AD7746
AD7746 IIO Capacitance to Digital Converter.
Supported Devices
Reference Circuits
Evaluation Boards
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
Files
Function |
File |
|---|---|
driver |
|
include |
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);
Adding Linux driver support
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 ring 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--]
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.
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_offset
-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_offset
-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_input
-r--r--r-- 1 root root 4096 Jan 1 11:17 in_temp1_input
--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_voltage_scale
-rw-r--r-- 1 root root 4096 Jan 1 11:17 in_voltage_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
Show device name
root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0> cat name
ad7746
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>
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 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_voltage_scale |
VIN_VDD_MON |
in_voltage_supply_scale |
root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0> cat in_capacitance_scale
0.000000488
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
Description: Scaled temperature measurement on channel in_temp0_input. Results are in millidegrees Celsius.
ADC Input Channel |
Channel name |
|---|---|
TEMP (internal sensor) |
in_temp0_input |
TEMP (external sensor diode) |
in_temp1_input |
root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0> cat in_temp0_input
28756
T = in_temp0_input = 28756 = 28.756 °C
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_capacitance1-capacitance3_offset |
CIN2(-) |
CAPDAC(-) |
in_capacitance0-capacitance2_offset |
Note
The AD7745/AD7746 CDC full-scale input range is ±4.096 pF. The part can accept a higher capacitance on the input and the common-mode or offset (not-changing component) capacitance can be balanced by programmable on-chip CAPDACs.
Example 4pF Offset: in_capacitance0_offset = Offset / in_capacitance_scale = 4pF / 0.000000488 = 8196721
root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0> echo 8196721 > in_capacitance0_offset
root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0> cat in_capacitance0_offset
8127504
Perform channel gain calibration
Description: Triggers gain 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_calibscale_calibration
root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0> cat in_capacitance1_calibscale
1.000000
Set channel gain coefficient
Description: Write gain coefficient for channel in_capacitance0 or in_capacitance1 Valid range is between 1.0 and 1.99999999.
root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0> echo 1.5 > in_capacitance1_calibscale
root:/sys/devices/platform/i2c-bfin-twi.0/i2c-0/0-0048/iio:device0> cat in_capacitance1_calibscale
1.500000
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