AD5421

AD5421 IIO DAC Linux Driver.

Supported Devices

AD5421

Reference Circuits

Evaluation Boards

EVAL-AD5421SDZ

Description

This is a Linux industrial I/O (Linux Industrial I/O Subsystem) subsystem driver, targeting multi-channel serial interface DACs. 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/dac/ad5421.c

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 ad5421 platform data is used to describe how the chip is configured on a specific board.

/**
 * enum ad5421_current_range - Current range the AD5421 is configured for.
 * @AD5421_CURRENT_RANGE_4mA_20mA: 4 mA to 20 mA (RANGE1,0 pins = 00)
 * @AD5421_CURRENT_RANGE_3mA8_21mA: 3.8 mA to 21 mA (RANGE1,0 pins = x1)
 * @AD5421_CURRENT_RANGE_3mA2_24mA: 3.2 mA to 24 mA (RANGE1,0 pins = 10)
 */

enum ad5421_current_range {
    AD5421_CURRENT_RANGE_4mA_20mA,
    AD5421_CURRENT_RANGE_3mA8_21mA,
    AD5421_CURRENT_RANGE_3mA2_24mA,
};

/**
 * struct ad5421_platform_data - AD5421 DAC driver platform data
 * @external_vref: whether an external reference voltage is used or not
 * @current_range: Current range the AD5421 is configured for
 */

struct ad5421_platform_data {
    bool external_vref;
    enum ad5421_current_range current_range;
};

If no platform data is given, the driver will assume an internal reference voltage and a current range of 4 mA to 20 mA.

struct ad5421_platform_data ad5421_pdata {
    .external_vref = true,
    .current_range = AD5421_CURRENT_RANGE_3mA2_24mA,
};

static int __init board_init(void)
{
    [--snip--]

    platform_add_devices(board_devices, ARRAY_SIZE(board_devices));

    [--snip--]

    return 0;
}
arch_initcall(board_init);

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

static struct spi_board_info board_spi_board_info[] __initdata = {
#if defined(CONFIG_AD5421_SPI) || \
    defined(CONFIG_AD5421_SPI_MODULE)
    {
        /* the modalias must be the same as spi device driver name */
        .modalias = "ad5421", /* Name of spi_driver for this device */
        .max_speed_hz = 30000000,     /* max spi clock (SCK) speed in HZ */
        .bus_num = 0, /* Framework bus number */
        .chip_select = 3, /* Framework chip select */
        .mode = SPI_MODE_2,
        .irq = IRQ_PF4,
        .platform_data = &ad5421_pdata,
    },
#endif
};

Adding Linux driver support

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

Note

The ad5421 Driver depends on CONFIG_SPI

Linux Kernel Configuration
    Device Drivers  --->
        ...
        <*>     Industrial I/O support --->
            --- Industrial I/O support
            ...
            Digital to analog converters  --->
                ...
                <*>  Analog Devices AD5421 DAC 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.

root:/> cd /sys/bus/iio/devices/
root:/sys/bus/iio/devices> ls
iio:device0

root:/sys/bus/iio/devices> cd iio:device0

root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> ls -l
-r--r--r--    1 root     root          4096 Jan  1 00:03 dev
-r--r--r--    1 root     root          4096 Jan  1 00:03 name
-rw-r--r--    1 root     root          4096 Jan  1 00:03 out_current0_calibbias
-rw-r--r--    1 root     root          4096 Jan  1 00:03 out_current0_calibscale
-rw-r--r--    1 root     root          4096 Jan  1 00:03 out_current0_raw
-rw-r--r--    1 root     root          4096 Jan  1 00:03 out_current_offset
-rw-r--r--    1 root     root          4096 Jan  1 00:03 out_current_scale
drwxr-xr-x    2 root     root             0 Jan  1 00:03 power
lrwxrwxrwx    1 root     root             0 Jan  1 00:03 subsystem -> ../../../../../bus/iio
-rw-r--r--    1 root     root          4096 Jan  1 00:03 uevent

Show device name

root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> cat name
ad5421

Show scale

Description:

/sys/bus/iio/devices/iio:deviceX/out_current_scale

scale to be applied to out_current0_raw in order to obtain the outputted current in milliampere.

root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> cat out_current_scale
0.000244

Show offset

Description:

/sys/bus/iio/devices/iio:deviceX/out_current_offset

offset to be applied to out_current0_raw in order to obtain the outputted current in milliampere.

root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> cat out_current_offset
16384

Set channel Y output current

Description: /sys/bus/iio/devices/iio:deviceX/out_currentY_raw

Raw (unscaled, no bias etc.) output current for channel Y.

root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> echo 10000 > out_current0_raw

I = ( out_current0_raw + out_current_offset ) * out_current_scale = (10000 + 16384) * 0.000244 mA = 6.437696 mA

Calibrate channel Y gain

Description: /sys/bus/iio/devices/iio:deviceX/out_currentY_calibscale

Each channel has an adjustable gain which can be used to calibrate the channel’s scale and compensate for full-scale errors. The default value is 65535.

If scale calibration is used the following formula can be used calculate the output current:

I = ( ( out_currentY_raw * ( out_currentY_calibscale + 1 ) ) / 2^16 + out_currentY_calibbias + out_current_offset ) * out_currentY_scale

Calibrate channel Y offset

Description: /sys/bus/iio/devices/iio:deviceX/out_currentY_calibbias

Each channel has an adjustable offset which can be used to calibrate the channel’s offset and compensate for zero-scale errors. The default value is 0.

If offset calibration is used the following formula can be used calculate the output current: