AD7298
AD7298 IIO Multi-Channel ADC Linux Driver.
Supported Devices
Evaluation Boards
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 |
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,
};
Tip
In case platform_data is not present or set to NULL, the driver will use the AD7298 internal reference.
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 = {
#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)
Note
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.
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
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.
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.
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
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.
Hint
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
/sys/bus/iio/devices/triggerY$
cat name
irqtrig56
Description: Make irqtrig56 (trigger using system IRQ56, likely a GPIO IRQ), to current trigger of deviceX
/sys/bus/iio/devices/iio:deviceX/trigger$
echo irqtrig56 > current_trigger
Description: Read current trigger source of deviceX
/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
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_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/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.