AD5755

AD5755 IIO Multi-Channel DAC Linux Driver.

Supported Devices

• AD5735

• AD5737

• AD5755

• AD5755-1

• AD5757

Evaluation Boards

• EVAL-AD5755SDZ

• EVAL-AD5755-1SDZ

• EVAL-AD5757SDZ

Reference Circuits

• CN0198

• CN0328

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/ad5755.c

Adding Linux driver support

Kernel configuration

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

Note

The AD5755 Driver depends on CONFIG_SPI

Linux Kernel Configuration
    Device Drivers  --->
        ...
        <*>     Industrial I/O support --->
            --- Industrial I/O support
            ...
            Digital to analog converters  --->
                ...
                <*>  Analog Devices AD5755/AD5755-1/AD5757/AD5735/AD5737 DAC driver
                ...
            ...
        ...

Platform-data configuration

Example platform device initialization

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 IS_ENABLED(CONFIG_AD5755)
    {
        /* the modalias must be the same as spi device driver name */
        .modalias = "ad5755", /* 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_2,
    },
#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

The mixed current/voltage channel transcript below requires explicit firmware channel configuration. With no firmware configuration, the driver defaults all four channels to 4–20 mA current mode; out_voltage2_* and out_voltage3_* then do not exist. Inspect the registered channel types and configured output ranges before using the examples.

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  2 21:54 dev
-r--r--r--    1 root     root          4096 Jan  2 21:54 name

-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_current0_calibbias
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_current0_calibscale
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_current0_offset
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_current0_raw
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_current0_scale
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_current0_powerdown
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_current1_calibbias
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_current1_calibscale
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_current1_offset
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_current1_raw
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_current1_scale
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_current1_powerdown
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_voltage2_calibbias
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_voltage2_calibscale
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_voltage2_offset
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_voltage2_raw
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_voltage2_scale
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_voltage2_powerdown
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_voltage3_calibbias
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_voltage3_calibscale
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_voltage3_offset
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_voltage3_raw
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_voltage3_scale
-rw-r--r--    1 root     root          4096 Jan  2 21:54 out_voltage3_powerdown
drwxr-xr-x    2 root     root             0 Jan  2 21:54 power
lrwxrwxrwx    1 root     root             0 Jan  2 21:54 subsystem -> ../../../../../bus/iio
-rw-r--r--    1 root     root          4096 Jan  2 21:54 uevent

Identify the device

root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> cat name
ad5755

Basic operation

Show scale

Description: /sys/bus/iio/devices/iio:deviceX/out_voltageY_scale /sys/bus/iio/devices/iio:deviceX/out_currentY_scale

Apply the relevant offset before scale. Voltage channels produce millivolts; current channels produce milliamps. The offset attributes are out_voltageY_offset or out_currentY_offset, not out_offsetY_raw.

Read the scale for a channel that actually exists; replace X with the IIO device index and select the registered voltage or current channel:

DEV=/sys/bus/iio/devices/iio:deviceX
cat "$DEV/out_current0_scale"
# Only if channel 2 is configured for voltage output:
cat "$DEV/out_voltage2_scale"

Calibration controls

The per-channel out_voltageY_calibscale or out_currentY_calibscale attribute accesses the hardware gain register. For AD5755/AD5757, the driver accepts integer gain codes from 0 to 65535.

The corresponding out_voltageY_calibbias or out_currentY_calibbias attribute accesses the hardware offset-calibration register. For AD5755/AD5757, the driver adds 32768 before writing the register, accepting signed codes from -32768 to 32767. This calibration control is distinct from the nominal IIO *_offset conversion attribute.

The AD5735/AD5737 use an additional four-bit shift for calibration-register access, so their calibration codes differ from the AD5755/AD5757 codes.

Show offset

The attributes are out_voltageY_offset and out_currentY_offset. They express the nominal conversion offset in raw-code units, applied before the corresponding voltage or current scale:

cat "$DEV/out_current0_offset"
# Only if channel 2 is configured for voltage output:
cat "$DEV/out_voltage2_offset"

Set channel Y output code

Write an unscaled DAC code to the registered out_voltageY_raw or out_currentY_raw attribute.

Without calibration correction, the output is voltage_mV = (raw + offset) × scale for a voltage channel, or current_mA = (raw + offset) × scale for a current channel.

Enable power down mode for the device

/sys/bus/iio/devices/iio:deviceX/out_voltageY_powerdown /sys/bus/iio/devices/iio:deviceX/out_currentY_powerdown

Description: Writing 1 causes channel Y to enter power down mode. Clearing returns to normal operation.

root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> echo 1 > out_voltage2_powerdown
root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> cat out_voltage2_powerdown
1
root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> echo 0 > out_voltage2_powerdown
root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> cat out_voltage2_powerdown
0

More Information

• IIO mailing list: linux-iio@vger.kernel.org

• IIO Linux Kernel Documentation sysfs-bus-iio-*

• IIO Documentation

• IIO Oscilloscope

• Libiio

• About libiio

• IIO High Speed

• Software Defined Radio using the IIO framework

• libiio introduction