[HH] Seeking help on Programming I2C bus in python on Raspberry Pi

[Ming kuo]

Thank you Tom, Jon, and Greg,

I am grateful for all the help while I go through this challenge.

Tom, I have previously responded to Jon and Greg privately trying to avoid
too many emails to the group.

But yes, your tracking of my progress of the project is right, I basically
have proof of concept with hardware, circuits and now it is just the
software programming..

Following the code in the previously mentioned example, I am trying to
adapt it to working with the Bar LED..

I have isolated the critical piece of code is


   for x in range(0,8):
     a = 1 << x
     set_led(a,0)
     time.sleep(delay)
   set_led(0,0)


in code :  set_led(x,y) x is the data and y is the bank.

so I am confused on x or data piece of the parameter..



in the above code set_led(a,0)

*a= 1 << x what does this statement do?*

*and I tried just setting a to various numbers 1,2,3,4,5,6,7,8,9 and
the led seem to light up in no understandable pattern, sometimes
individually and sometimes multiple LEDs go on.. *


So basically, once I get to know how to control the leds on the bar
graph, then I can start deciphering the code for letters and words


Thanks to the group


Ming



On Thu, Nov 15, 2012 at 2:36 PM, Tom Metro <tmetro+hhacking at gmail.com>wrote:

> Ming kuo wrote:
> > Hi Guys, I was recently introduced to this group, so here is my first
> post.
>
> Welcome to the group.
>
>
> > I figured out the wiring to get the bar to light up using the same codes
> > as above.
>
> Sounds like you've got the hard part done. If you've gotten that much to
> work, then you've proven the hardware works, and the basic software
> works. Now you just need to tweak the behavior.
>
>
> > I am stuck...on the python code to control the I2C.
> > This is my immediate challenge to figure out how to program the
> > I2C for the LED bar...
>
> Can you clarify?
>
> You said above that you succeeded in getting the bar to light up using
> the same setup as the article you referenced. That implies you have a
> working I2C driver and some Python code using it.
>
>
> > ...I need to program a python code to POV display the messages which
> > i can imaging that it get pretty challenging given how to form a
> > character with LED while timing the rotation.
>
> So POV is persistence of vision? You're looking to build something like:
> http://www.youtube.com/watch?v=fJBYOU-Y-ZU
>
> How do you plan to move the display?
>
> To build on what Jon Evans said in his reply, it sounds like what you
> have left to do is less hardware hacking and more basic Python coding.
> Looking at the sample code:
> http://www.skpang.co.uk/blog/wp-content/uploads/2012/06/ledchaser.py
>
> they provide a set_led() function, so you'll need to remove the two
> chunks of code used to strobe the LEDs ("# Move led left " and "# Move
> led right") and replace them with your character generating code.
>
> Not trivial but given that POV displays seem to be popular, likely you
> can find some existing code for the character generator. If not, you'll
> need to dig up a library or lookup table that maps characters to dot
> patterns. Basically a dot matrix font. Then build a function that when
> given a character, translates that into the equivalent array of dots.
>
> I'd be inclined to use an architecture where rows of dots are pushed
> onto a stack, and then some background process is responsible for
> pulling a row from the stack and lighting up the LEDs on a steady clock
> rate. (I'm not sure if that's easy to accomplish in Python.)
>
> I'm guessing that with most of these POV displays, there isn't any real
> synchronization between the display motion and the strobing. The builder
> simply tunes the strobe rate until it works most of the time for
> whatever RPM their display ends up running at.
>
>
> > So any help through email or in person would be greatly appreciated.
>
> Post some follow up questions as you get stuck and we'll see what we can
> do.
>
>  -Tom
>
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