diff --git a/Documentation/iio/iio_adc.rst b/Documentation/iio/iio_adc.rst new file mode 100644 index 00000000000000..43b8cad547c9fa --- /dev/null +++ b/Documentation/iio/iio_adc.rst @@ -0,0 +1,280 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========================= +IIO Abstractions for ADCs +========================= + +1. Overview +=========== + +The IIO subsystem supports many Analog to Digital Converters (ADCs). Some ADCs +have features and characteristics that are supported in specific ways by IIO +device drivers. This documentation describes common ADC features and explains +how they are (should be?) supported by the IIO subsystem. + +1. ADC Channel Types +==================== + +ADCs can have distinct types of inputs, each of them measuring analog voltages +in a slightly different way. An ADC digitizes the analog input voltage over a +span given by the provided voltage reference, the input type, and the input +polarity. The input range allowed to an ADC channel is needed to determine the +scale factor and offset needed to obtain the measured value in real-world +units (millivolts for voltage measurement, milliamps for current measurement, +etc.). + +There are three types of ADC inputs (single-ended, differential, +pseudo-differential) and two possible polarities (unipolar, bipolar). The input +type (single-ended, differential, pseudo-differential) is one channel +characteristic, and is completely independent of the polarity (unipolar, +bipolar) aspect. A comprehensive article about ADC input types (on which this +doc is heavily based on) can be found at +https://www.analog.com/en/resources/technical-articles/sar-adc-input-types.html. + +1.1 Single-ended channels +------------------------- + +Single-ended channels digitize the analog input voltage relative to ground and +can be either unipolar or bipolar. + +1.1.1 Single-ended Unipolar Channels +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +:: + + ---------- VREF ------------- + ´ ` ´ ` _____________ + / \ / \ / | + / \ / \ --- < IN ADC | + \ / \ / \ | + `-´ `-´ \ VREF | + -------- GND (0V) ----------- +-----------+ + ^ + | + External VREF + +The input voltage to a **single-ended unipolar** channel is allowed to swing +from GND to VREF (where VREF is a voltage reference with electrical potential +higher than system ground). The maximum input voltage is also called VFS +(full-scale input voltage), with VFS being determined by VREF. The voltage +reference may be provided from an external supply or derived from the chip power +source. + +A single-ended unipolar channel could be described in device tree like the +following example:: + + adc@0 { + ... + #address-cells = <1>; + #size-cells = <0>; + + channel@0 { + reg = <0>; + }; + }; + +See ``Documentation/devicetree/bindings/iio/adc/adc.yaml`` for the complete +documentation of ADC specific device tree properties. + + +1.1.2 Single-ended Bipolar Channels +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +:: + + ---------- +VREF ------------ + ´ ` ´ ` _____________________ + / \ / \ / | + / \ / \ --- < IN ADC | + \ / \ / \ | + `-´ `-´ \ +VREF -VREF | + ---------- -VREF ------------ +-------------------+ + ^ ^ + | | + External +VREF ------+ External -VREF + +For a **single-ended bipolar** channel, the analog voltage input can go from +-VREF to +VREF (where -VREF is the voltage reference that has the lower +electrical potential while +VREF is the reference with the higher one). Some ADC +chips derive the lower reference from +VREF, others get it from a separate +input. Often, +VREF and -VREF are symmetric but they don't need to be so. When +-VREF is lower than system ground, these inputs are also called single-ended +true bipolar. + +Here's an example device tree description of a single-ended bipolar channel. +:: + + adc@0 { + ... + #address-cells = <1>; + #size-cells = <0>; + + channel@0 { + reg = <0>; + bipolar; + }; + }; + +1.2 Differential channels +------------------------- + +A differential voltage measurement digitizes the voltage level at the positive +input (IN+) relative to the negative input (IN-) over the -VREF to +VREF span. +In other words, a differential channel measures how many volts IN+ is away from +IN- (IN+ - IN-). + +1.2.1 Differential Bipolar Channels +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +:: + + -------- +VREF ------ + ´ ` ´ ` +-------------------+ + / \ / \ / / | + `-´ `-´ --- < IN+ | + -------- -VREF ------ | | + | ADC | + -------- +VREF ------ | | + ´ ` ´ ` --- < IN- | + \ / \ / \ \ +VREF -VREF | + `-´ `-´ +-------------------+ + -------- -VREF ------ ^ ^ + | +---- External -VREF + External +VREF + +The analog signals to **differential bipolar** inputs are also allowed to swing +from -VREF to +VREF. If -VREF is below system GND, these are also called +differential true bipolar inputs. + +Device tree example of a differential bipolar channel:: + + adc@0 { + ... + #address-cells = <1>; + #size-cells = <0>; + + channel@0 { + reg = <0>; + bipolar; + diff-channels = <0 1>; + }; + }; + +In the ADC driver, `differential = 1` is set into `struct iio_chan_spec` for the +channel. See ``include/linux/iio/iio.h`` for more information. + +1.2.2 Differential Unipolar Channels +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +For **differential unipolar** channels, the analog voltage at the positive input +must also be higher than the voltage at the negative input. Thus, the actual +input range allowed to a differential unipolar channel is IN- to +VREF. Because +IN+ is allowed to swing with the measured analog signal and the input setup must +guarantee IN+ will not go below IN- (nor IN- will raise above IN+), most +differential unipolar channel setups have IN- fixed to a known voltage that does +not fall within the voltage range expected for the measured signal. This leads +to a setup that is equivalent to a pseudo-differential channel. Thus, +differential unipolar channels are actually pseudo-differential unipolar +channels. + +1.3 Pseudo-differential Channels +-------------------------------- + +There is a third ADC input type which is called pseudo-differential or +single-ended to differential configuration. A pseudo-differential channel is +similar to a differential channel in that it also measures IN+ relative to IN-. +However, unlike differential channels, the negative input is limited to a narrow +voltage range while only IN+ is allowed to swing. A pseudo-differential channel +can be made out from a differential pair of inputs by restricting the negative +input to a known voltage while allowing only the positive input to swing. Aside +from that, some parts have a COM pin that allows single-ended inputs to be +referenced to a common-mode voltage, making them pseudo-differential channels. + +1.3.1 Pseudo-differential Unipolar Channels +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +:: + + -------- +VREF ------ +-------------------+ + ´ ` ´ ` / | + / \ / \ / --- < IN+ | + `-´ `-´ | | + --------- IN- ------- | ADC | + | | + Common-mode voltage --> --- < IN- | + \ +VREF -VREF | + +-------------------+ + ^ ^ + | +---- External -VREF + External +VREF + +A **pseudo-differential unipolar** input has the limitations a differential +unipolar channel would have, meaning the analog voltage to the positive input +IN+ must stay within IN- to +VREF. The fixed voltage to IN- is sometimes called +common-mode voltage and it must be within -VREF to +VREF as would be expected +from the signal to any differential channel negative input. + +In pseudo-differential configuration, the voltage measured from IN+ is not +relative to GND (as it would be for a single-ended channel) but to IN-, which +causes the measurement to always be offset by IN- volts. To allow applications +to calculate IN+ voltage with respect to system ground, the IIO channel may +provide an `_offset` attribute to report the channel offset to user space. + +Device tree example for pseudo-differential unipolar channel:: + + adc@0 { + ... + #address-cells = <1>; + #size-cells = <0>; + + channel@0 { + reg = <0>; + single-channel = <0>; + common-mode-channel = <1>; + }; + }; + +Do not set `differential` in the channel `iio_chan_spec` struct of +pseudo-differential channels. + +1.3.2 Pseudo-differential Bipolar Channels +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +:: + + -------- +VREF ------ +-------------------+ + ´ ` ´ ` / | + / \ / \ / --- < IN+ | + `-´ `-´ | | + -------- -VREF ------ | ADC | + | | + Common-mode voltage --> --- < IN- | + \ +VREF -VREF | + +-------------------+ + ^ ^ + | +---- External -VREF + External +VREF + +A **pseudo-differential bipolar** input is not limited by the level at IN- but +it will be limited to -VREF or to GND on the lower end of the input range +depending on the particular ADC. Similar to their unipolar counter parts, +pseudo-differential bipolar channels may define an `_offset` attribute to +provide the read offset relative to GND. + +Device tree example for pseudo-differential bipolar channel:: + + adc@0 { + ... + #address-cells = <1>; + #size-cells = <0>; + + channel@0 { + reg = <0>; + bipolar; + single-channel = <0>; + common-mode-channel = <1>; + }; + }; + +Again, the `differential` field of `struct iio_chan_spec` is not set for +pseudo-differential channels. diff --git a/Documentation/iio/index.rst b/Documentation/iio/index.rst index 6f953e23ccf893..25a57ffeb6aa98 100644 --- a/Documentation/iio/index.rst +++ b/Documentation/iio/index.rst @@ -7,6 +7,7 @@ Industrial I/O .. toctree:: :maxdepth: 1 + iio_adc iio_configfs iio_devbuf iio_tools