Hi!
I have board based on AT91SAM9G20 (ARM926EJ-S CPU), Linux-4.9.36 kernel
and I2C RTC chip DS1340 (rtc-ds1307.c driver) without HW IRQ line connected.
Kernel configured to not use embedded functions for time getting at
startup and saving at shutdown:
CONFIG_RTC_CLASS=y
# CONFIG_RTC_HCTOSYS is not set
# CONFIG_RTC_SYSTOHC is not set
CONFIG_RTC_INTF_DEV_UIE_EMUL=y
CONFIG_RTC_DRV_DS1307=y
CONFIG_RTC_DRV_DS1307_CENTURY=y
The hwclock utility is from util-linux-2.29.1.
The OS does not have external time synchronization sources like NTP, PTP
or else.
Generally I need to achieve error within +-20 ms when RTC's time copied
into OS or back from OS into RTC.
I have made measurements during startup and shutdown of OS and have
found 500 ms delay introduced into RTC's time, when "hwclock --utc
--systohc" executed.
Logical analyzer show I2C-bus transactions and 500 ms delay corresponds
to a gap between moments when RTC's time polled for beginning of new
second and when time really saved into RTC.
Please explain, why this happens?
Is this due to absence of IRQ line for RTC or due to a bug in the
hwclock, or kernel bug or I have missed something else?
Best wishes.
--
Igor Plyatov
