AD7266

AD7266 Differential/Single-Ended Input, Dual, Simultaneous Sampling, 2 MSPS, 12-Bit, 3-Channel SAR A/D Converter 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

Source

Mainlined?

git

Yes

Files

Function

File

driver

drivers/iio/adc/ad7266.c

include

include/linux/platform_data/ad7266.h

Adding Linux driver support

Kernel configuration

Configure kernel with make menuconfig (alternatively use make xconfig or make qconfig)

Note

The AD7266 Driver depends on CONFIG_SPI

Linux Kernel Configuration
    Device Drivers  --->
        ...
        <*>     Industrial I/O support --->
            --- Industrial I/O support
            ...
            Analog to digital converters  --->
                ...
                <*>  Analog Devices AD7265/AD7266 ADC driver
                ...
            ...
        ...

Platform-data configuration

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 AD7265/AD7266 supports different operating modes, which can be configured via external pins. To properly interpret the data from the chip the driver has to know which mode it is configured for. This is done using platform data. If the AD (address) pins of the chip are hook up to GPIOs to be able to change the sampled inputs at runtime the GPIOs can be specified via the platform data as well. If GPIOs are used the `fixed_addr` field must be set to false.

/*
 * ad7266_range - AD7266 reference voltage range
 * @AD7266_RANGE_VREF: Device is configured for input range 0V - VREF
 *          (RANGE pin set to low)
 * @AD7266_RANGE_2VREF: Device is configured for input range 0V - 2VREF
 *          (RANGE pin set to high)
 */
enum ad7266_range {
    AD7266_RANGE_VREF,
    AD7266_RANGE_2VREF,
};

/*
 * ad7266_mode - AD7266 sample mode
 * @AD7266_MODE_SINGLE_ENDED: Device is configured for single-ended mode
 *              (SGL/DIFF pin set to high)
 * @AD7266_MODE_PSEUDO_DIFF: Device is configured for pseudo differential mode
 *              (SGL/DIFF pin set to low, AD0 pin set to high)
 * @AD7266_MODE_PSEUDO_DIFF: Device is configured for full differential mode
 *              (SGL/DIFF pin set to low, AD0 pin set to low)
 */
enum ad7266_mode {
    AD7266_MODE_SINGLE_ENDED,
    AD7266_MODE_PSEUDO_DIFF,
    AD7266_MODE_DIFF,
};

/*
 * ad7266_platform_data - Platform data for the AD7266 driver
 * @range: Reference voltage range the device is configured for
 * @mode: Sample mode the device is configured for
 * @fixed_addr: Whether the AD pins are hard-wired
 * @addr_gpios: GPIOs used for controlling the AD pins, only valid if fixed_addr
 *      is set to false.
 */
struct ad7266_platform_data {
    enum ad7266_range range;
    enum ad7266_mode mode;
    bool fixed_addr;
    /* GPIO descriptors named ad0, ad1 and ad2 are supplied through GPIO lookup mappings. */
};

static struct ad7266_platform_data pdata = {
    .range = AD7266_RANGE_2VREF,
    .mode = AD7266_MODE_SINGLE_ENDED,
    .fixed_addr = false,
    .addr_gpios = { GPIO_PF6, GPIO_PF7, GPIO_PF8 },
};

If no platform data is provided the driver assumes differential 0V-VREF operation with a fixed address. This corresponds to the following platform data.

static struct ad7266_platform_data ad7266_pdata_default = {
    .range = AD7266_RANGE_VREF,
    .mode = AD7266_MODE_DIFF,
    .fixed_addr = true,
};

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 = "ad7266",
        .max_speed_hz = 1000000,     /* max spi clock (SCK) speed in HZ */
        .bus_num = 0,
        .chip_select = GPIO_PF10 + MAX_CTRL_CS, /* CS, change it for your board */
        .platform_data = &ad7266_pdata, /* No spi_driver specific config */
        .mode = SPI_MODE_2,
    },
};

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 iio:device0

root:/sys/devices/platform/bfin-spi.0/spi0.12/iio:device0> ls -l
drwxr-xr-x    2 root     root             0 Jan  1 04:03 buffer
-r--r--r--    1 root     root          4096 Jan  1 04:03 dev
-rw-r--r--    1 root     root          4096 Jan  1 04:03 in_voltage-voltage_scale
-r--r--r--    1 root     root          4096 Jan  1 04:03 in_voltage0-voltage1_raw
-r--r--r--    1 root     root          4096 Jan  1 04:03 in_voltage2-voltage3_raw
-r--r--r--    1 root     root          4096 Jan  1 04:03 in_voltage4-voltage5_raw
-r--r--r--    1 root     root          4096 Jan  1 04:03 in_voltage6-voltage7_raw
-r--r--r--    1 root     root          4096 Jan  1 04:03 in_voltage8-voltage9_raw
-r--r--r--    1 root     root          4096 Jan  1 04:03 in_voltage10-voltage11_raw
-r--r--r--    1 root     root          4096 Jan  1 04:03 name
drwxr-xr-x    2 root     root             0 Jan  1 04:03 power
drwxr-xr-x    2 root     root             0 Jan  1 04:03 scan_elements
lrwxrwxrwx    1 root     root             0 Jan  1 04:03 subsystem -> ../../../../../bus/iio
drwxr-xr-x    2 root     root             0 Jan  1 04:03 trigger
-rw-r--r--    1 root     root          4096 Jan  1 04:03 uevent

root:/sys/devices/platform/bfin-spi.0/spi0.12/iio:device0>

Identify the device

root:/sys/devices/platform/bfin-spi.0/spi0.12/iio:device0> cat name
ad7266

Basic operation

Show scale for differential input channels

Description: in_voltage-voltage_scale

Scale to be applied to in_voltageY-voltageZ_raw in order to obtain the measured voltage in millivolts.

root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat in_voltage-voltage_scale
0.610350
root:/sys/devices/platform/bfin-spi.0/spi0.3/device0>

Show differential channel measurement

Description: in_voltageY-voltageZ_raw

Raw unscaled voltage measurement on channel pair X-Z in differential mode

ADC Input Pair

Channel name

VA1 - VA2

in_voltage0-voltage1_raw

VB1 - VB2

in_voltage2-voltage3_raw

VA2 - VA3

in_voltage4-voltage5_raw

VB2 - VB3

in_voltage6-voltage7_raw

VA4 - VA5

in_voltage8-voltage9_raw

VB4 - VB5

in_voltage10-voltage11_raw

 
root:/sys/devices/platform/bfin-spi.0/spi0.12/iio:device0> cat in_voltage0-voltage1_raw
100

U = in_voltage0-voltage1_raw * in_voltage-voltage_scale = 100 * 0.610350 = 61.035 mV

Show scale for single-ended input channels

Description: in_voltage_scale

Scale to be applied to in_voltageY in order to obtain the measured voltage in millivolts.

root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> cat in_voltage_scale
0.610350
root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0>

Show single-ended channel measurement

Description: in_voltageY_raw

Raw unscaled voltage measurement on channel in single ended mode

root:/sys/devices/platform/bfin-spi.0/spi0.12/iio:device0> cat in_voltage0_raw
100

The voltage is U_mV = (in_voltage0_raw + in_voltage_offset) × in_voltage_scale. In the single-ended or pseudo-differential 2×VREF range the offset is 2048 codes, otherwise zero. The scale depends on the selected mode and range.

Trigger management

root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> cat trigger/current_trigger
bfintmr3

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_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.

root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0/scan_elements> ls
in_voltage0-voltage1_en     in_voltage2-voltage3_en
in_voltage0-voltage1_index  in_voltage2-voltage3_index
in_voltage0-voltage1_type   in_voltage2-voltage3_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; s or u specifies signed (two’s complement) or unsigned data. bits is the number of meaningful bits and storagebits is the storage width. The optional repeat count describes repeated elements. After interpreting byte order, shift the storage value right by shift and mask it to bits; signed values also require sign extension. For example, be:u12/16>>1 describes 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.

More Information