Enabling ADC on a Beaglebone Black running FreeBSD 12.0-CURRENT (BEAGLEBONE)

[Dr. Rolf Jansen]

Am 30.03.2017 um 17:35 schrieb Emmanuel Vadot <manu at bidouilliste.com>:
> On Thu, 30 Mar 2017 13:28:58 -0700
> Oleksandr Tymoshenko <gonzo at bluezbox.com> wrote:
> 
>> Dr. Rolf Jansen (rj at obsigna.com) wrote:
>>> Today I updated once again my Beaglebone Black by
>>> merging-in the latest FreeBSD 12.0-CURRENT (BEAGLEBONE)
>>> snapshot, and once again I enabled the ADCs on the
>>> Beaglebone in the device tree blob am335x-boneblack.dtb,
>>> and the ADC is still working fine.
>>> 
>>> Even if it is not that a big hassle to modify the device
>>> tree blob, I am curious on why the ADC has been disabled
>>> in the blob in the first place, end even more, given the
>>> fact that the device ti_adc driver is built-in to kernel
>>> and once enabled, the ADC is functional.
>>> 
>>> Didn't it work at some time in the past? Now it is.
>>> 
>>> What is missing to activate the ADC in the device tree
>>> blob by default?
>> 
>> Few months ago FreeBSD switched to using upstream DTB files
>> instead of custom-made ones. For some reason ADC is disabled
>> in upstream. If you're running recent FreeBSD you can use
>> dtb overlays to enable ADC without hassle of maintaining
>> custom dts file. You can do following:
>> 
>> 1. Create am335x-beaglebone-tscadc.dts with following
>> content:
>> 
>> /dts-v1/;
>> /plugin/;
>> 
>> / {
>>    compatible = "ti,am335x-bone-green", "ti,am335x-bone-black", "ti,am335x-bone", "ti,am33xx";
>> 
>>    fragment at 0 {
>>        target = <&tscadc>;
>>        __overlay__ {
>>            status = "okay";
>>            adc {
>>                ti,adc-channels = <0 1 2 3 4 5 6>;
>>            };
>>        };
>>    };
>> };
>> 
>> 2. Compile overlay:
>>  $ dtc -I dts -O dtb -o am335x-beaglebone-tscadc.dtbo am335x-beaglebone-tscadc.dts
>> 
>> 3. Copy it to /boot/dtb/ directory on your BBB
>> 
>> 4. Enable overlay in /boot/loader.conf by adding following line:
>> 
>> fdt_overlays="am335x-beaglebone-tscadc.dtbo"
>> 
>> -- 
>> gonzo
> 
> I guess that ADC uses pins that can be used for other purpose so they
> didn't enabled it by default ? If it's not the case they should be
> enable by default.

AFAIK, the ADC pins are dedicated ones. See here:

    http://beagleboard.org/support/bone101

In every pinout scheme for the different programmable modes, the ADC pins are shown invariably at the same positions. 

> Note 1 : You need to use a recent dtc (i.e. the latest dtc imported in HEAD)

I use the latest one which came with the FreeBSD 12.0-CURRENT (BEAGLEBONE) snapshot, and it works.

> Note 2 : We really need to add some dtbo facility for people in the buildkernel process.


:-Q

Best regards

Rolf