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AD7791 IIO Low Power Sigma-Delta 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
/**
 * struct ad7791_platform_data - AD7791 device platform data
 * @buffered: If set to true configure the device for buffered input mode.
 * @burnout_current: If set to true the 100mA burnout current is enabled.
 * @unipolar: If set to true sample in unipolar mode, if set to false sample in
 *      bipolar mode.
 */
struct ad7791_platform_data {
	bool buffered;
	bool burnout_current;
	bool unipolar;
};
static struct ad7791_platform_data ad7791_pdata =  {
	.buffered = true,
	.burnout_current = false,
	.unipolar = false,
};

Declaring SPI slave devices

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: Documentation/spi/spi-summary

21 Oct 2010 16:10 · Michael Hennerich

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 = {
	{
		.modalias = "ad7791",
		.max_speed_hz = 1000000,     /* max spi clock (SCK) speed in HZ */
		.bus_num = 0,
		.chip_select = 3, /* CS, change it for your board */
		.platform_data = &ad7791_pdata, /* No spi_driver specific config */
		.mode = SPI_MODE_3,
		.irq = IRQ_PF6,
	},
};
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);

Adding Linux driver support

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

The AD7791 Driver depends on CONFIG_SPI

Linux Kernel Configuration
    Device Drivers  --->
        ...
        <*>     Industrial I/O support --->
            --- Industrial I/O support
            ...
            Analog to digital converters  ---> 
                ...
                <*>  Analog Devices AD7791 ADC driver
                ...
            ...
        ...

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/bfin-spi.0/spi0.3/iio:device0> ls -l
drwxr-xr-x    2 root     root             0 Jan  1 04:18 buffer
-r--r--r--    1 root     root          4096 Jan  1 04:18 dev
-rw-r--r--    1 root     root          4096 Jan  1 04:18 in_voltage-voltage_scale
-rw-r--r--    1 root     root          4096 Jan  1 04:18 in_voltage-voltage_shorted_scale
-rw-r--r--    1 root     root          4096 Jan  1 04:18 in_voltage0-voltage0_offset
-rw-r--r--    1 root     root          4096 Jan  1 04:18 in_voltage0-voltage0_raw
-rw-r--r--    1 root     root          4096 Jan  1 04:18 in_voltage0-voltage0_shorted_offset
-rw-r--r--    1 root     root          4096 Jan  1 04:18 in_voltage0-voltage0_shorted_raw
-rw-r--r--    1 root     root          4096 Jan  1 04:18 in_voltage1_supply_offset
-rw-r--r--    1 root     root          4096 Jan  1 04:18 in_voltage1_supply_raw
-rw-r--r--    1 root     root          4096 Jan  1 04:18 in_voltage_supply_scale
-r--r--r--    1 root     root          4096 Jan  1 04:18 name
drwxr-xr-x    2 root     root             0 Jan  1 04:18 power
-rw-r--r--    1 root     root          4096 Jan  1 04:18 sampling_frequency
-r--r--r--    1 root     root          4096 Jan  1 04:18 sampling_frequency_available
drwxr-xr-x    2 root     root             0 Jan  1 04:18 scan_elements
lrwxrwxrwx    1 root     root             0 Jan  1 04:18 subsystem -> ../../../../../../../bus/iio
drwxr-xr-x    2 root     root             0 Jan  1 04:18 trigger
-rw-r--r--    1 root     root          4096 Jan  1 04:18 uevent


root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0>

Show device name

This specifies any shell prompt running on the target

root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> cat name
ad7791

Show available sampling frequencies / update rates

This specifies any shell prompt running on the target

root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> cat sampling_frequency_available
120 100 33.3 20 16.7 16.6 13.3 9.5

Set sampling frequency / update rate

This specifies any shell prompt running on the target

root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> cat sampling_frequency
16.6
root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> echo 100 > sampling_frequency
root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> cat sampling_frequency
100
root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0>

Show supply voltage

Description:
Shows the AVDD supply voltage

This specifies any shell prompt running on the target

root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> cat in_voltage1_supply_raw
7817399
root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> cat in_voltage_supply_scale
0.000697374

U = (in_voltage1_supply_raw + in_voltage1_supply_offset) * in_voltage_supply_scale = 7817399 * 0.000697374 = 3270.95608958 mV

Show in_voltage0-voltage0_raw measurement

Description:
Raw unscaled voltage measurement on channel 0

ADC Input Pair Channel name
AIN(+) – AIN(–) in_voltage0-voltage0_raw
AIN(-) - AIN(-) in_voltage0-voltage0_shorted_raw
AVDD in_voltage1_supply_raw

This specifies any shell prompt running on the target

root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> cat in_voltage0-voltage0_raw
6710665

U = (in_voltage0-voltage0_raw + in_voltage0-voltage0_offset) * in_voltage-voltage_scale = 6710665 * 0.000596046 = 999.95619165 mV

Trigger management

This driver only supports it's own default trigger source ad7791-dev0

This specifies any shell prompt running on the target

root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat trigger/current_trigger 
ad7791-dev0

Buffer management

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/bfin-spi.0/spi0.3/iio:device0/scan_elements> ls
in_timestamp_en                     in_voltage0-voltage0_shorted_index
in_timestamp_index                  in_voltage0-voltage0_shorted_type
in_timestamp_type                   in_voltage0-voltage0_type
in_voltage0-voltage0_en             in_voltage1_supply_en
in_voltage0-voltage0_index          in_voltage1_supply_index
in_voltage0-voltage0_shorted_en     in_voltage1_supply_type

root:/sys/devices/platform/bfin-spi.0/spi0.3/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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