sht-sensor
----------

Python driver and command-line tool for Sensirion SHT1x and SHT7x
sensors connected to GPIO pins.

Pure-python module only requires /sys/class/gpio interface, provided by
the Linux kernel and should work on any device that has it (including
RPi, Beaglebone boards, Cubieboard, etc).

Its main purpose is reading temperature (in degrees Celsius) and
humidity (%RH) values from these devices, checking CRC8 checksums for
received data to make sure it was not corrupted in transfer.

SHT1x and SHT7x are fairly popular and accurate capacitive/band-gap
relative humidity and temperature sensor IC's, with digital output via
custom 2-wire serial interface. SHT1x differs from SHT7x in packaging,
with SHT1x being surface-mountable one and latter having pluggable FR4
package.

Sensors include additional functionality available via status register
(like VDD level check, enabling internal heating element, resolution,
OTP reload, etc) which may or may not also be implemented here, see
"Stuff that is not implemented" section at the end.

Usage
-----

Module can be imported from the python code or used via included
command-line tool, which should be installed along with the module (or
can be used via ./sht symlink in the repo root without installation).
See "Installation" section below on how to install the module.

GPIO pin numbers (to which SCK and DATA sensor pins are connected) must
be specified either on command-line (for cli tool) or on class init
(when using as a python module).

Example, for SCK pin 21 and DATA pin 17:

::

    % sht -v -trd 21 17
    temperature: 25.07
    rh: 26.502119362
    dew_point: 4.4847911176

For both the tool and module, be sure to check/specify correct voltage
that the sensor is connected to:

::

    % sht --voltage=5V --temperature 21 17
    25.08

This voltage value is used to pick coefficient (as presented in
datasheet table) for temperature calculation, and incorrect setting here
should result in incorrect values (all of them, as RH also uses T in
calculation).

If you're using non-SHT1x/SHT7x, but a similar sensor (e.g. some later
model), it might be a good idea to look at the Sht class in the code and
make sure all coefficients (taken from `SHT7x
datasheet <http://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/Humidity/Sensirion_Humidity_SHT7x_Datasheet_V5.pdf>`_)
there match your model's datasheet exactly.

See ``sht --help`` output for more command-line arguments for the tool.

Example usage from python code:

::

    from sht_sensor import Sht
    sht = Sht(21, 17)
    print 'Temperature', sht.read_t()
    print 'Relative Humidity', sht.read_rh()

Again, be sure to specify correct voltage value (on sensor's VDD pin,
exactly as it is presented in datasheet table) for calculations, if it's
not module-default '3.5V', for example:
``sht = Sht(21, 17, voltage='5V')``.

Some calculations (e.g. for RH) use other sensor-provided values, so
it's possible to pass these to corresponding read\_\* functions, to
avoid heating-up sensor with unnecessary extra measurements:

::

    t = sht.read_t()
    rh = sht.read_rh(t)
    dew_point = sht.read_dew_point(t, rh)

If included sht\_sensor.gpio module (accessing /sys/class/gpio directly)
should not be used, its interface ("get\_pin\_value" and
"set\_pin\_value" attrs/functions) can be re-implemented and passed as
gpio keyword argument on Sht class init.

ShtComms class is an implementation of 2-wire protocol that sensor uses
and probably should not be used directly, all the coefficients,
calculations and such high-level logic is defined in Sht class.

Installed python module can also be used from cli via the usual
``python -m sht_sensor ...`` convention.

Installation
------------

It's a regular package for Python 2.7 (not 3.X).

Using `pip <http://pip-installer.org/>`_ is the best way:

::

    % pip install sht-sensor

If you don't have it, use:

::

    % easy_install pip
    % pip install sht-sensor

Alternatively (see also `pip2014.com <http://pip2014.com/>`_ and
`install
guide <http://www.pip-installer.org/en/latest/installing.html>`_):

::

    % curl https://raw.github.com/pypa/pip/master/contrib/get-pip.py | python
    % pip install sht-sensor

Or, if you absolutely must:

::

    % easy_install sht-sensor

But, you really shouldn't do that.

Current-git version can be installed like this:

::

    % pip install 'git+https://github.com/mk-fg/sht-sensor.git#egg=sht-sensor'

Note that to install stuff in system-wide PATH and site-packages,
elevated privileges are often required. Use "install --user",
`~/.pydistutils.cfg <http://docs.python.org/install/index.html#distutils-configuration-files>`_
or `virtualenv <http://pypi.python.org/pypi/virtualenv>`_ to do
unprivileged installs into custom paths.

Alternatively, ``./sht`` tool can be run right from the checkout tree
without any installation, if that's the only thing you need there.

Links
-----

Other drivers for these sensors that I know of and might be more
suitable for some particular case:

-  `rpiSht1x <https://pypi.python.org/pypi/rpiSht1x>`_ (python package)

   Based on RaspberryPi-specific RPi.GPIO module, does not check CRC8
   checksums for received data, uses hard-coded 5V temperature
   conversion coefficients.

-  sht1x module in `Linux kernel <https://www.kernel.org/>`_

   Looks very mature and feature-complete, probably used a lot for
   various platforms' hardware monitoring drivers.

   Seem to be only for internal use (i.e. from other kernel modules) at
   the moment (3.17.x), but should be possible (and easy) to add Device
   Tree hooks there, which would allow to specify how it is connected
   (gpio pins) via Device Tree.

-  `SHT1x module for
   Arduino <https://github.com/practicalarduino/SHT1x>`_

   C++ code, rpiSht1x above is based on this one.

Stuff that is not implemented
-----------------------------

-  Everything related to the Status Register.

   In particular, commands like VDD level check, enabling internal
   heating element, resolution, OTP reload, etc.

-  Temerature measurements in degrees Fahrenheit.

   These just use different calculation coefficients, which can be
   overidden in the Sht class. Or degrees-Celsius value can easily be
   converted to F after the fact.

   Metric system is used here, so I just had no need for these.

-  Lower-resolution measurements.

   Sensor supports returning these after changing the value in the
   Status Register, so interface to that one should probably be
   implemented/tested first.

-  Skipping CRC8 checksum validation.

   Code is there, as ShtComms.\_skip\_crc() method, but no idea why it
   might be preferrable to skip this check.

-  Changing SCK clock rate.

   Might be desirable for slower boards or more electric-noisy
   environments.


