AD7780

AD7780 IIO Low Power Sigma-Delta ADC Linux Driver.

Supported Devices

Evaluation Boards

Reference Circuits

Description

This is a Linux industrial I/O (Linux Industrial I/O Subsystem) 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 Linux Industrial I/O Subsystem for more information.

Source Code

Status

Source

Mainlined?

git

Yes

Files

Function

File

driver

drivers/iio/adc/ad7780.c

Adding Linux driver support

Kernel configuration

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

Note

The AD7780 driver depends on CONFIG_SPI and CONFIG_GPIOLIB (or CONFIG_COMPILE_TEST) and selects CONFIG_AD_SIGMA_DELTA.

Linux Kernel Configuration
    Device Drivers  --->
        <*> Industrial I/O support --->
            Analog to digital converters --->
                <*> Analog Devices AD7780 and similar ADCs 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 driver obtains reference/supply voltage from the avdd regulator and GPIOs by descriptor lookup (powerdown, adi,gain and adi,filter). It does not use an ad7780_platform_data structure. For a board-file configuration, register GPIO descriptor lookup mappings for the SPI device as required by the connected hardware. The examples below show regulator and SPI registration only; they do not specify board-specific GPIO mappings.

Note

The AD7780 doesn’t feature a dedicated SPI chip select, in addition it features a dual use data out ready DOUT/RDY output. In order to avoid contentions on the SPI bus, it’s therefore necessary to use spi bus locking combined with a dedicated GPIO to control the power down reset signal PDRST of the AD7780.

The AD7780 DOUT/RDY output typically connected to SPI MISO must also be wired to an interrupt capable GPIO.

Specifying reference voltage via the regulator framework

The driver requires an avdd regulator. Register the board’s actual supply with that consumer name.

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 ad7780_consumer_supplies[] = {
    REGULATOR_SUPPLY("avdd", "spi0.3"),
};

static struct regulator_init_data stamp_avdd_reg_init_data = {
    .constraints    = {
        .name   = "3V3",
        .valid_ops_mask = REGULATOR_CHANGE_STATUS,
    },
    .consumer_supplies = ad7780_consumer_supplies,
    .num_consumer_supplies = ARRAY_SIZE(ad7780_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);

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_AD7780) || \
    defined(CONFIG_AD7780_MODULE)
    {
        /* the modalias must be the same as spi device driver name */
        .modalias = "ad7780", /* 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 */
        .irq = IRQ_PF6,
        .mode = SPI_MODE_0,
    },
#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);

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

root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> ls -l
-r--r--r--    1 root     root          4096 Jan  3 12:41 in_voltage0_raw
-r--r--r--    1 root     root          4096 Jan  3 12:41 in_scale
-r--r--r--    1 root     root          4096 Jan  3 12:41 name
lrwxrwxrwx    1 root     root             0 Jan  3 12:41 subsystem -> ../../../../../bus/iio
-rw-r--r--    1 root     root          4096 Jan  3 12:41 uevent

Identify the device

root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat name
ad7780

Basic operation

Show scale

Description: scale to be applied to in_voltage0_raw in order to obtain the measured voltage in millivolts.

root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat in_scale
0.00029

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
-1

The raw value is unsigned offset binary, so U_mV = (in_voltage0_raw + in_voltage0_offset) × in_voltage_scale, with an offset of -8388608.

More Information