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This version (01 Oct 2012 18:10) was approved by larsc.The Previously approved version (01 Oct 2012 15:29) is available.Diff

AD7780 IIO Low Power Sigma-Delta ADC Linux Driver

Supported Devices

Reference Circuits

Evaluation Boards

Description

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

Source Code

Status

Source Mainlined?
git Yes

Files

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

file: drivers/staging/iio/adc/ad7780.h

struct ad7780_platform_data {
	u16				vref_mv;
	int				gpio_pdrst;
};
static struct ad7780_platform_data __initdata ad7780_pdata = {
	.vref_mv  	= 2500,
	.gpio_pdrst	= GPIO_PF10,
};

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

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

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("vcc", "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);

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_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 */
		.platform_data = &ad7780_pdata,
		.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);

Adding Linux driver support

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

The AD7780 Driver depends on CONFIG_SPI

Linux Kernel Configuration
	Device Drivers  --->
		[*] Staging drivers  --->
			<*>     Industrial I/O support --->
			    --- Industrial I/O support
			    -*-   Enable ring buffer support within IIO
			    -*-     Industrial I/O lock free software ring
			    -*-   Enable triggered sampling support

			          *** Analog to digital converters ***
			    [--snip--]

			    <*>   Analog Devices AD7780 AD7781 ADC driver

			    [--snip--]

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.

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/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

Show device name

This specifies any shell prompt running on the target

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

Show scale

Description:
scale to be applied to in_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/device0> cat in_scale
0.00029

Show channel 0 measurement

Description:
Raw unscaled voltage measurement on channel 0

This specifies any shell prompt running on the target

root:/sys/devices/platform/bfin-spi.0/spi0.3/device0> cat in0_raw
-1

U = in_voltage0_raw * in_scale = -1 * 0.00029 = -0.00029 mV

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