diff mbox series

[v3,1/2] dt-bindings: iio: afe: add current-sense-shunt and voltage-divider

Message ID 20180410152802.30958-2-peda@axentia.se
State Not Applicable, archived
Headers show
Series iio: add unit converter | expand

Commit Message

Peter Rosin April 10, 2018, 3:28 p.m. UTC 
An ADC is often used to measure other quantities indirectly. These
bindings describe two cases, a current through a shunt resistor, and
a "big" voltage measured with the help of a voltage divider.

Signed-off-by: Peter Rosin <peda@axentia.se>
---
 .../bindings/iio/afe/current-sense-shunt.txt       | 41 ++++++++++++++++++++
 .../bindings/iio/afe/voltage-divider.txt           | 45 ++++++++++++++++++++++
 MAINTAINERS                                        |  7 ++++
 3 files changed, 93 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/iio/afe/current-sense-shunt.txt
 create mode 100644 Documentation/devicetree/bindings/iio/afe/voltage-divider.txt

Comments

Rob Herring April 13, 2018, 9:42 p.m. UTC | #1 
On Tue, Apr 10, 2018 at 05:28:01PM +0200, Peter Rosin wrote:
> An ADC is often used to measure other quantities indirectly. These
> bindings describe two cases, a current through a shunt resistor, and
> a "big" voltage measured with the help of a voltage divider.
> 
> Signed-off-by: Peter Rosin <peda@axentia.se>
> ---
>  .../bindings/iio/afe/current-sense-shunt.txt       | 41 ++++++++++++++++++++
>  .../bindings/iio/afe/voltage-divider.txt           | 45 ++++++++++++++++++++++
>  MAINTAINERS                                        |  7 ++++
>  3 files changed, 93 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/iio/afe/current-sense-shunt.txt
>  create mode 100644 Documentation/devicetree/bindings/iio/afe/voltage-divider.txt

Reviewed-by: Rob Herring <robh@kernel.org>
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Peter Rosin April 16, 2018, 2 p.m. UTC | #2 
On 2018-04-10 17:28, Peter Rosin wrote:
> +Example:
> +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 = "voltage-divider";
> +	io-channels = <&maxadc 1>;
> +
> +	/* Multiply the ADC voltage by 222/22 to get the system voltage. */
> +	numerator = <222>; /* 200 + 22 */
> +	denominator = <22>;
> +};

While I already got a reviewed-by from Rob, and maybe I shouldn't be
stirring the pot, but I had an umpteenth look and I now think this
one looks a bit odd. It shows a bit that it originates from when the
compatible was the very generic "io-channel-unit-converter" in v1
of the series. What I mean is that a voltage divider presumable always
gets you a lower voltage. Therefore, one would assume that the
denominator should be larger than the numerator. The fact that this
translates into the inverted fraction when calculating backwards
through the voltage divider should probably not affect the binding.

So, in the above example, I think it would make more sense to have

	numerator = <22>;
	denominator = <222>; /* 200 + 22 */

(and then, naturally, adjust the driver to invert the fraction)

Comments?

Cheers,
Peter
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Jonathan Cameron April 21, 2018, 2:34 p.m. UTC | #3 
On Mon, 16 Apr 2018 16:00:44 +0200
Peter Rosin <peda@axentia.se> wrote:

> On 2018-04-10 17:28, Peter Rosin wrote:
> > +Example:
> > +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 = "voltage-divider";
> > +	io-channels = <&maxadc 1>;
> > +
> > +	/* Multiply the ADC voltage by 222/22 to get the system voltage. */
> > +	numerator = <222>; /* 200 + 22 */
> > +	denominator = <22>;
> > +};  
> 
> While I already got a reviewed-by from Rob, and maybe I shouldn't be
> stirring the pot, but I had an umpteenth look and I now think this
> one looks a bit odd. It shows a bit that it originates from when the
> compatible was the very generic "io-channel-unit-converter" in v1
> of the series. What I mean is that a voltage divider presumable always
> gets you a lower voltage. Therefore, one would assume that the
> denominator should be larger than the numerator. The fact that this
> translates into the inverted fraction when calculating backwards
> through the voltage divider should probably not affect the binding.
> 
> So, in the above example, I think it would make more sense to have
> 
> 	numerator = <22>;
> 	denominator = <222>; /* 200 + 22 */
> 
> (and then, naturally, adjust the driver to invert the fraction)
> 
> Comments?
Agreed - it is odd as we currently have it.  I wouldn't have
a particular problem with renaming this compatible as voltage
scaler (voltage multiplier has a specific meaning we should avoid
stepping on!)  Anyhow, either way is fine with me, but I agree
the current version is odd.

Jonathan

> 
> Cheers,
> Peter
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diff mbox series

Patch

diff --git a/Documentation/devicetree/bindings/iio/afe/current-sense-shunt.txt b/Documentation/devicetree/bindings/iio/afe/current-sense-shunt.txt
new file mode 100644
index 000000000000..4d842aa85040
--- /dev/null
+++ b/Documentation/devicetree/bindings/iio/afe/current-sense-shunt.txt
@@ -0,0 +1,41 @@ 
+Current Sense Shunt
+===================
+
+When an io-channel measures the voltage over a current sense shunt,
+the interesting mesaurement is almost always the current through the
+shunt, not the voltage over it. This binding describes such a current
+sense circuit.
+
+Required properties:
+- compatible : "current-sense-shunt"
+- io-channels : Channel node of a voltage io-channel.
+- shunt-resistor-micro-ohms : The shunt resistance in microohms.
+
+Example:
+The system current is measured by measuring the voltage over a
+3.3 ohms shunt resistor.
+
+sysi {
+	compatible = "current-sense-shunt";
+	io-channels = <&tiadc 0>;
+
+	/* Divide the voltage by 3300000/1000000 (or 3.3) for the current. */
+	shunt-resistor-micro-ohms = <3300000>;
+};
+
+&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/Documentation/devicetree/bindings/iio/afe/voltage-divider.txt b/Documentation/devicetree/bindings/iio/afe/voltage-divider.txt
new file mode 100644
index 000000000000..fd4a215d9e6d
--- /dev/null
+++ b/Documentation/devicetree/bindings/iio/afe/voltage-divider.txt
@@ -0,0 +1,45 @@ 
+Voltage divider
+===============
+
+When an io-channel measures the midpoint of a voltage divider, the
+interesting voltage is often the voltage over the full resistance
+of the divider. This binding describes the voltage divider in such
+a curcuit.
+
+Required properties:
+- compatible : "voltage-divider"
+- io-channels : Channel node of a voltage io-channel.
+
+Optional properties:
+- numerator : The io-channel scale is multiplied by this value (default 1).
+- denominator : The io-channel scale is divided by this value (default 1).
+
+Example:
+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 = "voltage-divider";
+	io-channels = <&maxadc 1>;
+
+	/* 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>;
+	};
+};
diff --git a/MAINTAINERS b/MAINTAINERS
index 6e950b8b4a41..237fcdfdddc6 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -6887,6 +6887,13 @@  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/afe/current-sense-shunt.txt
+F:	Documentation/devicetree/bindings/iio/afe/voltage-divider.txt
+
 IKANOS/ADI EAGLE ADSL USB DRIVER
 M:	Matthieu Castet <castet.matthieu@free.fr>
 M:	Stanislaw Gruszka <stf_xl@wp.pl>