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This version (06 Feb 2014 11:53) was approved by larsc.The Previously approved version (18 Feb 2013 14:56) is available.Diff

AD5791 IIO DAC Linux Driver

Supported Devices

Reference Circuits

Evaluation Boards

Description

This is a Linux industrial I/O (IIO) subsystem driver, targeting single-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 IIO for more information.

Source Code

Status

Source Mainlined?
git Yes

Files

Function File
driver drivers/iio/dac/ad5791.c
include drivers/iio/dac/ad5791.h

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.

21 Oct 2010 16:10 · Michael Hennerich

The reference voltage may vary between boards and models. The platform_data for the device's “struct device” holds this information.

/**
 * struct ad5791_platform_data - platform specific information
 * @vref_pos_mv:	Vdd Positive Analog Supply Volatge (mV)
 * @vref_neg_mv:	Vdd Negative Analog Supply Volatge (mV)
 * @use_rbuf_gain2:	ext. amplifier connected in gain of two configuration
 */
 
struct ad5791_platform_data {
	u16				vref_pos_mv;
	u16				vref_neg_mv;
	bool				use_rbuf_gain2;
};

This driver supports also an alternative way of specifying the reference voltage, by using the Linux regulator framework.

Specifying reference voltage via the regulator framework

Below example specifies a 30 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 ad5791_consumer_supplies_vdd[] = {
	REGULATOR_SUPPLY("vdd", "spi0.3"),
};
 
static struct regulator_init_data stamp_avdd_reg_init_data = {
	.constraints	= {
		.name	= "10V0",
		.valid_ops_mask = REGULATOR_CHANGE_STATUS,
	},
	.consumer_supplies = ad5791_consumer_supplies_vdd,
	.num_consumer_supplies = ARRAY_SIZE(ad5791_consumer_supplies_vdd),
};
 
static struct fixed_voltage_config stamp_vdd_pdata = {
	.supply_name	= "board+10V0",
	.microvolts	= 10000000,
	.gpio		= -EINVAL,
	.enabled_at_boot = 0,
	.init_data	= &stamp_avdd_reg_init_data,
};
static struct platform_device brd_voltage_regulator_vdd = {
	.name		= "reg-fixed-voltage",
	.id		= 0,
	.num_resources	= 0,
	.dev		= {
		.platform_data	= &stamp_vdd_pdata,
	},
};
 
static struct regulator_consumer_supply ad5791_consumer_supplies_vss[] = {
	REGULATOR_SUPPLY("vss", "spi0.3"),
};
 
static struct regulator_init_data stamp_avss_reg_init_data = {
	.constraints	= {
		.name	= "-10V0",
		.valid_ops_mask = REGULATOR_CHANGE_STATUS,
	},
	.consumer_supplies = ad5791_consumer_supplies_vss,
	.num_consumer_supplies = ARRAY_SIZE(ad5791_consumer_supplies_vss),
};
 
static struct fixed_voltage_config stamp_vss_pdata = {
	.supply_name	= "board-10V0",
	.microvolts	= 10000000,
	.gpio		= -EINVAL,
	.enabled_at_boot = 0,
	.init_data	= &stamp_avss_reg_init_data,
};
static struct platform_device brd_voltage_regulator_vss = {
	.name		= "reg-fixed-voltage",
	.id		= 1,
	.num_resources	= 0,
	.dev		= {
		.platform_data	= &stamp_vss_pdata,
	},
};
#endif
static struct platform_device *board_devices[] __initdata = {
#if defined(CONFIG_REGULATOR_FIXED_VOLTAGE) || defined(CONFIG_REGULATOR_FIXED_VOLTAGE_MODULE)
	&brd_voltage_regulator_vss,
	&brd_voltage_regulator_vdd,
#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

21 Oct 2010 16:10 · Michael Hennerich

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_AD5791) || \
 	defined(CONFIG_AD5791
_MODULE)
	{
		/* the modalias must be the same as spi device driver name */
		.modalias = "ad5791", /* 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 */
		.platform_data = &ad5791_pdata,
		.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);

Adding Linux driver support

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

The AD5791 Driver depends on CONFIG_SPI

Linux Kernel Configuration
    Device Drivers  --->
        ...
        <*>     Industrial I/O support --->
            --- Industrial I/O support
            ...
            Digital to analog converters  ---> 
                ...
                <*>   Analog Devices AD5760/AD5780/AD5781/AD5790/AD5791 DAC SPI driver
                ...
            ...
        ...

Hardware configuration

ad5791_eval_lr.jpg

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.

02 Mar 2011 15:16 · Michael Hennerich

This specifies any shell prompt running on the target

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/iio:device0> ls -l
drwxr-xr-x    2 root     root             0 Jan  4 12:28 .
drwxr-xr-x    3 root     root             0 Jan  4 12:28 ..
-r--r--r--    1 root     root          4096 Jan  4 12:28 name
-rw-r--r--    1 root     root          4096 Jan  4 12:29 out_voltage0_powerdown
-rw-r--r--    1 root     root          4096 Jan  4 12:30 out_voltage0_raw
-rw-r--r--    1 root     root          4096 Jan  4 12:28 out_voltage_powerdown_mode
-r--r--r--    1 root     root          4096 Jan  4 12:28 out_voltage_powerdown_mode_available
-r--r--r--    1 root     root          4096 Jan  4 12:28 out_voltage_scale
lrwxrwxrwx    1 root     root             0 Jan  4 12:28 subsystem -> ../../../../../bus/iio
-rw-r--r--    1 root     root          4096 Jan  4 12:28 uevent

Show device name

This specifies any shell prompt running on the target

root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> cat name
ad5791

Show scale

Description:
scale to be applied to out_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/iio:device0> cat out_voltage_scale
0.019

Set channel Y output voltage

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

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

This specifies any shell prompt running on the target

root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> echo 131579 > out_voltage0_raw

U = out_voltage0_raw * out_voltage_scale = 131579 * 0.019 = 2500.001 mV

This specifies any shell prompt running on the target

root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> echo -131579 > out_voltage0_raw

U = out_voltage0_raw * out_voltage_scale = -131579 * 0.019 = -2500.001 mV

List available power down modes

/sys/bus/iio/devices/iio:deviceX/out_voltage_powerdown_mode_available

Description:
Lists all available output power down modes.

This specifies any shell prompt running on the target

root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> cat out_voltage_powerdown_mode_available
6kohm_to_gnd three_state

Set power down mode

/sys/bus/iio/devices/iio:deviceX/out_voltage_powerdown_mode

Description:
Specifies the output power down mode. DAC output stage is disconnected from the amplifier and

6kohm_to_gnd connected to ground via an 6kOhm resistor
three_state left floating

For a list of available output power down options read out_voltage_powerdown_mode_available.

This specifies any shell prompt running on the target

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

Enable power down mode on output Y

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

Description:
Writing 1 causes output Y to enter the power down mode specified by the corresponding out_voltageY_powerdown_mode. Clearing returns to normal operation. Y may be suppressed if all outputs are controlled together.

This specifies any shell prompt running on the target

root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> echo 1 > out_voltage0_powerdown
root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> cat out_voltage0_powerdown
1
root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> echo 0 > out_voltage0_powerdown
root:/sys/devices/platform/bfin-spi.0/spi0.3/iio:device0> cat out_voltage0_powerdown
0

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