Add IIO example to the kernelnewbies wiki

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Hi,

I'm testing an AD7746 eval board using a Diolan DLN-2 adapter. I wrote
some basic notes about the installation an instanciation process for my
own use. But then I thought it could be useful for someone else. So
maybe I could put this in the kernel newbies page under the IIO section?

If you think is worth it, I would be glad to add it to the wiki page.

Best regards,

Gaston

Notes:

The setup
=========

	AD7746-EVAL BOARD --> Diolan DLN-2 Adapter --> Thinkpad T430

The connection between the AD7746 eval board and adapter is as detailed
in [2] I2C Interface Configuration Options, External connection (pag 4).

The connection between Diolan DLN-2 and the T430 is USB straighforward.

Drivers
=======

Both in mainline kernel:

 - Diolan DLN-2 driver: I2C_DLN2 (prompt Diolan DLN-2 USB I2C adapter)
	Loc: drivers/i2c/busses/
 - AD7746: AD7746
	Loc: drivers/staging/iio/cdc/

Bus of interest
===============

 $  sudo i2cdetect -l

We get something like:

 ...
 i2c-10  i2c             dln2-i2c-1-1.2:1.0-0                    I2C adapter
 ...

So our bus is the number 10.

Getting the address
===================

 $ sudo i2cdetect -y -r 10

We get something like:

      0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- 48 -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

3. Device instantiation:

 # echo add7746 0x48 > /sys/bus/i2c/devices/i2c-10/new_device

We must see something like:

$ cd /sys/bus/i2c/devices/i2c-10/10-0048/
$ ls
driver  iio:device0  modalias  name  power  subsystem  uevent

$ cd iio:device0
$ ls
dev                                      in_capacitance-capacitance_calibbias
in_capacitance0_calibbias_calibration    in_capacitance-capacitance_sampling_frequency
in_capacitance0_calibscale               in_capacitance-capacitance_scale
in_capacitance0_calibscale_calibration   in_capacitance_sampling_frequency
in_capacitance0-capacitance2_calibscale  in_capacitance_sampling_frequency_available
in_capacitance0-capacitance2_offset      in_capacitance_scale
in_capacitance0-capacitance2_raw         in_temp0_input
in_capacitance0_offset                   in_temp1_input
in_capacitance0_raw                      in_voltage0_calibscale_calibration
in_capacitance1_calibbias_calibration    in_voltage0_raw
in_capacitance1_calibscale               in_voltage1_supply_raw
in_capacitance1_calibscale_calibration   in_voltage_sampling_frequency
in_capacitance1-capacitance3_calibscale  in_voltage_sampling_frequency_available
in_capacitance1-capacitance3_offset      in_voltage_scale
in_capacitance1-capacitance3_raw         name
in_capacitance1_offset                   power
in_capacitance1_raw                      subsystem
in_capacitance_calibbias                 uevent

Checking the permits:

$ ll
-r--r--r-- 1 root root 4096 mar 31 09:57 dev
--w------- 1 root root 4096 mar 31 09:57 in_capacitance0_calibbias_calibration
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_capacitance0_calibscale
--w------- 1 root root 4096 mar 31 09:57 in_capacitance0_calibscale_calibration
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_capacitance0-capacitance2_calibscale
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_capacitance0-capacitance2_offset
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_capacitance0-capacitance2_raw
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_capacitance0_offset
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_capacitance0_raw
--w------- 1 root root 4096 mar 31 09:57 in_capacitance1_calibbias_calibration
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_capacitance1_calibscale
--w------- 1 root root 4096 mar 31 09:57 in_capacitance1_calibscale_calibration
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_capacitance1-capacitance3_calibscale
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_capacitance1-capacitance3_offset
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_capacitance1-capacitance3_raw
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_capacitance1_offset
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_capacitance1_raw
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_capacitance_calibbias
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_capacitance-capacitance_calibbias
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_capacitance-capacitance_sampling_frequency
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_capacitance-capacitance_scale
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_capacitance_sampling_frequency
-r--r--r-- 1 root root 4096 mar 31 09:57 in_capacitance_sampling_frequency_available
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_capacitance_scale
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_temp0_input
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_temp1_input
--w------- 1 root root 4096 mar 31 09:57 in_voltage0_calibscale_calibration
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_voltage0_raw
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_voltage1_supply_raw
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_voltage_sampling_frequency
-r--r--r-- 1 root root 4096 mar 31 09:57 in_voltage_sampling_frequency_available
-rw-r--r-- 1 root root 4096 mar 31 09:57 in_voltage_scale
-r--r--r-- 1 root root 4096 mar 31 09:57 name
drwxr-xr-x 2 root root    0 mar 31 09:57 power
lrwxrwxrwx 1 root root    0 mar 31 09:57 subsystem -> ../../../../../../../../../../../bus/iio
-rw-r--r-- 1 root root 4096 mar 31 09:57 uevent

4. Capacitance

According to [2], the capacitive length sensor is connected to CIN2+ and
CIN2-. In the driver, the values are mapped to capacitance0.

Raw values vs sensor length:

Note: (+) values means the sensor was moved to the left, (-) values
means the sensor was moved to the right. I.e. +1 cm means the rule was
moved 1 to the left, while -1 means the rule was moved 1 cm to the
right.

Note2: note the raw value for -10 is 8224733, while +10 is -8224726, the
difference is because when the rule is moved 10 cm to the left, the is
still 2 o 3 cm of dielectric between the plates. The design is obviously
thought to move the rule to the right.


	Length (cm) |	Raw Value Cap0
	------------------------------
	-10		-8224733
	-9 		-8224730
	-8		-8224728
	-7		-8224725
	-6		-8224724
	-5		-8224722
	-4		-8224720
	-3 		-8224718
	-2 		-8224716
	-1 		-8224714
	+0		-8224711
	+1		-8224711
	+2		-8224711
	+3		-8224711
	+4		-8224713
	+5		-8224715
	+6		-8224717
	+7		-8224719
	+8		-8224721
	+9		-8224723
	+10		-8224726


References
==========

[1] http://www.analog.com/media/en/technical-documentation/data-sheets/AD7745_7746.pdf
[2] http://www.analog.com/media/en/technical-documentation/evaluation-documentation/EVAL-AD7746EB.PDF
[3] https://i2c.wiki.kernel.org/index.php/Main_Page
[4] https://kernelsense.wordpress.com/2016/01/22/achievement-unlocked-first-iio-driver/

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