  • Internal integrated oscillating capacitor
  • drain open circuit interrupt pin

10.2. RTC驱动​

10.3. Interface usage​

Linux provides three user-space call interfaces. The corresponding path in the ITX-3568Q development board is:

  • SYSFS Interface : /sys/class/rtc/rtc0/
  • PROCFS Interface : /proc/driver/rtc
  • IOCTL Interface : /dev/rtc0

10.3.1. SYSFS Interface​

You can directly use the interface below cat and echo operations /sys/class/rtc/rtc0/.

For example, check the date and time of the current RTC:

# cat /sys/class/rtc/rtc0/date
2013-01-18
# cat /sys/class/rtc/rtc0/time
09:36:10

Set the startup time, such as starting up after 120 seconds:

#Start the machine regularly after 120 seconds
echo +120 >  /sys/class/rtc/rtc0/wakealarm
# View boot time
cat /sys/class/rtc/rtc0/wakealarm
#To turn it off
reboot -p

10.3.2. PROCFS Interface​

Print RTC related information:

# cat /proc/driver/rtc
rtc_time        : 09:34:59
rtc_date        : 2013-01-18
alrm_time       : 08:52:45
alrm_date       : 2013-01-18
alarm_IRQ       : no
alrm_pending    : no
update IRQ enabled      : no
periodic IRQ enabled    : no
periodic IRQ frequency  : 1
max user IRQ frequency  : 64
24hr            : yes

10.3.3. IOCTL Interface​

You can use ioctl to control /dev/rtc0.

Please refer to the document kernel/Documentation/rtc.txt for detailed instructions.

10.4. FAQs​

10.4.1. Q1: The time is out of sync after the development board is powered on ?​

A1 : Check that the RTC battery is properly connected