AD7746

AD7746 IIO Capacitance to Digital Converter.

Supported Devices

Evaluation Boards

Reference Circuits

CN0346

Description

This is a Linux industrial I/O (Linux Industrial I/O Subsystem) subsystem driver, targeting single and multi channel serial interface Capacitance to Digital Converters. 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/cdc/ad7746.c

Unlike PCI or USB devices, I2C devices are not enumerated at the hardware level. Instead, the software must know which devices are connected on each I2C bus segment, and what address these devices are using. For this reason, the kernel code must instantiate I2C devices explicitly. There are different ways to achieve this, depending on the context and requirements. However the most common method is to declare the I2C devices by bus number.

This method is appropriate when the I2C bus is a system bus, as in many embedded systems, wherein each I2C bus has a number which is known in advance. It is thus possible to pre-declare the I2C devices that inhabit this bus. This is done with an array of struct i2c_board_info, which is registered by calling i2c_register_board_info().

So, to enable such a driver one need only edit the board support file by adding an appropriate entry to i2c_board_info.

For more information see: How to instantiate I2C devices

Depending on the converter IC used, you may need to set the I2C_BOARD_INFO name accordingly, matching your part name.

ADI part number	I2C_BOARD_INFO Name
AD7745	ad7745
AD7746	ad7746
AD7747	ad7747

static struct i2c_board_info __initdata board_i2c_board_info[] = {
#if defined(CONFIG_AD7746) || defined(CONFIG_AD7746_MODULE)
    {
        I2C_BOARD_INFO("ad7746", 0x48),
    },
#endif
};

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

    i2c_register_board_info(0, board_i2c_board_info,
                ARRAY_SIZE(board_i2c_board_info));
    [--snip--]

    return 0;
}
arch_initcall(board_init);

Hardware configuration

Use floating capacitive sensors with AD7745/AD7746: both sensor plates must remain isolated from ground, with the sensor connected between excitation and capacitance input. AD7747 instead measures sensors with one plate connected to ground. These variants are not interchangeable in the sensor wiring.

https://wiki.analog.com/_media/resources/tools-software/linux-drivers/iio-cdc/ad7746_eval_lr.jpg

Adding Linux driver support

Kernel configuration

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

Note

The driver depends on CONFIG_I2C

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

                [--snip--]

                *** Analog to digital converters ***
                <*>   Analog Devices AD7745, AD7746 AD7747 capacitive sensor driver

                [--snip--]

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/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0> ls -l
-r--r--r--    1 root     root          4096 Jan  1 11:17 dev
-rw-r--r--    1 root     root          4096 Jan  1 11:17 in_capacitance-capacitance_calibbias
-rw-r--r--    1 root     root          4096 Jan  1 11:17 in_capacitance-capacitance_scale
-rw-r--r--    1 root     root          4096 Jan  1 11:17 in_capacitance0-capacitance2_calibscale
-rw-r--r--    1 root     root          4096 Jan  1 11:17 in_capacitance0-capacitance2_zeropoint
-r--r--r--    1 root     root          4096 Jan  1 11:17 in_capacitance0-capacitance2_raw
--w-------    1 root     root          4096 Jan  1 11:17 in_capacitance0_calibbias_calibration
-rw-r--r--    1 root     root          4096 Jan  1 11:17 in_capacitance0_calibscale
--w-------    1 root     root          4096 Jan  1 11:17 in_capacitance0_calibscale_calibration
-rw-r--r--    1 root     root          4096 Jan  1 11:19 in_capacitance0_offset
-r--r--r--    1 root     root          4096 Jan  1 11:17 in_capacitance0_raw
-rw-r--r--    1 root     root          4096 Jan  1 11:17 in_capacitance1-capacitance3_calibscale
-rw-r--r--    1 root     root          4096 Jan  1 11:17 in_capacitance1-capacitance3_zeropoint
-r--r--r--    1 root     root          4096 Jan  1 11:17 in_capacitance1-capacitance3_raw
--w-------    1 root     root          4096 Jan  1 11:17 in_capacitance1_calibbias_calibration
-rw-r--r--    1 root     root          4096 Jan  1 11:17 in_capacitance1_calibscale
--w-------    1 root     root          4096 Jan  1 11:17 in_capacitance1_calibscale_calibration
-rw-r--r--    1 root     root          4096 Jan  1 11:17 in_capacitance1_offset
-r--r--r--    1 root     root          4096 Jan  1 11:17 in_capacitance1_raw
-rw-r--r--    1 root     root          4096 Jan  1 11:17 in_capacitance_calibbias
-rw-r--r--    1 root     root          4096 Jan  1 11:17 in_capacitance_sampling_frequency
-r--r--r--    1 root     root          4096 Jan  1 11:17 in_capacitance_sampling_frequency_available
-rw-r--r--    1 root     root          4096 Jan  1 11:17 in_capacitance_scale
-r--r--r--    1 root     root          4096 Jan  1 11:17 in_temp0_raw
-r--r--r--    1 root     root          4096 Jan  1 11:17 in_temp1_raw
--w-------    1 root     root          4096 Jan  1 11:17 in_voltage0_calibscale_calibration
-r--r--r--    1 root     root          4096 Jan  1 11:17 in_voltage0_raw
-r--r--r--    1 root     root          4096 Jan  1 11:17 in_voltage1_supply_raw
-rw-r--r--    1 root     root          4096 Jan  1 11:17 in_voltage_sampling_frequency
-r--r--r--    1 root     root          4096 Jan  1 11:17 in_voltage_sampling_frequency_available
-rw-r--r--    1 root     root          4096 Jan  1 11:17 in_voltage0_scale
-rw-r--r--    1 root     root          4096 Jan  1 11:17 in_voltage1_supply_scale
-r--r--r--    1 root     root          4096 Jan  1 11:17 name
drwxr-xr-x    2 root     root             0 Jan  1 11:17 power
lrwxrwxrwx    1 root     root             0 Jan  1 11:17 subsystem -> ../../../../../../bus/iio
-rw-r--r--    1 root     root          4096 Jan  1 11:17 uevent

Identify the device

root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0> cat name
ad7746

Basic operation

Show scales for single and differential input channels

Lists all available scales for the input and input pairs:

ADC Input	Channel name
CIN1(+)	in_capacitance_scale
CIN2(+)	in_capacitance_scale
CIN1(+) - CIN1(-)	in_capacitance-capacitance_scale
CIN2(+) - CIN2(-)	in_capacitance-capacitance_scale
VIN1	in_voltage0_scale
VIN_VDD_MON	in_voltage1_supply_scale

root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0> cat in_capacitance_scale
0.000000488

Show available sampling frequencies / update rates

root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0> cat in_capacitance_sampling_frequency_available
91 84 50 26 16 13 11 9

root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0> cat in_voltage_sampling_frequency_available
50 31 16 8
root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0>

Perform channel gain calibration

Description: Triggers gain calibration on channel in_capacitance0 or in_capacitance1. For AD7745/AD7746, connect an external full-scale calibration capacitance to the selected input before starting gain calibration. Power-up or reset restores the factory gain coefficient; the datasheet specifications apply to that factory coefficient. On AD7746 the gain-calibration register is shared by both capacitance channels.

root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0> echo 1 > in_capacitance1_calibscale_calibration
root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0> cat in_capacitance1_calibscale
1.000000

Perform channel offset calibration

Description: Triggers offset calibration on channel in_capacitance0 or in_capacitance1

root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0> echo 1 > in_capacitance1_calibbias_calibration
root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0> cat in_capacitance_calibbias
0

Set sampling frequency / update rate

root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0> cat in_capacitance_sampling_frequency
91
root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0> echo 50 > in_capacitance_sampling_frequency
root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0> cat in_capacitance_sampling_frequency
50
root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0>

Show channel CIN1(+) measurement

Description: Raw unscaled capacitance measurement on channel in_capacitance0_raw

ADC Input	Channel name
CIN1(+)	in_capacitance0_raw
CIN2(+)	in_capacitance1_raw
CIN1(+) - CIN1(-)	in_capacitance0-capacitance2_raw
CIN2(+) - CIN2(-)	in_capacitance1-capacitance3_raw
VIN1	in_voltage0_raw
VIN_VDD_MON	in_voltage1_supply_raw

root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0> cat in_capacitance0_raw
14914

U = in_capacitance0_raw * in_capacitance_scale = 14914 * 0.000000488 = 0.007278032 pF

Show internal temperature measurement

Read in_temp0_raw for the internal sensor or in_temp1_raw for the external diode. Multiply by the shared in_temp_scale to obtain temperature in millidegrees Celsius.

ADC Input Channel	Channel name
TEMP (internal sensor)	in_temp0_raw
TEMP (external sensor diode)	in_temp1_raw

Run these commands in the device’s sysfs directory to calculate the internal sensor’s temperature from actual readbacks:

raw=$(cat in_temp0_raw)
scale=$(cat in_temp_scale)
awk -v raw="$raw" -v scale="$scale" 'BEGIN { print raw * scale }'

Set channel common mode offset (CAPDAC)

Description: If known for a device, offset to be added to <type>[Y]_raw prior to scaling by <type>[Y]_scale in order to obtain value in the <type> units as specified in <type>[y]_raw documentation.

Input	CAPDAC	Channel name
CIN1(+)	CAPDAC(+)	in_capacitance0_offset
CIN2(+)	CAPDAC(+)	in_capacitance1_offset
CIN1(-)	CAPDAC(-)	in_capacitance0-capacitance2_zeropoint
CIN2(-)	CAPDAC(-)	in_capacitance1-capacitance3_zeropoint