RTC

Introduction

CAM-C1126S2U 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
  .0~5.5V
• Low resting current: Typical 0.25μA(VDD =3.0V, TA =25°C)
• Internal integrated oscillating capacitor
• drain open circuit interrupt pin

Driver RTC

DTS configuration reference in Linux SDK:kernel/arch/arm/boot/dts/rv1109-firefly-ai-cam.dtsi

&i2c0 {
	status = "okay";
	hym8563: hym8563@51 {
		compatible = "haoyu,hym8563";
		reg = <0x51>;
		#clock-cells = <0>;
		clock-frequency = <32768>;
		clock-output-names = "rtc_32k";
	};
};

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

Interface usage

Linux provides three user-space call interfaces. The corresponding path in the CAM-C1126S2U 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:

# 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

PROCFS Interface

Print RTC related information:

# cat /proc/driver/rtc
rtc_time        : 06:53:50
rtc_date        : 2022-06-21
alrm_time       : 06:55:05
alrm_date       : 2022-06-21
alarm_IRQ       : yes
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.

Please refer to the document rtc.txt for detailed instructions.

FAQs

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

A1 : Check that the RTC battery is properly connected