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Table of Contents
AD7476A IIO Single Channel Serial ADC Linux Driver
Supported Devices
Reference Circuits
Evaluation Boards
Description
This is a Linux industrial I/O (IIO) subsystem driver, targeting single 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/ad7476.c |
Example platform device initialization
Specifying reference voltage via the regulator framework
Below example specifies a 3.3 Volt reference for the SPI device 3 on SPI-Bus 0. (spi0.3)
#if defined(CONFIG_REGULATOR_FIXED_VOLTAGE) || defined(CONFIG_REGULATOR_FIXED_VOLTAGE_MODULE) static struct regulator_consumer_supply ad7476_consumer_supplies[] = { REGULATOR_SUPPLY("vcc", "spi0.3"), }; static struct regulator_init_data stamp_avdd_reg_init_data = { .constraints = { .name = "3V3", .valid_ops_mask = REGULATOR_CHANGE_STATUS, }, .consumer_supplies = ad7476_consumer_supplies, .num_consumer_supplies = ARRAY_SIZE(ad7476_consumer_supplies), }; static struct fixed_voltage_config stamp_vdd_pdata = { .supply_name = "board-3V3", .microvolts = 3300000, .gpio = -EINVAL, .enabled_at_boot = 0, .init_data = &stamp_avdd_reg_init_data, }; static struct platform_device brd_voltage_regulator = { .name = "reg-fixed-voltage", .id = -1, .num_resources = 0, .dev = { .platform_data = &stamp_vdd_pdata, }, }; #endif
static struct platform_device *board_devices[] __initdata = { #if defined(CONFIG_REGULATOR_FIXED_VOLTAGE) || defined(CONFIG_REGULATOR_FIXED_VOLTAGE_MODULE) &brd_voltage_regulator #endif };
static int __init board_init(void) { [--snip--] platform_add_devices(board_devices, ARRAY_SIZE(board_devices)); [--snip--] return 0; } arch_initcall(board_init);
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_AD7476) || \ defined(CONFIG_AD7476_MODULE) { /* the modalias must be the same as spi device driver name */ .modalias = "ad7476", /* Name of spi_driver for this device */ .max_speed_hz = 1000000, /* max spi clock (SCK) speed in HZ */ .bus_num = 0, /* Framework bus number */ .chip_select = 3, /* Framework chip select */ .mode = SPI_MODE_3, }, #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);
Devicetree
Required devicetree properties:
- compatible: Needs to be “adi,” followed by the name of the device. E.g. “adi,ad7476a”
- reg: The chipselect number used for the device
- spi-max-frequency: Maximum SPI clock frequency.
- spi-cpha: Needs to be set for the correct SPI mode
- spi-cpol: Needs to be set for the correct SPI mode
- vcc-supply: Phandle to the supply regulator
adc_supply: fixedregulator {
compatible = "regulator-fixed";
regulator-name = "fixed-supply";
regulator-min-microvolt = <3300000>;
regulator-max-microvolt = <3300000>;
};
spi: spi@e0007000 {
#address-cells = <1>;
#size-cells = <0>;
compatible = "xlnx,ps7-spi-1.00.a";
...
ad7476@0 {
compatible = "adi,ad7476a";
reg = <0>;
spi-max-frequency = <1000000>;
spi-cpha;
spi-cpol;
vcc-supply = <&adc_supply>;
};
};
Adding Linux driver support
Configure kernel with “make menuconfig” (alternatively use “make xconfig” or “make qconfig”)
The AD7476 Driver depends on CONFIG_SPI
Linux Kernel Configuration
Device Drivers --->
...
<*> Industrial I/O support --->
--- Industrial I/O support
...
Analog to digital converters --->
...
<*> Analog Devices AD7476 and similar 1-channel ADCs 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.
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 buffer -r--r--r-- 1 root root 4096 Jan 1 00:00 in_voltage0_raw -r--r--r-- 1 root root 4096 Jan 1 00:00 in_voltage_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 drwxr-xr-x 2 root root 0 Jan 1 00:00 trigger -rw-r--r-- 1 root root 4096 Jan 1 00:00 uevent
Show device name
This specifies any shell prompt running on the target
root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat name ad7476
Show scale
Description:
scale to be applied to in_voltage0_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_voltage_scale 0.805
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 in_voltage0_raw 2001
U = in_voltage0_raw * in_voltage_scale = 2001 * 0.805 = 1610,805 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 in_type timestamp_index in0_index timestamp_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
/srv/wiki.analog.com/data/pages/resources/tools-software/linux-drivers/iio-adc/ad7476a.txt · Last modified: 31 Oct 2024 20:25 by David Lechner