This version (05 Jan 2021 17:01) was approved by Ioana Chelaru.The Previously approved version (11 Feb 2016 20:47) is available.Diff

AD7998 IIO ADC Linux Driver

Supported Devices

Reference Circuits

Evaluation Boards


This is a Linux industrial I/O (IIO) 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 IIO for more information.

Source Code


Source Mainlined?
git Yes


Function File
driver drivers/iio/adc/ad799x.c

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.

21 Oct 2010 16:10

Specifying reference voltage via the regulator framework

In case the AD799x on-chip reference is not used, this driver requires specifying the reference voltage, by using the Linux regulator framework.

Below example specifies a 2.5 Volt reference for the I2C device 0-0024 on I2C-Bus 0. (0-0024)

static struct regulator_consumer_supply ad7998_consumer_supplies[] = {
	REGULATOR_SUPPLY("vref", "0-0024"),
static struct regulator_init_data board_avdd_reg_init_data = {
	.constraints	= {
		.name	= "2V5",
		.valid_ops_mask = REGULATOR_CHANGE_STATUS,
	.consumer_supplies = ad7998_consumer_supplies,
	.num_consumer_supplies = ARRAY_SIZE(ad7998_consumer_supplies),
static struct fixed_voltage_config board_vref_pdata = {
	.supply_name	= "board-2V5",
	.microvolts	= 2500000,
	.gpio		= -EINVAL,
	.enabled_at_boot = 0,
	.init_data	= &board_avdd_reg_init_data,
static struct platform_device brd_voltage_regulator = {
	.name		= "reg-fixed-voltage",
	.id		= -1,
	.num_resources	= 0,
	.dev		= {
		.platform_data	= &board_vref_pdata,
static struct platform_device *board_devices[] __initdata = {
static int __init board_init(void)
	platform_add_devices(board_devices, ARRAY_SIZE(board_devices));
	return 0;

Declaring I2C devices

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: Documentation/i2c/instantiating-devices.rst

21 Oct 2010 16:10

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),
		.platform_data = &ad799x_pdata,
		.irq = IRQ_PG0,
static int __init board_init(void)
	i2c_register_board_info(0, board_i2c_board_info,
	return 0;


Required devicetree properties:

  • compatible: Needs to be “adi,” followed by the name of the device. E.g. “adi,ad7798”
  • 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>;

       adc_vref: fixedregulator {
               compatible = "regulator-fixed";
               regulator-name = "fixed-supply";
               regulator-min-microvolt = <2500000>;
               regulator-max-microvolt = <2500000>;
       i2c: i2c@e0007000 {
               #address-cells = <1>;
               #size-cells = <0>;
               compatible = "xlnx,xps-iic-2.00.a";

               ad7798@24 {
                       compatible = "adi,ad7798";
                       reg = <0x24>;
                       vcc-supply = <&adc_supply>;
                       vref-supply = <&adc_vref>;

Adding Linux driver support

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

The AD7998 Driver depends on CONFIG_I2C

Linux Kernel Configuration
	Device Drivers  --->
		[*] Staging drivers  --->
			<*>     Industrial I/O support --->
			    --- Industrial I/O support
			    -*-   Enable ring buffer support within IIO
			    -*-     Industrial I/O lock free software ring
			    -*-   Enable triggered sampling support

			          *** Analog to digital converters ***

			    <*>   Analog Devices AD799x ADC driver
			    -*-     Analog Devices AD799x: use ring buffer


Hardware configuration

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.

02 Mar 2011 15:16

This specifies any shell prompt running on the target

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

Show device name

This specifies any shell prompt running on the target

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

Show scale

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

This specifies any shell prompt running on the target

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

Show channel 0 measurement

Raw unscaled voltage measurement on channel 0

This specifies any shell prompt running on the target

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

U = in_voltage0_raw * in_voltage_scale = 1491 * 1.000 = 1491,00 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/device/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.

Trigger Consumers:
Currently triggers are only used for the filling of software ring buffers and as such any device supporting INDIO_RING_TRIGGERED has the consumer interface automatically created.

Description: Read name of triggerY

This specifies any shell prompt running on the target

root:/sys/bus/iio/devices/triggerY/> cat name

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

This specifies any shell prompt running on the target

root:/sys/bus/iio/devices/iio:deviceX/trigger> echo irqtrig56 > current_trigger

Description: Read current trigger source of deviceX

This specifies any shell prompt running on the target

root:/sys/bus/iio/devices/iio:deviceX/trigger> cat current_trigger

02 Mar 2011 15:16

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.
02 Mar 2011 15:16

Buffer management

This specifies any shell prompt running on the target

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

The Industrial I/O subsystem provides support for various ring buffer based data acquisition methods. Apart from device specific hardware buffer support, the user can chose between two different software ring buffer implementations. One is the IIO lock free software ring, and the other is based on Linux kfifo. Devices with buffer support feature an additional sub-folder in the /sys/bus/iio/devices/deviceX/ folder hierarchy. Called deviceX:bufferY, where Y defaults to 0, for devices with a single buffer.

Every buffer implementation features a set of files:

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

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

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.

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.

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.

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

02 Mar 2011 15:16

This specifies any shell prompt running on the target

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

in_voltageX_en / in_voltageX-voltageY_en / 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 / timestamp_type:
Description of the scan element data storage within the buffer and therefore in the form in which it is read from user-space. Form is [s|u]bits/storage-bits. s or u specifies if signed (2's complement) or unsigned. bits is the number of bits of data and storage-bits is the space (after padding) that it occupies in the buffer. Note that some devices will have additional information in the unused bits so to get a clean value, the bits value must be used to mask the buffer output value appropriately. The storage-bits value also specifies the data alignment. So u12/16 will be a unsigned 12 bit integer stored in a 16 bit location aligned to a 16 bit boundary. For other storage combinations this attribute will be extended appropriately.

in_voltageX_index / in_voltageX-voltageY_index / timestamp_index:
A single positive integer specifying the position 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.

02 Mar 2011 15:16

More Information

resources/tools-software/linux-drivers/iio-adc/ad7998.txt · Last modified: 05 Jan 2021 16:01 by Ioana Chelaru