RTC Development and Testing

Basic operation

ls -l /dev/rtc*
cat /proc/driver/rtc
hwclock -r
hwclock -w    # system -> RTC
hwclock -s    # RTC -> system
date -s '2026-07-17 12:00:00'

On Android, also check whether init services, the time detector, network time, or policy changes the system time again.

ioctl interface

An application can open /dev/rtc0 and use RTC_RD_TIME, RTC_SET_TIME, RTC_ALM_READ, and RTC_ALM_SET. Confirm alarm and wakeup support before relying on an RTC alarm.

Calibration approach

Measure accumulated error against a stable reference for at least 24 hours. Determine whether the cause is crystal tolerance, load capacitance, temperature drift, or software synchronization. Do not change calibration from a few seconds of short-term observation.

Common build and flash workflow

cd <ANDROID_TOP>/longan
./build.sh kernel
./build.sh pack

After flashing, confirm the kernel/DTB build timestamp and version so that new changes are not evaluated on an old image.