RTC

Introduction

ROC-RK3506J-CC development BOARD uses TT8563RH as RTC(Real Time Clock), TT8563RH 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.

• Timing can be based on 32.768kHz crystals in seconds, minutes, hours, weeks, days, months and years
• Wide working voltage range
  .0~5.5V
• Low resting current: Typical 0.25μA(VDD =3.0V, TA =25°C)
• Internal integrated oscillating capacitor
• drain open circuit interrupt pin

ROC-RK3506J-CC needs to be connected to the RTC battery to power the RTC chip to ensure that the RTC can operate normally after a short system power outage.

Driver RTC

DTS Reference: kernel/arch/arm/boot/dts/rk3506b-firefly-roc-rk3506b-cc.dtsi

Driver Reference: kernel/drivers/rtc/rtc-hym8563.c

Interface usage

Linux provides three user-space call interfaces. The corresponding path in the ROC-RK3506J-CC development board is:

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

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:

root@rk3506-buildroot:/# cat /sys/class/rtc/rtc0/date
2021-01-01

root@rk3506-buildroot:/# cat /sys/class/rtc/rtc0/time
17:18:14

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

PROCFS Interface

Print RTC related information:

root@rk3506-buildroot:/# cat /proc/driver/rtc
rtc_time        : 17:20:04
rtc_date        : 2021-01-01
alrm_time       : 16:31:00
alrm_date       : 2021-01-02
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

IOCTL Interface

You can use ioctl to control /dev/rtc0.

For detailed instructions, please refer to the document kernel/Documentation/admin-guide/rtc.rst.

FAQs

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

A1 : Check that the RTC battery is properly connected