Python bindings (ctypes)

[Matthew Baker]

Hi there,

I recently threw together some python bindings to bcm2835 that I thought I'd share with the group. The bindings have been constructed in pure python using ctypes (http://docs.python.org/library/ctypes.html) which provides functionality to dynamically load shared libraries and, hence, requires bcm2835 to be built as a shared library object. You can find the bindings here https://github.com/mubeta06/py-libbcm2835 with instructions on installation and usage. The advantage of ctypes is it give a relatively "straight through" approach to the underlying c code. As an example I ported the example spi.c file as per below.

from _bcm2835 import *

if not bcm2835_init():
        return

    bcm2835_spi_begin()
    bcm2835_spi_setBitOrder(BCM2835_SPI_BIT_ORDER_MSBFIRST)      # The default
    bcm2835_spi_setDataMode(BCM2835_SPI_MODE0)                   # The default
    bcm2835_spi_setClockDivider(BCM2835_SPI_CLOCK_DIVIDER_65536) # The default
    bcm2835_spi_chipSelect(BCM2835_SPI_CS0)                      # The default
    bcm2835_spi_setChipSelectPolarity(BCM2835_SPI_CS0, LOW)      # the default

    # Send a byte to the slave and simultaneously read a byte back from the slave
    # If you tie MISO to MOSI, you should read back what was sent
    data = bcm2835_spi_transfer(0x23);
    print "Read from SPI: %02X\n" % data

    bcm2835_spi_end()
    bcm2835_close()

If there is any questions I'd be happy to field them.

Cheers,

matb

[Mike McCauley]

Hi Matthew,

thanks for your contribution.
I have added a link to your module to the bcm2835 documentation for the next
release.

Cheers.

--
Mike McCauley VK4AMM mi...@airspayce.com
Airspayce Pty Ltd 9 Bulbul Place Currumbin Waters QLD 4223 Australia
http://www.airspayce.com
Phone +61 7 5598-7474 Fax +61 7 5598-7070

[Matthew Baker]

Hi Mike,

Thanks for the inclusion. On another note seeing as though the bindings require a shared library object would you accept a patch that sees the autotools configure file made more contemporary so both static and shared libs can be built via the build system?

Cheers,

matb

[Mike McCauley]

Sure.

Cheers.

[DGC]

Matt, I'm trying to use your library and I'm having problems.  I've cloned the git repository and run "sudo python ./setup.py install".  It seems to have installed correctly; the following files were installed:

/usr/local/lib/python2.7/dist-packages/libbcm2835/_bcm2835.py
/usr/local/lib/python2.7/dist-packages/libbcm2835/_bcm2835.pyc

but I get the following error when I try to include the module:

  File "therm.py", line 2, in <module>
    from _bcm2835 import *
ImportError: No module named _bcm2835

Any ideas what I'm doing wrong?

Thanks,
DG

[DGC]

In case anybody is following, I got this to work by copying libbcm2835/_bcm2835.py into my project directory.  I wasn't expecting to have to do this but I'm a Python neophyte so maybe I'm doing something wrong.

[ahlawy red]

Hi Matthew,
thanks for your help.
but what if i wanna to specify a certain channel to read from
how can i do it using _bcm2835 ??

[Dan H]

Hi Mike/Matthew

The bcm2835_init() call works when using the C libraries.

However, when using the python (3.4) libraries it refuses to initialize. It returns 1 instead of 0.

Also the spi.py test returns 0 instead of 35 (hex23).

The problem may be the Waveshare AD/DA HAT that I'm using. However ads1256_test.c from Waveshare works and it goes through bcm2835_init()

I'm using version 1.52 of the lib on Raspi 2.0 with the latest Jessie (nice wall paper) and I have rebuilt al libs that I could find. 

It seems that the libbcm2835.so must be made with gcc and copied to /usr/local/lib

Please help if you can.

Regards

Dan

[Mike McCauley]

Hi. I'm traveling at the moment and won't get to look at this for a few weeks

Sent from my iPhone

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