ds3231 rtc i2c module on bpi-m5

[glambert]

Hello,

I am trying to connect a ds3231 module to get time during boot (dns does'nt like 1970) and without internet connexion. I load a recent build UBUNTU 26.2.1 on the M5 but unfortunatly after 2 days of research with chatgpt , I ask to knowledge experience people if they know if it's possible and which kind of dts can make this module up and running. I did'nt identify a dtbo on the distribution , a made a lot of investigation , pin 3 and 5 og the 40pins connector is connect to the cpu with pullup resistor , i try a lot dts conf and nothing gave result. If some one did this kind of connexion thanks in advance.

Gérard

[tparys]

So, I went through this recently on an NVidia device, and device tree overlays really aren't that hard. But there's a few things you'll need to get set up first, and not having an M5 to check, you'll probably have to do some reading / verification. I make no claim this will work out of the box for you. But I imagine this will save you a lot of reading.

 

There's an example .dtbo at https://github.com/KF0ARE/i2c0-rtc.dtbo/blob/main/i2c0-rtc.dtbo you should look at, and you can decompile it with the following:

 

dtc -O dts -o i2c0-rtc.dts i2c0-rtc.dtbo

 

Which contains a fragment you'll be interested in, explicitly for the DS3231 ...

 

/dts-v1/;

/ {
	compatible = "brcm,bcm2708";

	... snip ...
          
	fragment@3 {
		target = <0xffffffff>;

		__dormant__ {
			#address-cells = <0x01>;
			#size-cells = <0x00>;
			status = "okay";

			ds3231@68 {
				compatible = "maxim,ds3231";
				reg = <0x68>;
				status = "okay";
				phandle = <0x04>;
			};
		};
	};

	... snip ...

};

 

But you can't use this directly, because "compatible" doesn't include your M5, and "target" doesn't point anywhere useful in your DT. But it does call out the ds3231, and that it's at I2C address 0x68, so you can use that to find where the kernel thinks your RTC is.

 

Take a look at where your I2C buses are (/dev/i2c-*) and just scan each of them like this to see if you can find it:

 

tparys@pebble:~$ sudo i2cdetect -y -r 0                 # NOTE: I2C bus #0 is /dev/i2c-0
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:                         -- -- -- -- -- -- -- -- 
10: UU -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
70: -- -- -- -- -- -- -- --           

 

In the above, address 0x10 is unusable as there's something else bound at that register. But check your buses to see if you can find 0x68. If you see something other than "--" or "UU" there, that's probably it. If you can't find it, double check that it's connected, powered on, and that your I2C bus has been enabled (might need to enable via DTB).

 

Once you find it, you should be able to register it by doing something similar to the below as root (driver and I2C bus may be different):

 

echo ds3231 0x68 > /sys/bus/i2c/devices/i2c-0/new_device

 

If that works, and creates a /dev/rtc0 device, test that it's working with "hwclock".

 

Once it's working, it's time to make your .dtbo. You'll probably want to dump your main .dtb with the same dtc command above, and look for i2c-0 (or whatever other bus you found), as well as a .dtbo or two for your M5 to find text to enter for "compatible".

 

And you'll end up with something like this, saved as "m5-ds3231.dts" (or whatever):

 

/dts-v1/;
/plugin/;

/ {
	compatible = "brcm,bcm2708";	// <---- Change this

	fragment@0 {
		target = "i2c-0";	// <---- Change this

		__overlay__ {
			#address-cells = <0x01>;
			#size-cells = <0x00>;
			status = "okay";

			ds3231@68 {
				compatible = "maxim,ds3231";
				reg = <0x68>;
				status = "okay";
				phandle = <0x04>;
			};
		};
	};
};

 

And you'll compile it like this:

 

dtc -@ -O dtb -o m5-ds3231.dtbo m5-ds3231.dts

 

And check it against your M5's DTB like below. Note that "target" or "target-path" has some odd syntax requirements. Quotes with a leading slash is a full path to your I2C device. Quotes without a leading slash is an alias. And angle brackets with an ampersand is a label (not everything has an explicit label). Worst case scenariou, call out the whole path in "target-path" with a leading slash, and call it a day. I beat my head against a wall for a while here before I realized I could test this without rebooting ...

 

fdtoverlay -i /path/to/your/used.dtb -o modified.dtb m5-ds3231.dtbo

 

And then add it to your overlay directory (/boot/dtb/CHIPNAME/overlays), enable it armbianEnv.txt, and give it a go?

 

There's also two kernel build configs that may be of interest as well:

 

CONFIG_RTC_HCTOSYS_DEVICE="rtc0"
CONFIG_RTC_SYSTOHC_DEVICE="rtc0"

 

The first triggers the kernel to load time from a given RTC when it first shows up. The second will tell the system to update RTC when locked against NTP. FYI, it seems that you can change these only with a new kernel build.

Edited 1 hour ago by tparys