This version is outdated by a newer approved version.
This version (11 Oct 2012 11:02) was approved by Lars-Peter Clausen.The Previously approved version (05 Sep 2012 16:06) is available.
This version (11 Oct 2012 11:02) was approved by Lars-Peter Clausen.The Previously approved version (05 Sep 2012 16:06) is available.This is an old revision of the document!
Table of Contents
AD7793 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
Files
| Function | File |
|---|---|
| driver | drivers/staging/iio/adc/ad7793.c |
| include | drivers/staging/iio/adc/ad7793.h |
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.
The reference voltage may vary between boards and models. The platform_data for the device's “struct device” holds this information.
<source trunk/drivers/staging/iio/adc/ad7793.h:ad7793_platform_data{} c linux-kernel>
static struct ad7793_platform_data ad7793_pdata = { .vref_mv = 2500, .mode = AD7793_MODE_CLKSRC(AD7793_CLK_INT) | AD7793_MODE_RATE(12), .conf = AD7793_CONF_REFSEL | AD7793_CONF_BUF | AD7793_CONF_GAIN(0) | AD7793_CONF_UNIPOLAR, };
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.rst
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 = { #if defined(CONFIG_AD7793) \ || defined(CONFIG_AD7793_MODULE) { .modalias = "ad7793", .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 = &ad7793_pdata, /* No spi_driver specific config */ .mode = SPI_MODE_3, .irq = IRQ_PF6, }, #endif };
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 AD7793 Driver depends on CONFIG_SPI
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 AD7792 AD7793 ADC driver [--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.
<box 100% green|shell prompt running on the target>
root:/> cd /sys/bus/iio/devices/ root:/sys/bus/iio/devices> ls device0 device0:buffer0 trigger0 root:/sys/bus/iio/devices> cd device0 root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> ls -l drwxr-xr-x 4 root root 0 Jan 4 09:29 device0:buffer0 -rw-r--r-- 1 root root 4096 Jan 4 09:30 in-in_scale -r--r--r-- 1 root root 4096 Jan 4 09:30 in-in_scale_available -r--r--r-- 1 root root 4096 Jan 4 09:30 in0-in0_raw -r--r--r-- 1 root root 4096 Jan 4 09:30 in0-in0_shorted_raw -r--r--r-- 1 root root 4096 Jan 4 09:30 in1-in1_raw -r--r--r-- 1 root root 4096 Jan 4 09:30 in2-in2_raw -r--r--r-- 1 root root 4096 Jan 4 09:30 in4_supply_raw -rw-r--r-- 1 root root 4096 Jan 4 09:30 in4_supply_scale -r--r--r-- 1 root root 4096 Jan 4 09:30 name drwxr-xr-x 2 root root 0 Jan 4 09:30 power -rw-r--r-- 1 root root 4096 Jan 4 09:30 sampling_frequency -r--r--r-- 1 root root 4096 Jan 4 09:30 sampling_frequency_available lrwxrwxrwx 1 root root 0 Jan 4 09:30 subsystem -> ../../../../../bus/iio -r--r--r-- 1 root root 4096 Jan 4 09:30 temp0_raw -rw-r--r-- 1 root root 4096 Jan 4 09:30 temp0_scale drwxr-xr-x 2 root root 0 Jan 4 09:30 trigger -rw-r--r-- 1 root root 4096 Jan 4 09:30 uevent root:/sys/devices/platform/bfin-spi.0/spi0.3/device0></box>
Show device name
<box 100% green|shell prompt running on the target>
root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat name ad7793</box>
Show available sampling frequencies / update rates
<box 100% green|shell prompt running on the target>
root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat sampling_frequency_available 470 242 123 62 50 39 33 19 17 16 12 10 8 6 4</box>
Set sampling frequency / update rate
<box 100% green|shell prompt running on the target>
root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat sampling_frequency 10 root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> echo 50 > sampling_frequency root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat sampling_frequency 50 root:/sys/devices/platform/bfin-spi.0/spi0.3/device0></box>
Show supply voltage
Description:
Shows the AVDD supply voltage
<box 100% green|shell prompt running on the target>
root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat in4_supply_raw 7817399 root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat in4_supply_scale 0.000418420</box>
U = in4_supply_raw * in4_supply_scale = 7817399 * 0.000418420 = 3270.95608958 mV
Show available scales for differential input channels
Lists all available scales for the differential input pairs:
| ADC Input Pair | Channel name |
|---|---|
| AIN1(+) - AIN1(-) | in0-in0_raw |
| AIN2(+) - AIN2(-) | in1-in1_raw |
| AIN3(+) - AIN3(-) | in1-in1_raw |
| AIN1(-) - AIN1(-) | in0-in0_shorted_raw |
Setting these directly influences the ADC input range, by altering the GAIN amplifier.
<box 100% green|shell prompt running on the target>
root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat in-in_scale_available 0.000149010 0.000074500 0.000037250 0.000018620 0.000009310 0.000004650 0.000002320 0.000001160</box>
Set scale for differential input channels
Scale to be applied to in0-in0_raw, in1-in1_raw, in2-in2_raw in order to obtain the measured voltage in millivolts. Allows the user to select one scale out of the available scales. If the written scale differs from the current scale. The driver performs full and zero offset calibration on all differential input channels.
<box 100% green|shell prompt running on the target>
root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat in-in_scale 0.000149010 root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> echo 0.000074500 > in-in_scale root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat in-in_scale 0.000074500 root:/sys/devices/platform/bfin-spi.0/spi0.3/device0></box>
Show channel in0-in0 measurement
Description:
Raw unscaled voltage measurement on channel 0
| ADC Input Pair | Channel name |
|---|---|
| AIN1(+) - AIN1(-) | in0-in0_raw |
| AIN2(+) - AIN2(-) | in1-in1_raw |
| AIN3(+) - AIN3(-) | in1-in1_raw |
| AIN1(-) - AIN1(-) | in0-in0_shorted_raw |
<box 100% green|shell prompt running on the target>
root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat in0-in0_raw 6710665</box>
U = in0-in0_raw * in-in_scale = 6710665 * 0.000149010 = 999.95619165 mV
Trigger management
This driver only supports it's own default trigger source ad7793-dev0
<box 100% green|shell prompt running on the target>
root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat trigger/current_trigger ad7793-dev0</box>
Buffer management
<box 100% green|shell prompt running on the target>
root:/sys/devices/platform/bfin-spi.0/spi0.3/device0/device0:buffer0> ls bytes_per_datum enable power subsystem dev length scan_elements uevent root:/sys/devices/platform/bfin-spi.0/spi0.3/device0/device0:buffer0></box>
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.
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.
<box 100% green|shell prompt running on the target>
root:/sys/devices/platform/bfin-spi.0/spi0.3/device0/device0:buffer0/scan_elements> ls in0-in0_en in1-in1_index in4_supply_type in0-in0_index in1-in1_type temp0_en in0-in0_shorted_en in2-in2_en temp0_index in0-in0_shorted_index in2-in2_index temp0_type in0-in0_shorted_type in2-in2_type timestamp_en in0-in0_type in4_supply_en timestamp_index in1-in1_en in4_supply_index timestamp_type root:/sys/devices/platform/bfin-spi.0/spi0.3/device0/device0:buffer0/scan_elements></box>
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.
More Information
- IIO mailing list: linux [dash] iio [at] vger [dot] kernel [dot] org
resources/tools-software/linux-drivers/iio-adc/ad7793.1349946111.txt.gz · Last modified: 11 Oct 2012 11:01 by Lars-Peter Clausen