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SubjectRe: [PATCH 2/3] dt-bindings: iio: wrapper: add io-channel-unit-converter
On Mon, 19 Mar 2018 18:02:45 +0100
Peter Rosin <peda@axentia.se> wrote:

> Allow linear scaling and modification of the type of an io-channel.
>
> When an ADC channel measures the midpoint of a voltage divider, the
> interesting voltage is often the voltage over the full resistance
> of the divider. Likewise, measuring the voltage over a resistor is
> often a way to get to the current through it.
>
> This binding allows description of such hardware which is external
> to the ADC.
>
> Signed-off-by: Peter Rosin <peda@axentia.se>
> ---
> .../iio/wrapper/io-channel-unit-converter.txt | 84 ++++++++++++++++++++++
Hmm. I'm not convinced by the naming really though I can see where you care
coming from as it can effectively use a voltage ADC to measure a current.

Lets see if the devicetree people or anyone else has a suggestion on this.

Could go with AFE as that is how a chip doing this would normally be described.
It's just that here we are doing it in old fashioned resistors...

There are a few unusual elements in here binding wise so definitely looking
for input on the bindings!

Thanks,

Jonathan

> MAINTAINERS | 6 ++
> 2 files changed, 90 insertions(+)
> create mode 100644 Documentation/devicetree/bindings/iio/wrapper/io-channel-unit-converter.txt
>
> diff --git a/Documentation/devicetree/bindings/iio/wrapper/io-channel-unit-converter.txt b/Documentation/devicetree/bindings/iio/wrapper/io-channel-unit-converter.txt
> new file mode 100644
> index 000000000000..23af661abe32
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/iio/wrapper/io-channel-unit-converter.txt
> @@ -0,0 +1,84 @@
> +I/O channel unit converter bindings
> +
> +Allow linear scaling and modification of the type of an io-channel.
I can certainly conceive that we will have simple non linear cases in future
though they get awfully hard to describe so we can tackle that when it
happens.

> +
> +When an ADC channel measures the midpoint of a voltage divider, the
> +interesting voltage is often the voltage over the full resistance
> +of the divider. Likewise, measuring the voltage over a resistor is
> +often a way to get to the current through it.
> +
> +Required properties:
> +- compatible : "io-channel-unit-converter"
> +- io-channels : Channel node of the parent channel.
> +- io-channel-names : Should be "parent".
> +
> +Optional properties:
> +- numerator : The parent channel scale is multiplied by this value (default 1).
> +- denominator : The parent channel scale is divided by this value (default 1).
> +- type : The type of the wrapped channel is modified to this type. The default
> + is to use the same type as the parent channel. Recognized types are:
> + "voltage"
> + "current"
> +
> +Example 1:
> +The system voltage is circa 12V, but divided down with a 22/200
> +voltage divider to adjust it to the ADC range.
> +
> +SYSV ADC GND
> + + + +
> + | .-----. | .----. |
> + '--| 200 |-+-| 22 |--'
> + '-----' '----'
> +
> +sysv {
> + compatible = "io-channel-unit-converter";
> + io-channles = <&maxadc 1>;
> + io-channel-names = "parent";
> +
> + /* multiply the ADC voltage by 222/22 to get the system voltage */
> + numerator = <222>; /* 200 + 22 */
> + denominator = <22>;
> +}
> +
> +&spi {
> + maxadc: adc@0 {
> + compatible = "maxim,max1027";
> + reg = <0>;
> + #io-channel-cells = <1>;
> + interrupt-parent = <&gpio5>;
> + interrupts = <15 IRQ_TYPE_EDGE_RISING>;
> + spi-max-frequency = <1000000>;
> + };
> +};
> +
> +Example 2:
> +The system current is measured by measuring the voltage over a
> +3.3 ohm resistor.
> +
> +sysi {
> + compatible = "io-channel-unit-converter";
> + io-channles = <&tiadc 0>;
> + io-channel-names = "parent";
> +
> + /* divide the ADC voltage by 33/10 (i.e. 3.3) to get current */
> + numerator = <10>;
> + denominator = <33>;
> + type = "current";
> +}
> +
> +&i2c {
> + tiadc: adc@48 {
> + compatible = "ti,ads1015";
> + reg = <0x48>;
> + #io-channel-cells = <1>;
> +
> + #address-cells = <1>;
> + #size-cells = <0>;
> +
> + channel@0 { /* IN0,IN1 differential */
> + reg = <0>;
> + ti,gain = <1>;
> + ti,datarate = <4>;
> + };
> + };
> +};
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 96e5503bfb60..5dd555c7b1b0 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -6884,6 +6884,12 @@ F: drivers/staging/iio/
> F: include/linux/iio/
> F: tools/iio/
>
> +IIO UNIT CONVERTER
> +M: Peter Rosin <peda@axentia.se>
> +L: linux-iio@vger.kernel.org
> +S: Maintained
> +F: Documentation/devicetree/bindings/iio/wrapper/io-channel-unit-converter.txt
> +
> IKANOS/ADI EAGLE ADSL USB DRIVER
> M: Matthieu Castet <castet.matthieu@free.fr>
> M: Stanislaw Gruszka <stf_xl@wp.pl>

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