AD7791
AD7791 IIO Low Power Sigma-Delta 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
Files
Function |
File |
|---|---|
driver |
|
include |
Hardware configuration
Adding Linux driver support
Kernel configuration
Configure kernel with make menuconfig (alternatively use make xconfig or
make qconfig)
Note
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
...
...
...
Platform-data configuration
Example platform device initialization
Example platform device initialization Provide a refin regulator consumer and a data-ready IRQ; probe requires both. Platform-data buffered enables the analog input buffer, independently of IIO buffered capture. For DT, the equivalent properties are adi,buffered-mode-enable, adi,burnout-current-enable and adi,unipolar-mode-enable; omitted booleans are false, unlike the non-DT fallback buffered=true.
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.
/**
* struct ad7791_platform_data - AD7791 device platform data
* @buffered: If set to true configure the device for buffered input mode.
* @burnout_current: If true, enable the 100 nA burnout current sources.
* Burnout detection requires buffered input mode.
* @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,
};
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 = {
{
.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);
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/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>
Identify the device
root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> cat name
ad7791
Basic operation
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 |
root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> cat in_voltage0-voltage0_raw
6710665
In this bipolar setup, U_mV = (6710665 - 8388608) × 0.000596046 ≈ -1000.13 mV.
Show available sampling frequencies / update rates
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
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
AD7791 registers the supply input as channel 1. Read its raw value and offset,
and the shared supply scale, from the device. Calculate the voltage in
millivolts as (raw + offset) * scale; do not omit the offset in bipolar mode.
The following commands run from the device’s sysfs directory and calculate the voltage:
raw=$(cat in_voltage1_supply_raw)
offset=$(cat in_voltage1_supply_offset)
scale=$(cat in_voltage_supply_scale)
awk -v raw="$raw" -v offset="$offset" -v scale="$scale" \
'BEGIN { printf "%.6f mV\n", (raw + offset) * scale }'
Trigger management
Note
This driver only supports it’s own default trigger source ad7791-dev0
root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat trigger/current_trigger
ad7791-dev0
Buffer management
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_bytesproperty.- 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/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 / 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;soruspecifies signed (two’s complement) or unsigned data.bitsis the number of meaningful bits andstoragebitsis the storage width. The optional repeat count describes repeated elements. After interpreting byte order, shift the storage value right byshiftand mask it tobits; signed values also require sign extension. For example,be:u12/16>>1describes 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.