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AD7998 IIO ADC Linux Driver

Supported Devices

Reference Circuits

Evaluation Boards

Description

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

Status

Source Mainlined?
git Yes

Files

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 · Michael Hennerich

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

file: drivers/staging/iio/adc/ad799x.h

struct ad799x_platform_data {
	u16				vref_mv;
};
static struct ad799x_platform_data ad7998_pdata = {
	.vref_mv = 2500,
};

In case platform_data is not present or set to NULL, the driver will use the AD7998 internal reference.

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

21 Oct 2010 16:10 · Michael Hennerich

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,
	},
#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);

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 ***
			    [--snip--]

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

			    [--snip--]

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 · Michael Hennerich

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
ad7998

Show scale

Description:
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
1.000

Show channel 0 measurement

Description:
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
1491

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
irqtrig56

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
irqtrig56

02 Mar 2011 15:16 · Michael Hennerich

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 · Michael Hennerich

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

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:

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.

scan_elements
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 · Michael Hennerich

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

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 · Michael Hennerich

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