[SOLVED} Re: Activate a Real Time Clock chip on I2C in Raspberry Pi 4B with Debian (not Raspbian)

[Rick Thomas]

Turns out that the PiHut has a tutorial webpage for just this problem!


https://thepihut.com/blogs/raspberry-pi-tutorials/17209332-adding-a-real-time-clock-to-your-raspberry-pi

Thanks for all the help!
Rick

On Tue, Apr 18, 2023, at 11:06 PM, Rick Thomas wrote:
> Thanks very much, Pabs!  I will follow up on those links and hopefully 
> get a better understanding of what is going on and why.  Perhaps the 
> next time someone has a question like mine, I'll be able to be as 
> helpful to them as the folks here have been to me!
>
> Rick
>
> On Tue, Apr 18, 2023, at 4:57 PM, Paul Wise wrote:
>> On Mon, 2023-04-17 at 22:21 -0700, Rick Thomas wrote:
>>
>>> What would have been even greater is if the error messages include
>>> references to some documentation that explained the problem and gave
>>> some clues to the larger context in which the problem arose.
>>
>> The messages I sent that patch adding are for very simple issues;
>> "/proc not mounted", "/sys not mounted" and "i2c-dev not loaded" and
>> since they all have very simple solutions, I just added the commands
>> to fix them instead of pointers to documentation about each error.
>>
>> Probably the i2c manual pages should get some links to the Linux kernel
>> documentation and wiki for I2C. I'm unlikely to work on that though,
>> hopefully someone else is willing to do that.
>>
>>> For example, I'd like to know why it chose bus3 rather than one of
>>> the other busses (0,1,2,4)?
>>
>> I expect that depends on the bus driver code for your device,
>> those do not seem to document their ordering though and it
>> looks like your particular i2c bus driver has no documentation.
>>
>> https://www.kernel.org/doc/html/latest/i2c/busses/index.html
>>
>>> I'd also like to know what the various modules do and what kinds of
>>> parameters there are to influence the detailed behavior?
>>
>> Looks like the devices are defined by DeviceTree, ACPI, board files,
>> dynamically by drivers for other devices, hardware probing or sysfs:
>>
>> https://www.kernel.org/doc/html/latest/i2c/instantiating-devices.html
>>
>>> And where is the documentation for the magic writing into /sys ?
>>
>> Seems to be here:
>>
>> https://www.kernel.org/doc/html/latest/i2c/instantiating-devices.html#method-4-instantiate-from-user-space
>> https://www.kernel.org/doc/html/latest/i2c/i2c-sysfs.html
>>
>> These may also be useful:
>>
>> https://www.kernel.org/doc/html/latest/i2c/
>> https://archive.kernel.org/oldwiki/i2c.wiki.kernel.org/
>>
>> PS: I am subscribed, no need to CC me, please respect Reply-To.
>>
>> -- 
>> bye,
>> pabs
>>
>> https://wiki.debian.org/PaulWise
>>
>> Attachments:
>> * signature.asc

Re: Activate a Real Time Clock chip on I2C in Raspberry Pi 4B with Debian (not Raspbian)

[Rick Thomas]

Thanks very much, Pabs!  I will follow up on those links and hopefully get a 
better understanding of what is going on and why.  Perhaps the next time 
someone has a question like mine, I'll be able to be as helpful to them as the 
folks here have been to me!

Rick

On Tue, Apr 18, 2023, at 4:57 PM, Paul Wise wrote:
> On Mon, 2023-04-17 at 22:21 -0700, Rick Thomas wrote:
>
>> What would have been even greater is if the error messages include
>> references to some documentation that explained the problem and gave
>> some clues to the larger context in which the problem arose.
>
> The messages I sent that patch adding are for very simple issues;
> "/proc not mounted", "/sys not mounted" and "i2c-dev not loaded" and
> since they all have very simple solutions, I just added the commands
> to fix them instead of pointers to documentation about each error.
>
> Probably the i2c manual pages should get some links to the Linux kernel
> documentation and wiki for I2C. I'm unlikely to work on that though,
> hopefully someone else is willing to do that.
>
>> For example, I'd like to know why it chose bus3 rather than one of
>> the other busses (0,1,2,4)?
>
> I expect that depends on the bus driver code for your device,
> those do not seem to document their ordering though and it
> looks like your particular i2c bus driver has no documentation.
>
> https://www.kernel.org/doc/html/latest/i2c/busses/index.html
>
>> I'd also like to know what the various modules do and what kinds of
>> parameters there are to influence the detailed behavior?
>
> Looks like the devices are defined by DeviceTree, ACPI, board files,
> dynamically by drivers for other devices, hardware probing or sysfs:
>
> https://www.kernel.org/doc/html/latest/i2c/instantiating-devices.html
>
>> And where is the documentation for the magic writing into /sys ?
>
> Seems to be here:
>
> https://www.kernel.org/doc/html/latest/i2c/instantiating-devices.html#method-4-instantiate-from-user-space
> https://www.kernel.org/doc/html/latest/i2c/i2c-sysfs.html
>
> These may also be useful:
>
> https://www.kernel.org/doc/html/latest/i2c/
> https://archive.kernel.org/oldwiki/i2c.wiki.kernel.org/
>
> PS: I am subscribed, no need to CC me, please respect Reply-To.
>
> -- 
> bye,
> pabs
>
> https://wiki.debian.org/PaulWise
>
> Attachments:
> * signature.asc

Re: Activate a Real Time Clock chip on I2C in Raspberry Pi 4B with Debian (not Raspbian)

[Paul Wise]

On Mon, 2023-04-17 at 22:21 -0700, Rick Thomas wrote:

> What would have been even greater is if the error messages include
> references to some documentation that explained the problem and gave
> some clues to the larger context in which the problem arose.

The messages I sent that patch adding are for very simple issues;
"/proc not mounted", "/sys not mounted" and "i2c-dev not loaded" and
since they all have very simple solutions, I just added the commands
to fix them instead of pointers to documentation about each error.

Probably the i2c manual pages should get some links to the Linux kernel
documentation and wiki for I2C. I'm unlikely to work on that though,
hopefully someone else is willing to do that.

> For example, I'd like to know why it chose bus3 rather than one of
> the other busses (0,1,2,4)?

I expect that depends on the bus driver code for your device,
those do not seem to document their ordering though and it
looks like your particular i2c bus driver has no documentation.

https://www.kernel.org/doc/html/latest/i2c/busses/index.html

> I'd also like to know what the various modules do and what kinds of
> parameters there are to influence the detailed behavior?

Looks like the devices are defined by DeviceTree, ACPI, board files,
dynamically by drivers for other devices, hardware probing or sysfs:

https://www.kernel.org/doc/html/latest/i2c/instantiating-devices.html

> And where is the documentation for the magic writing into /sys ?

Seems to be here:

https://www.kernel.org/doc/html/latest/i2c/instantiating-devices.html#method-4-instantiate-from-user-space
https://www.kernel.org/doc/html/latest/i2c/i2c-sysfs.html

These may also be useful:

https://www.kernel.org/doc/html/latest/i2c/
https://archive.kernel.org/oldwiki/i2c.wiki.kernel.org/

PS: I am subscribed, no need to CC me, please respect Reply-To.

-- 
bye,
pabs

https://wiki.debian.org/PaulWise


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Re: Activate a Real Time Clock chip on I2C in Raspberry Pi 4B with Debian (not Raspbian)

[Uwe Kleine-König]

On 4/16/23 10:01, Diederik de Haas wrote:

On Sunday, 16 April 2023 04:47:37 CEST Rick Thomas wrote:

I've got a Raspberry Pi 4B (4GB) with a DS3231 RTC module.  I can make the
combo work with Ubuntu and RaspberryPI-OS.  I'd like to try it with the
plain-vanilla Debian from  but I
can't find instructions for activating the I2C and RTC hardware in that OS.


Documentation/devicetree/bindings/rtc/rtc-ds1307.yaml in the kernel source
describes how one could use it ('compatible = "maxim,ds3231"' for DS3231).
I think you should make a DeviceTree Overlay, but I don't know how to do that.


An overlay is the nice way to add the RTC to the system. With the device 
trees provided by Debian this is a bit more complicated than with 
RaspberryPI-OS, because they are not compiled using dtc -@.


If the sysfs stuff that Georg Gast suggested works, I can help you to 
create a dt overlay.


Best regards
Uwe


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Re: Activate a Real Time Clock chip on I2C in Raspberry Pi 4B with Debian (not Raspbian)

[Georg Gast]

I searched for other ways but this was the only way to do it from userspace 
without device tree changes ... The online device tree overlays don't work 
without kernel support and a DT that got compiled with the "-@" option. That is 
not the case with stock Debian kernels. In my opinion, dtcs are nowadays more 
like firmware as they change more or less often between kernel versions. In the 
original meaning they were thought to stay the same even among different os 
like Linux and FreeBSD.

Bye

Georg

18.04.2023 01:59:14 Paul Wise :

> On Sun, 2023-04-16 at 11:17 +0200, Georg Gast wrote:
> 
>> ExecStart=/bin/sh -c "modprobe i2c_bcm2835 && modprobe i2c_bcm2708 &&
>> modprobe rtc_ds1307 && sleep 1 && echo ds1307 0x68 >
>> /sys/class/i2c-adapter/i2c-1/new_device && /sbin/hwclock -s"
> 
> I feel like there should be a better way to do this than manually doing
> /sys changes? Is there no i2c command for wrapping /sys i2c changes?
> 
> -- 
> bye,
> pabs
> 
> https://wiki.debian.org/PaulWise

Re: Activate a Real Time Clock chip on I2C in Raspberry Pi 4B with Debian (not Raspbian)

[Rick Thomas]

On Mon, Apr 17, 2023, at 7:55 PM, Paul Wise wrote:
> On Mon, 2023-04-17 at 13:39 +0300, Reco wrote:
>> On Mon, Apr 17, 2023 at 02:48:44AM -0700, Rick Thomas wrote:
>> > Sadly,   when I do:
>> >     i2cdetect -l
>> > I get nothing back.  Leading me to conclude that there are no busses 
>> > available.
>> 
>> "modprobe i2c-dev" should fix that.
>
> This seems like a usability issue, so I have sent a patch that
> adds a bunch of error messages in situations of silent failure:
>
> https://lore.kernel.org/linux-i2c/20230418023248.250685-1-pa...@bonedaddy.net/T/

That would have been great while I was trying to track down the solution to 
this problem.
What would have been even greater is if the error messages include references 
to some documentation that explained the problem and gave some clues to the 
larger context in which the problem arose.

I'm still hoping that someone here can point me to documentation that can give 
me answers to questions like these:  For example, I'd like to know why it chose 
bus3 rather than one of the other busses (0,1,2,4)?  I'd also like to know what 
the various modules do and what kinds of parameters there are to influence the 
detailed behavior?  And where is the documentation for the magic writing into 
/sys ?

Big Thanks! to all who have contributed to this discussion.
Rick

Re: Activate a Real Time Clock chip on I2C in Raspberry Pi 4B with Debian (not Raspbian)

[Paul Wise]

On Mon, 2023-04-17 at 13:39 +0300, Reco wrote:
> On Mon, Apr 17, 2023 at 02:48:44AM -0700, Rick Thomas wrote:
> > Sadly,   when I do:
> >     i2cdetect -l
> > I get nothing back.  Leading me to conclude that there are no busses 
> > available.
> 
> "modprobe i2c-dev" should fix that.

This seems like a usability issue, so I have sent a patch that
adds a bunch of error messages in situations of silent failure:

https://lore.kernel.org/linux-i2c/20230418023248.250685-1-pa...@bonedaddy.net/T/

-- 
bye,
pabs

https://wiki.debian.org/PaulWise


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Re: Activate a Real Time Clock chip on I2C in Raspberry Pi 4B with Debian (not Raspbian)

[Paul Wise]

On Sun, 2023-04-16 at 11:17 +0200, Georg Gast wrote:

> ExecStart=/bin/sh -c "modprobe i2c_bcm2835 && modprobe i2c_bcm2708 &&
> modprobe rtc_ds1307 && sleep 1 && echo ds1307 0x68 >
> /sys/class/i2c-adapter/i2c-1/new_device && /sbin/hwclock -s"

I feel like there should be a better way to do this than manually doing
/sys changes? Is there no i2c command for wrapping /sys i2c changes?

-- 
bye,
pabs

https://wiki.debian.org/PaulWise


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Re: Activate a Real Time Clock chip on I2C in Raspberry Pi 4B with Debian (not Raspbian)

[Rick Thomas]

That helps some...

root@pi:~# i2cdetect -l
i2c-3   i2c bcm2835 (i2c@7e804000)  I2C 
adapter
i2c-1   i2c Broadcom STB :  I2C 
adapter
i2c-2   i2c bcm2835 (i2c@7e205000)  I2C 
adapter
i2c-0   i2c Broadcom STB :  I2C 
adapter

and:
root@pi:~# i2cdetect -y 3
 0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:   -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- 68 -- -- -- -- -- -- -- 
70: -- -- -- -- -- -- -- --

All other busses (0 1 and 2 as arguments to i2cdetect give no devices)

So, if I change "i2c-1" to "i2c-3" in the "echo > /sys/class ..."  I find that 
the hwclock command now works!

Now all I have to do is get all that into a systemd service file so it is done 
at boot time.

It's late now.  I'll try that tomorrow!

Thanks very much to all who contributed!
Rick

PS:  Anybody who can explain why it has to be bus 3 -- and how I could have 
predicted that -- will get a hundred Internet guru-points!

On Mon, Apr 17, 2023, at 3:39 AM, Reco wrote:
> Hi.
>
> On Mon, Apr 17, 2023 at 02:48:44AM -0700, Rick Thomas wrote:
>> Sadly,   when I do:
>> i2cdetect -l
>> I get nothing back.  Leading me to conclude that there are no busses 
>> available.
>
> "modprobe i2c-dev" should fix that.
>
> Reco

Re: Activate a Real Time Clock chip on I2C in Raspberry Pi 4B with Debian (not Raspbian)

[Reco]

Hi.

On Mon, Apr 17, 2023 at 02:48:44AM -0700, Rick Thomas wrote:
> Sadly,   when I do:
> i2cdetect -l
> I get nothing back.  Leading me to conclude that there are no busses 
> available.

"modprobe i2c-dev" should fix that.

Reco

Re: Activate a Real Time Clock chip on I2C in Raspberry Pi 4B with Debian (not Raspbian)

[Rick Thomas]

Sadly,   when I do:
i2cdetect -l
I get nothing back.  Leading me to conclude that there are no busses available.

FWIW, when I do:
/sbin/hwclock -vr
I get:
hwclock from util-linux 2.36.1
System Time: 1681724077.614748
Trying to open: /dev/rtc0
Trying to open: /dev/rtc
Trying to open: /dev/misc/rtc
No usable clock interface found.
hwclock: Cannot access the Hardware Clock via any known method.

Also FWIW, when I do:
ls -l /dev/i2c*
I get:
ls: cannot access '/dev/i2c*': No such file or directory

Further confirming that there are no available busses.

Is it possible that I need to modprobe a different (or additional) module than 
"i2c_bcm2835"?

Thanks!
Rick

PS:  I'm still wondering if there is some kind of documentation of these 
modules and what they do that would give me a bit of background for this 
project.


On Mon, Apr 17, 2023, at 1:10 AM, Reco wrote:
> Hi.
>
> On Sun, Apr 16, 2023 at 06:41:53PM -0700, Rick Thomas wrote:
>> I found this in journalctl :
>> Apr 16 18:23:33 pi kernel: rtc-ds1307: probe of 1-0068 failed with error 
>> -121
>> Apr 16 18:23:33 pi kernel: i2c i2c-1: new_device: Instantiated device 
>> ds1307 at 0x68
>> 
>> Is rtc_ds1307 the wrong module for my DS3231 rtc hardware?
>
> No, because ds1307 kernel module should work with DS3231.
> But the address of the device (0x68) on I2C bus could be different,
> or the device is actually connected to another I2C bus (i.e. - not 1st).
>
> Consider running i2cdetect to clarify things.
>
> Reco

Re: Activate a Real Time Clock chip on I2C in Raspberry Pi 4B with Debian (not Raspbian)

[Reco]

Hi.

On Sun, Apr 16, 2023 at 06:41:53PM -0700, Rick Thomas wrote:
> I found this in journalctl :
> Apr 16 18:23:33 pi kernel: rtc-ds1307: probe of 1-0068 failed with error 
> -121
> Apr 16 18:23:33 pi kernel: i2c i2c-1: new_device: Instantiated device 
> ds1307 at 0x68
> 
> Is rtc_ds1307 the wrong module for my DS3231 rtc hardware?

No, because ds1307 kernel module should work with DS3231.
But the address of the device (0x68) on I2C bus could be different,
or the device is actually connected to another I2C bus (i.e. - not 1st).

Consider running i2cdetect to clarify things.

Reco

Re: Activate a Real Time Clock chip on I2C in Raspberry Pi 4B with Debian (not Raspbian)

[Rick Thomas]

Thanks for your response, Georg!

Is there somewhere that these modules are documented that I can read-up for a 
more detailed understanding?

As an experiment, I tried running the following  (It didn't work, but maybe you 
can suggest a modification that will.  I'm probably not fully understanding 
something!):
modprobe i2c_bcm2835 &&
modprobe rtc_ds1307 &&
sleep 1 &&
echo ds1307 0x68 > /sys/class/i2c-adapter/i2c-1/new_device &&
/sbin/hwclock -r
This is what I got when I tried it:
hwclock: Cannot access the Hardware Clock via any known method.

and when I tried:
i2cdetect -y 1
I got"
Error: Could not open file `/dev/i2c-1' or `/dev/i2c/1': No such file or 
directory

I found this in journalctl :
Apr 16 18:23:33 pi kernel: rtc-ds1307: probe of 1-0068 failed with error 
-121
Apr 16 18:23:33 pi kernel: i2c i2c-1: new_device: Instantiated device 
ds1307 at 0x68

Is rtc_ds1307 the wrong module for my DS3231 rtc hardware?

Thanks!
Rick

=
On Sun, Apr 16, 2023, at 2:17 AM, Georg Gast wrote:
> Am Sun, 16 Apr 2023 10:01:08 +0200
> schrieb Diederik de Haas :
>
>> On Sunday, 16 April 2023 04:47:37 CEST Rick Thomas wrote:
>> > I've got a Raspberry Pi 4B (4GB) with a DS3231 RTC module.  I can
>> > make the combo work with Ubuntu and RaspberryPI-OS.  I'd like to
>> > try it with the plain-vanilla Debian from
>> >  but I can't find
>> > instructions for activating the I2C and RTC hardware in that OS.  
>> 
>> Documentation/devicetree/bindings/rtc/rtc-ds1307.yaml in the kernel
>> source describes how one could use it ('compatible = "maxim,ds3231"'
>> for DS3231). I think you should make a DeviceTree Overlay, but I
>> don't know how to do that.
>
> I got this working with that systemd unit on a Rpi3 but should also
> work on a RPi4. Maybe you need to adjust the modprobe sequence. I guess
> the 2708 is 2835 on the Pi4.
>
> cat /etc/systemd/system/hwclock-pi.service
> [Unit]
> Description=HWClock at the I2C Bus of the Raspberry pi
> Before=basic.target
> Conflicts=shutdown.target
> DefaultDependencies=no
>
> [Service]
> Type=oneshot
> ExecStart=/bin/sh -c "modprobe i2c_bcm2835 && modprobe i2c_bcm2708 &&
> modprobe rtc_ds1307 && sleep 1 && echo ds1307 0x68 >
> /sys/class/i2c-adapter/i2c-1/new_device && /sbin/hwclock -s"
> ExecStop=/sbin/hwclock -w RemainAfterExit=yes
>
> [Install]
> WantedBy=basic.target

Re: Activate a Real Time Clock chip on I2C in Raspberry Pi 4B with Debian (not Raspbian)

[Diederik de Haas]

On Sunday, 16 April 2023 11:17:43 CEST Georg Gast wrote:
> ExecStart=/bin/sh -c "modprobe i2c_bcm2835 && modprobe i2c_bcm2708 &&
> modprobe rtc_ds1307 && sleep 1 && echo ds1307 0x68 >
> /sys/class/i2c-adapter/i2c-1/new_device && /sbin/hwclock -s"

Nice :-)
I wonder whether this (essentially) is what a DTO does.
I'm pretty sure that based on the 'compatible' the appropriate kernel module 
gets loaded (rtc_ds1307 in this case) and what you described above, seems 
reasonable to be the rest of what it does/triggers.

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Re: Activate a Real Time Clock chip on I2C in Raspberry Pi 4B with Debian(not Raspbian)

[gene heskett]

On 4/16/23 04:01, Diederik de Haas wrote:

On Sunday, 16 April 2023 04:47:37 CEST Rick Thomas wrote:

I've got a Raspberry Pi 4B (4GB) with a DS3231 RTC module.  I can make the
combo work with Ubuntu and RaspberryPI-OS.  I'd like to try it with the
plain-vanilla Debian from  but I
can't find instructions for activating the I2C and RTC hardware in that OS.


Documentation/devicetree/bindings/rtc/rtc-ds1307.yaml in the kernel source
describes how one could use it ('compatible = "maxim,ds3231"' for DS3231).
I think you should make a DeviceTree Overlay, but I don't know how to do that.


The last time I made a realtime preempt kernel for an armhf, the 
makefile did that.  Std output from a make menuconfig, an rpi4b runs it 
just as if it doesn't know any better, has been since Feb 20, 2020.


Cheers, Gene Heskett.
--
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author, 1940)
If we desire respect for the law, we must first make the law respectable.
 - Louis D. Brandeis
Genes Web page 

Re: Activate a Real Time Clock chip on I2C in Raspberry Pi 4B with Debian (not Raspbian)

[Georg Gast]

Am Sun, 16 Apr 2023 10:01:08 +0200
schrieb Diederik de Haas :

> On Sunday, 16 April 2023 04:47:37 CEST Rick Thomas wrote:
> > I've got a Raspberry Pi 4B (4GB) with a DS3231 RTC module.  I can
> > make the combo work with Ubuntu and RaspberryPI-OS.  I'd like to
> > try it with the plain-vanilla Debian from
> >  but I can't find
> > instructions for activating the I2C and RTC hardware in that OS.  
> 
> Documentation/devicetree/bindings/rtc/rtc-ds1307.yaml in the kernel
> source describes how one could use it ('compatible = "maxim,ds3231"'
> for DS3231). I think you should make a DeviceTree Overlay, but I
> don't know how to do that.

I got this working with that systemd unit on a Rpi3 but should also
work on a RPi4. Maybe you need to adjust the modprobe sequence. I guess
the 2708 is 2835 on the Pi4.

cat /etc/systemd/system/hwclock-pi.service
[Unit]
Description=HWClock at the I2C Bus of the Raspberry pi
Before=basic.target
Conflicts=shutdown.target
DefaultDependencies=no

[Service]
Type=oneshot
ExecStart=/bin/sh -c "modprobe i2c_bcm2835 && modprobe i2c_bcm2708 &&
modprobe rtc_ds1307 && sleep 1 && echo ds1307 0x68 >
/sys/class/i2c-adapter/i2c-1/new_device && /sbin/hwclock -s"
ExecStop=/sbin/hwclock -w RemainAfterExit=yes

[Install]
WantedBy=basic.target

Re: Activate a Real Time Clock chip on I2C in Raspberry Pi 4B with Debian (not Raspbian)

[Diederik de Haas]

On Sunday, 16 April 2023 04:47:37 CEST Rick Thomas wrote:
> I've got a Raspberry Pi 4B (4GB) with a DS3231 RTC module.  I can make the
> combo work with Ubuntu and RaspberryPI-OS.  I'd like to try it with the
> plain-vanilla Debian from  but I
> can't find instructions for activating the I2C and RTC hardware in that OS.

Documentation/devicetree/bindings/rtc/rtc-ds1307.yaml in the kernel source 
describes how one could use it ('compatible = "maxim,ds3231"' for DS3231).
I think you should make a DeviceTree Overlay, but I don't know how to do that.

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Activate a Real Time Clock chip on I2C in Raspberry Pi 4B with Debian (not Raspbian)

[Rick Thomas]

I've got a Raspberry Pi 4B (4GB) with a DS3231 RTC module.  I can make the 
combo work with Ubuntu and RaspberryPI-OS.  I'd like to try it with the 
plain-vanilla Debian from  but I can't 
find instructions for activating the I2C and RTC hardware in that OS.

Any suggestions?  Thanks!

Rick