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"If you are getting an error message like this when trying to read/write to the RTC, make sure you have a good coin cell battery installed."
I have use five different "Energizer CR1220 Low Drain 3V lithuim Batteries" - all have had a minimum of 3.2 Volts.
No Luck - PLEASE ADVISE!
OS: Raspbian GNU/Linux 12 (bookworm)

--- /lib/udev/hwclock-set
--- DOES NOT HAVE ALL INFORMATION SHOWN IN TUTORTIAL "https://learn.adafruit.com/adding-a-real-time-clock-to-raspberry-pi/set-rtc-time"

#!/bin/sh
# Reset the System Clock to UTC if the hardware clock from which it
# was copied by the kernel was in localtime.

dev=$1

#if [ -e /run/systemd/system ] ; then
# exit 0
#fi

#/sbin/hwclock --rtc=$dev --systz
/sbin/hwclock --rtc=$dev --hctosys

____________________________________________________
COMMAND LINE: sudo hwclock -r --verbose
hwclock from util-linux 2.38.1
System Time: 1715689241.189133
Trying to open: /dev/rtc0
Using the rtc interface to the clock.
Last drift adjustment done at 1715687826 seconds after 1969
Last calibration done at 1715687826 seconds after 1969
Hardware clock is on UTC time
Assuming hardware clock is kept in UTC time.
Waiting for clock tick...
ioctl(4, RTC_UIE_ON, 0): Invalid argument
Waiting in loop for time from /dev/rtc0 to change
hwclock: ioctl(RTC_RD_TIME) to /dev/rtc0 to read the time failed: Invalid argument
...synchronization failed

Have you disabled the fake hardware clock?

Did you run:
before running:

Yes I have disabled and removed the fake hardware clock.

sudo hwclock -w
--- NO ERROR

sudo hwclock -r
ioctl(RTC_RD_TIME) to /dev/rtc0 to read the time failed: Invalid argument

Okay, I'll do some testing with my Pi4's today. I'm guessing this is a bookworm change we need to adapt to.

I have tried with Bookworm and Bullseye

I had to order a STEMMA breakout version of PCF8523 RTC.

However, I did have a Adafruit Adalogger on hand which also has built in RTC so I wired that up to my Pi5.

Note that I am using a battery (are you)?

I see it on the i2c bus:

The clock shows up so if I load the kernel module and want to read and write this is how I did it: