AD7298

AD7298 IIO Multi-Channel ADC Linux Driver.

Supported Devices

• AD7298

Evaluation Boards

• EVAL-AD7298SDZ

Description

This is a Linux industrial I/O (Linux Industrial I/O Subsystem) subsystem driver, targeting multi channel serial interface ADCs. The industrial I/O subsystem provides a unified framework for drivers for many different types of converters and sensors using a number of different physical interfaces (i2c, spi, etc). See Linux Industrial I/O Subsystem for more information.

Source Code

Status

Source	Mainlined?
git	Yes

Files

Function	File
driver	drivers/iio/adc/ad7298.c
include	include/linux/platform_data/ad7298.h

Adding Linux driver support

Kernel configuration

Configure kernel with make menuconfig (alternatively use make xconfig or make qconfig)

Note

The AD7298 Driver depends on CONFIG_SPI

Linux Kernel Configuration
    Device Drivers  --->
        <*>     Industrial I/O support --->
            --- Industrial I/O support
            *** Analog to digital converters ***
            [--snip--]
            <*>   Analog Devices AD7298 ADC driver
            [--snip--]

Platform-data configuration

Example platform device initialization

For compile time configuration, it’s common Linux practice to keep board- and application-specific configuration out of the main driver file, instead putting it into the board support file.

For devices on custom boards, as typical of embedded and SoC-(system-on-chip) based hardware, Linux uses platform_data to point to board-specific structures describing devices and how they are connected to the SoC. This can include available ports, chip variants, preferred modes, default initialization, additional pin roles, and so on. This shrinks the board-support packages (BSPs) and minimizes board and application specific #ifdefs in drivers.

The reference voltage may vary between boards and models. The platform_data for the device’s struct device holds this information.

/**
 * struct ad7298_platform_data - Platform data for the ad7298 ADC driver
 * @ext_ref: Whether to use an external reference voltage.
 **/
struct ad7298_platform_data {
    bool ext_ref;
};

static struct ad7298_platform_data ad7298_pdata = {
    .ext_ref = true,
};

Reference voltage

The driver selects an external reference when a vref regulator is supplied and enabled. Otherwise it uses the internal 2.5 V reference.

Unlike PCI or USB devices, SPI devices are not enumerated at the hardware level. Instead, the software must know which devices are connected on each SPI bus segment, and what slave selects these devices are using. For this reason, the kernel code must instantiate SPI devices explicitly. The most common method is to declare the SPI devices by bus number.

This method is appropriate when the SPI bus is a system bus, as in many embedded systems, wherein each SPI bus has a number which is known in advance. It is thus possible to pre-declare the SPI devices that inhabit this bus. This is done with an array of struct spi_board_info, which is registered by calling spi_register_board_info().

For more information see: Overview of Linux kernel SPI support

Depending on the converter IC used, you may need to set the modalias accordingly, matching your part name. It may also required to adjust max_speed_hz. Please consult the datasheet, for maximum spi clock supported by the device in question.

static struct spi_board_info board_spi_board_info[] __initdata = {
#if defined(CONFIG_AD7298) || \
    defined(CONFIG_AD7298_MODULE)
    {
        .modalias = "ad7298",
        .max_speed_hz = 1000000,     /* max spi clock (SCK) speed in HZ */
        .bus_num = 0,
        .chip_select = 3, /* CS, change it for your board */
        .controller_data = &spi_ad7298_chip_info,
        .mode = SPI_MODE_1,
    },
#endif
};

static int __init board_init(void)
{
    [--snip--]

    spi_register_board_info(board_spi_board_info, ARRAY_SIZE(board_spi_board_info));

    [--snip--]

    return 0;
}
arch_initcall(board_init);

Driver testing

Each and every IIO device, typically a hardware chip, has a device folder under /sys/bus/iio/devices/iio:deviceX. Where X is the IIO index of the device. Under every of these directory folders reside a set of files, depending on the characteristics and features of the hardware device in question. These files are consistently generalized and documented in the IIO ABI documentation. In order to determine which IIO deviceX corresponds to which hardware device, the user can read the name file /sys/bus/iio/devices/iio:deviceX/name. In case the sequence in which the iio device drivers are loaded/registered is constant, the numbering is constant and may be known in advance.

The driver exposes in_voltageN_raw, in_voltage_scale, in_temp0_raw, in_temp0_offset and in_temp0_scale instead of inN_raw, in_scale and temp0_input.

root:/> cd /sys/bus/iio/devices/
root:/sys/bus/iio/devices> ls
device0                  device0:buffer0:access0  trigger0
device0:buffer0          device0:buffer0:event0

root:/sys/bus/iio/devices> cd device0

root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> ls -l
drwxr-xr-x    5 root     root             0 Jan  1 00:00 device0:buffer0
-r--r--r--    1 root     root          4096 Jan  1 00:00 in0_raw
-r--r--r--    1 root     root          4096 Jan  1 00:00 in1_raw
-r--r--r--    1 root     root          4096 Jan  1 00:00 in2_raw
-r--r--r--    1 root     root          4096 Jan  1 00:00 in3_raw
-r--r--r--    1 root     root          4096 Jan  1 00:00 in4_raw
-r--r--r--    1 root     root          4096 Jan  1 00:00 in5_raw
-r--r--r--    1 root     root          4096 Jan  1 00:00 in6_raw
-r--r--r--    1 root     root          4096 Jan  1 00:00 in7_raw
-r--r--r--    1 root     root          4096 Jan  1 00:00 in_scale
-r--r--r--    1 root     root          4096 Jan  1 00:00 name
lrwxrwxrwx    1 root     root             0 Jan  1 00:00 subsystem -> ../../../../../bus/iio
-r--r--r--    1 root     root          4096 Jan  1 00:00 temp0_input
drwxr-xr-x    2 root     root             0 Jan  1 00:00 trigger
-rw-r--r--    1 root     root          4096 Jan  1 00:00 uevent
root:/sys/devices/platform/bfin-spi.0/spi0.3/device0>

Identify the device

root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat name
ad7298

Basic operation

Show scale

Description: scale to be applied to in0_raw in order to obtain the measured voltage in millivolts.

root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat in_scale
0.610

Show internal temperature

Description: Shows high accuracy band gap temperature sensor temperature in milli degrees Celsius.

Read temperature directly, with buffering stopped, in the correct IIO device directory. Temperature cannot be included in the buffered voltage scan; the result below is in millidegrees Celsius.

set -eu
raw=$(cat in_temp0_raw)
offset=$(cat in_temp0_offset)
scale=$(cat in_temp0_scale)
awk -v raw="$raw" -v offset="$offset" -v scale="$scale" 'BEGIN { printf "%.3f mC\n", (raw + offset) * scale }'

For example, an output of 29750.000 mC corresponds to T = 29.75 °C.

Show channel 0 measurement

Description: Raw unscaled voltage measurement on channel 0

root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat in0_raw
2158

The voltage is in_voltage0_raw × in_voltage_scale. With the internal 2.5 V reference, scale = 2500/4096 ≈ 0.610351562 mV/code, so a raw value of 2158 is approximately 1317.14 mV.

Trigger management

If deviceX supports triggered sampling, it’s a so called trigger consumer and there will be an additional folder /sys/bus/iio/devices/iio:deviceX/trigger. In this folder there is a file called current_trigger, allowing controlling and viewing the current trigger source connected to deviceX. Available trigger sources can be identified by reading the name file /sys/bus/iio/devices/triggerY/name. The same trigger source can connect to multiple devices, so a single trigger may initialize data capture or reading from a number of sensors, converters, etc.

Hint

Trigger Consumers:

Triggers are used to fill buffers, and any device supporting INDIO_BUFFER_TRIGGERED has the consumer interface automatically created.

Description: Read name of triggerY

/sys/bus/iio/devices/triggerY$

cat name

 irqtrig56

Description: Make irqtrig56 (trigger using system IRQ56, likely a GPIO IRQ), to current trigger of deviceX

/sys/bus/iio/devices/iio:deviceX/trigger$

echo irqtrig56 > current_trigger

Description: Read current trigger source of deviceX

/sys/bus/iio/devices/iio:deviceX/trigger$

cat current_trigger

 irqtrig56

Available standalone trigger drivers

Name	Description
iio-trig-gpio	Provides support for using GPIO pins as IIO triggers.
iio-trig-rtc	Provides support for using periodic capable real time clocks as IIO triggers.
iio-trig-sysfs	Provides support for using SYSFS entry as IIO triggers.
iio-trig-bfin-timer	Provides support for using a Blackfin timer as IIO triggers.

Buffer management

Use /sys/bus/iio/devices/iio:deviceX/buffer (or buffer0) for buffer controls, scan_elements/in_voltageN_en for voltage scan selection, and /dev/iio:deviceX for sample data. Replace X with the actual IIO device index. These commands inspect metadata; they do not assert capture results:

DEV=/sys/bus/iio/devices/iio:deviceX
cat "$DEV/name"
ls "$DEV/buffer"
ls "$DEV/scan_elements"

Voltage samples are big-endian unsigned 12-bit values in 16-bit storage. Read each channel’s *_type and *_index metadata, and account for alignment padding if the timestamp is enabled.

The Industrial I/O subsystem supports buffered data acquisition. Buffer implementation and capabilities depend on the driver. Devices with buffer support expose each buffer at /sys/bus/iio/devices/iio:deviceX/bufferY, where Y is the buffer index, starting at 0. The first buffer is additionally exposed through the legacy buffer directory, which contains its buffer-control attributes. Its scan configuration is exposed through the legacy scan_elements directory, while multi-buffer systems expose the scan attributes in the corresponding bufferY directory. Read samples from the device character node /dev/iio:deviceX; the selected buffer is determined by the active buffer configuration.

length

Get/set the number of sample sets that may be held by the buffer.

enable

Enables/disables the buffer. This file should be written last, after length and selection of scan elements.

watermark

A single positive integer specifying the maximum number of scan elements to wait for. Poll will block until the watermark is reached. Blocking read will wait until the minimum between the requested read amount or the low water mark is available. Non-blocking read will retrieve the available samples from the buffer even if there are less samples then watermark level. This allows the application to block on poll with a timeout and read the available samples after the timeout expires and thus have a maximum delay guarantee.

data_available

A read-only value indicating the bytes of data available in the buffer. In the case of an output buffer, this indicates the amount of empty space available to write data to. In the case of an input buffer, this indicates the amount of data available for reading.

length_align_bytes

Using the high-speed interface. DMA buffers may have an alignment requirement for the buffer length. Newer versions of the kernel will report the alignment requirements associated with a device through the length_align_bytes property.

scan_elements

The scan_elements directory contains interfaces for elements that will be captured for a single triggered sample set in the buffer.

For example, inspect the first voltage channel and timestamp:

cat "$DEV/scan_elements/in_voltage0_type"
cat "$DEV/scan_elements/in_voltage0_index"
cat "$DEV/scan_elements/in_timestamp_type"
cat "$DEV/scan_elements/in_timestamp_index"

in_voltageX_en / in_voltageX-voltageY_en / in_timestamp_en:

Scan element control for triggered data capture. Writing 1 will enable the scan element, writing 0 will disable it

in_voltageX_type / in_voltageX-voltageY_type / in_timestamp_type:

Describes the scan element’s representation in the buffer. The format is [be|le]:[s|u]bits/storagebits[Xrepeat]>>shift. The prefix gives byte order; s or u specifies signed (two’s complement) or unsigned data. bits is the number of meaningful bits and storagebits is the storage width. The optional repeat count describes repeated elements. After interpreting byte order, shift the storage value right by shift and mask it to bits; signed values also require sign extension. For example, be:u12/16>>1 describes an unsigned 12-bit value in big-endian 16-bit storage, shifted right by one bit. Account for alignment padding when combining enabled elements into a complete scan.

in_voltageX_index / in_voltageX-voltageY_index / in_timestamp_index:

A nonnegative integer specifying the ordering of this scan element in the buffer. Note these are not dependent on what is enabled and may not be contiguous. Thus for user-space to establish the full layout these must be used in conjunction with all _en attributes to establish which channels are present, and the relevant _type attributes to establish the data storage format.

More Information

• IIO mailing list: linux-iio@vger.kernel.org

• IIO Linux Kernel Documentation sysfs-bus-iio-*

• IIO Documentation

• IIO Oscilloscope

• Libiio

• About libiio

• IIO High Speed

• Software Defined Radio using the IIO framework

• libiio introduction