How to access the dither and toggle functionalities of the on-board ltc2688

Hi Massimo44 ,

I remember testing these for the driver author as it was being developed. You can add the toggle / dither features to the CN0554 device tree overlay, then the attributes will show up. There's a screenshot in the pull request:

https://github.com/analogdevicesinc/linux/pull/1876

And you can use the LTC2668 example overlay as a starting point:

https://github.com/analogdevicesinc/linux/pull/1876/files

https://github.com/analogdevicesinc/linux/blob/rpi-6.1.y/arch/arm/boot/dts/overlays/rpi-ltc2688-overlay.dts

-Mark

Hi Mark,

Thanks for your reply. Unfortunately I can't compile dts files.  andieo  provided a detailed procedure for the compilation here, but when I try it on my RPI it fails at the precompilation step. It complains that it cannot find the file <dt-binding/gpio/gpio.h>. The issue seems that the folder ../arch/arm/boot/dts does not exist.

Thanks.

Yup, if the dts has includes, the easiest thing to do is clone the whole repository, then do:

make dtbs

The device tree blobs will be next to the sources.

Note that if you make a new dts with a new name, add it to:

https://github.com/analogdevicesinc/linux/blob/rpi-6.1.y/arch/arm/boot/dts/overlays/Makefile

You can do this right on the Raspberry Pi, Kuiper Linux has dtc and git preinstalled.

-Mark

Hi Mark,

Thanks for the guidance. Following yours and andieo  instructions I managed to compile the dts file. After I modified the lines pertaining to the ltc2688 section, however, the ltc2688 was no longer recognized as a device. 

All I did was to add the entries that were in the ltc2688 dts file and not in the cn0554 one. I also added the  override section at the end of the ltc2688 file. I did try several combinations (for instance, the ltc2688 dts file has definitions only for channels 0, 1, 2, 3, and 7, so I did the same for the cn0554), but I was not able to find the right one.

I do get some warnings during the compilation, but those are relative to a different section that I did not modify. The unedited rpi-cn0554-overlay.dts file also gets the same warnings, but it works (at least the ltc2688 is recognized as a device).

Any suggestions?

Thanks

Hi Massimo44 ,

That's strange - can you try making a single, small change to the original rpi-cn0554-overlay.dts, like adding toggle functionality to channel0? Post the dtc errors here, as well as the output from dmesg? (Post the output as a file, it's pretty long.)

Also post the output from iio_info, just to see if the AD7124 is still there.

-Mark

Hi Mark,

As per your request, I modified the original rpi-cn0554-overlay.dts file by inserting the second line (in blue) in the channel0 section of the ltc2688:

channel@0 {
reg = <0>;
adi,toggle-dither-input = <1>;
adi,output-range-microvolt = <(-15000000) 15000000>;
};

I named the modified file as  rpi-cn0554-ch0toggle-overlay.dts and processed it following andieo's instructions. During the compilation the following warnings were issued:

rpi-cn0554-ch0toggle-overlay.dts:18.25-24.4: Warning (unit_address_vs_reg): /fragment@0/__overlay__/fixedregulator@0: node has a unit name, but no reg property
rpi-cn0554-ch0toggle-overlay.dts:25.24-30.4: Warning (unit_address_vs_reg): /fragment@0/__overlay__/fixedregulator@1: node has a unit name, but no reg property
rpi-cn0554-ch0toggle-overlay.dts:34.23-38.4: Warning (unit_address_vs_reg): /fragment@1/__overlay__/clock@0: node has a unit name, but no reg property
rpi-cn0554-ch0toggle-overlay.dts:40.24-45.4: Warning (unit_address_vs_reg): /fragment@1/__overlay__/clock@1: node has a unit name, but no reg property
rpi-cn0554-ch0toggle-overlay.dts:47.24-52.4: Warning (unit_address_vs_reg): /fragment@1/__overlay__/clock@2: node has a unit name, but no reg property

(As I said in the previous post, I get the same warnings when I compile the original dts file).

The compiled file (rpi-cn0554-ch0toggle.dtbo) was copied to the /boot/overlays folder and the config.txt file modified accordingly. After rebooting, the ltc2688 was no longer in the devices's list:

~ $ iio_attr -d
IIO context has 4 devices:
hwmon0, cpu_thermal: found 0 device attributes
hwmon1, rpi_volt: found 0 device attributes
iio:device0, ad7124-8: found 1 device attributes
trigger0, ad7124-8-dev0: found 0 device attributes

Looking at the dmesg command output, I see there is an error related to the ltc2688 "tgp clock" (line 394). 

I also tried compiling and loading the rpi-ltc2688-overlay.dts overlay file (instead of the cn0554's), and it seems to work (at least the ltc2688 is recognized and the  python file ltc2688_example.py does run almost to the very end, if I remember correctly). However, during compilation I get the same warnings (only three in this case, as the other two were relative to the ad7124).

Any further help is appreciated. Thanks!!

Looks like you also need to assign a clock to the toggle function, as in the LTC2688 overlay:

https://github.com/analogdevicesinc/linux/blob/rpi-6.1.y/arch/arm/boot/dts/overlays/rpi-ltc2688-overlay.dts#L71

Can you try adding this?

The clocks are defined in the CN0554 overlay already:

https://github.com/analogdevicesinc/linux/blob/rpi-6.1.y/arch/arm/boot/dts/overlays/rpi-cn0554-overlay.dts#L40

so you should just be able to add:

clocks = <&ltc2688_tgp2>;

(or tgp1). Also note that the clock frequencies are arbitrarily defined, you can apply whatever frequency you want, from DC to 500 kHz (the maximum implied by t13 and t14 in the datasheet).

It works!!

Thanks!!

Great! Thanks for confirming.

-Mark