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AD7298 IIO多通道ADC Linux漂流器
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Table of Contents
AD7298 IIO Multi-Channel ADC Linux Driver
Supported Devices
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/iio/adc/ad7298.c |
| include | include/linux/platform_data/ad7298.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.
/** * struct ad7298_platform_data - Platform data for the ad7298 ADC driver * @ext_ref: Whether to use an external reference voltage. **/ struct ad7298_platform_data { bool ext_ref; };
static struct ad7298_platform_data ad7298_pdata = { .ext_ref = true, };
In case platform_data is not present or set to NULL, the driver will use the AD7298 internal reference.
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
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_AD7298) || / defined(CONFIG_AD7298_MODULE) { .modalias = "ad7298", .max_speed_hz = 1000000, /* max spi clock (SCK) speed in HZ */ .bus_num = 0, .chip_select = 3, /* CS, change it for your board */ .controller_data = &spi_ad7298_chip_info, .mode = SPI_MODE_1, }, #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 AD7298 Driver depends on CONFIG_SPI
Linux Kernel Configuration Device Drivers ---> <*> Industrial I/O support ---> --- Industrial I/O support *** Analog to digital converters *** [--snip--] <*> Analog Devices AD7298 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.
This specifies any shell prompt running on the target
root:/> cd /sys/bus/iio/devices/ root:/sys/bus/iio/devices> ls device0 device0:buffer0:access0 trigger0 device0:buffer0 device0:buffer0:event0 root:/sys/bus/iio/devices> cd device0 root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> ls -l drwxr-xr-x 5 root root 0 Jan 1 00:00 device0:buffer0 -r--r--r-- 1 root root 4096 Jan 1 00:00 in0_raw -r--r--r-- 1 root root 4096 Jan 1 00:00 in1_raw -r--r--r-- 1 root root 4096 Jan 1 00:00 in2_raw -r--r--r-- 1 root root 4096 Jan 1 00:00 in3_raw -r--r--r-- 1 root root 4096 Jan 1 00:00 in4_raw -r--r--r-- 1 root root 4096 Jan 1 00:00 in5_raw -r--r--r-- 1 root root 4096 Jan 1 00:00 in6_raw -r--r--r-- 1 root root 4096 Jan 1 00:00 in7_raw -r--r--r-- 1 root root 4096 Jan 1 00:00 in_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 -r--r--r-- 1 root root 4096 Jan 1 00:00 temp0_input drwxr-xr-x 2 root root 0 Jan 1 00:00 trigger -rw-r--r-- 1 root root 4096 Jan 1 00:00 uevent root:/sys/devices/platform/bfin-spi.0/spi0.3/device0>
Show device name
This specifies any shell prompt running on the target
root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat name ad7298
Show internal temperature
Description:
Shows high accuracy band gap temperature sensor temperature in milli degrees Celsius.
This specifies any shell prompt running on the target
root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat temp0_input 29750
T = 29.75 °C
Show scale
Description:
scale to be applied to in0_raw in order to obtain the measured voltage in millivolts.
This specifies any shell prompt running on the target
root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat in_scale 0.610
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/bfin-spi.0/spi0.3/device0> cat in0_raw 2158
U = in0_raw * in_scale = 2158 * 0.610 = 1316,38 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
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. |
Buffer management
This specifies any shell prompt running on the target
root:/sys/devices/platform/bfin-spi.0/spi0.3/device0/device0:buffer0> ls bytes_per_datum enable subsystem device0:buffer0:access0 length uevent device0:buffer0:event0 scan_elements root:/sys/devices/platform/bfin-spi.0/spi0.3/device0/device0:buffer0>
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.
This specifies any shell prompt running on the target
root:/sys/devices/platform/bfin-spi.0/spi0.3/device0/device0:buffer0/scan_elements> ls in0_en in2_index in5_en in7_index in0_index in3_en in5_index in_type in1_en in3_index in6_en timestamp_en in1_index in4_en in6_index timestamp_index in2_en in4_index in7_en timestamp_type root:/sys/devices/platform/bfin-spi.0/spi0.3/device0/device0:buffer0/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.
More Information
- IIO mailing list: linux [dash] iio [at] vger [dot] kernel [dot] org
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