AD7998

AD7998 IIO ADC Linux Driver.

Supported Devices

Evaluation Boards

Reference Circuits

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/ad799x.c

Adding Linux driver support

Kernel configuration

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

Note

The AD7998 Driver depends on CONFIG_I2C

Linux Kernel Configuration
    Device Drivers  --->
            <*>     Industrial I/O support --->
                --- Industrial I/O support

                      *** Analog to digital converters ***
                [--snip--]

                <*>   Analog Devices AD799x ADC driver

                [--snip--]

Devicetree configuration

Required devicetree properties:

  • compatible: Needs to be adi, followed by the name of the device, e.g. adi,ad7998

  • reg: The slave address used for the device

  • vcc-supply: Phandle to the supply regulator

adc_supply: fixedregulator {
        compatible = "regulator-fixed";
        regulator-name = "fixed-supply";
        regulator-min-microvolt = <3300000>;
        regulator-max-microvolt = <3300000>;
};

i2c: i2c@e0007000 {
        #address-cells = <1>;
        #size-cells = <0>;
        compatible = "xlnx,xps-iic-2.00.a";
        ...

        adc@24 {
                compatible = "adi,ad7998";
                reg = <0x24>;
                vcc-supply = <&adc_supply>;
        };
 };

AD7991, AD7995 and AD7999 can also use an external reference through vref-supply; other variants, including AD7998, only use vcc-supply.

Device enumeration

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.

Specifying reference voltage via the regulator framework

Provide a vcc regulator for every part. Only AD7991/AD7995/AD7999 use the optional vref regulator as an external reference. Other supported variants, including AD7998, use vcc for their scale.

For devicetree-based AD7998 configuration, the device node’s supply property is:

vcc-supply = <&board_vcc>;

board_vcc must identify the regulator actually powering the device, with voltage and constraints suitable for that part. Do not supply an AD7998 reference using vref-supply: this driver does not acquire that consumer for AD7998. For board-file registration, the corresponding regulator consumer name is vcc for the I2C device, not vref.

Unlike PCI or USB devices, I2C devices are not enumerated at the hardware level. Instead, the software must know which devices are connected on each I2C bus segment, and what address these devices are using. For this reason, the kernel code must instantiate I2C devices explicitly. There are different ways to achieve this, depending on the context and requirements. However the most common method is to declare the I2C devices by bus number.

This method is appropriate when the I2C bus is a system bus, as in many embedded systems, wherein each I2C bus has a number which is known in advance. It is thus possible to pre-declare the I2C devices that inhabit this bus. This is done with an array of struct i2c_board_info, which is registered by calling i2c_register_board_info().

So, to enable such a driver one need only edit the board support file by adding an appropriate entry to i2c_board_info.

For more information see: How to instantiate I2C devices

Depending on the converter IC used, you may need to set the I2C_BOARD_INFO name accordingly, matching your part name.

static struct i2c_board_info __initdata board_i2c_board_info[] = {
#if defined(CONFIG_AD799X) || defined(CONFIG_AD799X_MODULE)
    {
        I2C_BOARD_INFO("ad7998", 0x24),
        .irq = IRQ_PG0,
    },
#endif
};

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

    i2c_register_board_info(0, board_i2c_board_info,
                ARRAY_SIZE(board_i2c_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.

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

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

root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0024/iio:device0> ls -l
drwxr-xr-x    5 root     root             0 Jan  1 00:00 buffer
drwxr-xr-x    2 root     root             0 Jan  1 00:00 events
-r--r--r--    1 root     root          4096 Jan  1 00:00 in_voltage0_raw
-r--r--r--    1 root     root          4096 Jan  1 00:00 in_voltage1_raw
-r--r--r--    1 root     root          4096 Jan  1 00:00 in_voltage2_raw
-r--r--r--    1 root     root          4096 Jan  1 00:00 in_voltage3_raw
-r--r--r--    1 root     root          4096 Jan  1 00:00 in_voltage4_raw
-r--r--r--    1 root     root          4096 Jan  1 00:00 in_voltage5_raw
-r--r--r--    1 root     root          4096 Jan  1 00:00 in_voltage6_raw
-r--r--r--    1 root     root          4096 Jan  1 00:00 in_voltage7_raw
-r--r--r--    1 root     root          4096 Jan  1 00:00 in_voltage_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
drwxr-xr-x    2 root     root             0 Jan  1 00:00 trigger
-rw-r--r--    1 root     root          4096 Jan  1 00:00 uevent

Identify the device

root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0024/iio:device0> cat name
ad7998

Basic operation

Show scale

Description: scale to be applied to in_voltage0_raw in order to obtain the measured voltage in millivolts.

root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0024/iio:device0> cat in_voltage_scale
1.000

Show channel 0 measurement

Description: Raw unscaled voltage measurement on channel 0

root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0024/iio:device0> cat in_voltage0_raw
1491

With a 3.3 V vcc, in_voltage_scale is 3300/4096 ≈ 0.805664062 mV/code, so a raw value of 1491 is approximately 1201.245 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

root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0024/iio:device0/buffer> ls
enable  length
root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0024/iio:device0/buffer>

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.

root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0024/iio:device0/scan_elements> ls
in_voltage0_en           in_voltage2_index        in_voltage5_en           in_voltage7_index
in_voltage0_index        in_voltage3_en           in_voltage5_index        in_voltage_type
in_voltage1_en           in_voltage3_index        in_voltage6_en           timestamp_en
in_voltage1_index        in_voltage4_en           in_voltage6_index        timestamp_index
in_voltage2_en           in_voltage4_index        in_voltage7_en           timestamp_type
root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0024/iio:device0/scan_elements>
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.

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