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dt-bindings: iio: afe: add bindings for temperature transducers

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An ADC is often used to measure other quantities indirectly.
This binding describe one case, the measurement of a temperature
through a temperature transducer (either voltage or current).

Signed-off-by: Liam Beguin <liambeguin@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Peter Rosin <peda@axentia.se>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220213025739.2561834-11-liambeguin@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
This commit is contained in:
Liam Beguin 2022-02-12 21:57:39 -05:00 committed by Jonathan Cameron
parent d065a283fe
commit 66724c31c0
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Documentation/devicetree/bindings/iio/afe/temperature-transducer.yaml Normal file
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# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
%YAML 1.2
---
$id: http://devicetree.org/schemas/iio/afe/temperature-transducer.yaml#
$schema: http://devicetree.org/meta-schemas/core.yaml#
title: Temperature Transducer
maintainers:
- Liam Beguin <liambeguin@gmail.com>
description: |
A temperature transducer is a device that converts a thermal quantity
into any other physical quantity. This binding applies to temperature to
voltage (like the LTC2997), and temperature to current (like the AD590)
linear transducers.
In both cases these are assumed to be connected to a voltage ADC.
When an io-channel measures the output voltage of a temperature analog front
end such as a temperature transducer, the interesting measurement is almost
always the corresponding temperature, not the voltage output. This binding
describes such a circuit.
The general transfer function here is (using SI units)
V(T) = Rsense * Isense(T)
T = (Isense(T) / alpha) + offset
T = 1 / (Rsense * alpha) * (V + offset * Rsense * alpha)
When using a temperature to voltage transducer, Rsense is set to 1.
The following circuits show a temperature to current and a temperature to
voltage transducer that can be used with this binding.
VCC
-----
|
+---+---+
| AD590 | VCC
+---+---+ -----
| |
V proportional to T +----+----+
| D+ --+ |
+---- Vout | LTC2997 +--- Vout
| D- --+ |
+---+----+ +---------+
| Rsense | |
+---+----+ -----
| GND
-----
GND
properties:
compatible:
const: temperature-transducer
io-channels:
maxItems: 1
description: |
Channel node of a voltage io-channel.
'#io-channel-cells':
const: 0
sense-offset-millicelsius:
description: |
Temperature offset.
This offset is commonly used to convert from Kelvins to degrees Celsius.
In that case, sense-offset-millicelsius would be set to <(-273150)>.
default: 0
sense-resistor-ohms:
description: |
The sense resistor.
By default sense-resistor-ohms cancels out the resistor making the
circuit behave like a temperature transducer.
default: 1
alpha-ppm-per-celsius:
description: |
Sometimes referred to as output gain, slope, or temperature coefficient.
alpha is expressed in parts per million which can be micro-amps per
degrees Celsius or micro-volts per degrees Celsius. The is the main
characteristic of a temperature transducer and should be stated in the
datasheet.
additionalProperties: false
required:
- compatible
- io-channels
- alpha-ppm-per-celsius
examples:
- |
ad950: temperature-sensor-0 {
compatible = "temperature-transducer";
#io-channel-cells = <0>;
io-channels = <&temp_adc 3>;
sense-offset-millicelsius = <(-273150)>; /* Kelvin to degrees Celsius */
sense-resistor-ohms = <8060>;
alpha-ppm-per-celsius = <1>; /* 1 uA/K */
};
- |
znq_tmp: temperature-sensor-1 {
compatible = "temperature-transducer";
#io-channel-cells = <0>;
io-channels = <&temp_adc 2>;
sense-offset-millicelsius = <(-273150)>; /* Kelvin to degrees Celsius */
alpha-ppm-per-celsius = <4000>; /* 4 mV/K */
};
...