Re: Raspberry Pi support for Rev-2 & BL

Hi Markus

On Thursday 24 January 2013, Markus Dolze wrote:
> After making myself more familiar with analog circuits (again) I now
> know why my backlight didn't work.
>
> My current problem is that all circuits you referenced above use NPN
> transistors that light the backlight if the BL is 'high'. The
> original patch however, turns the BL pin 'low' if backlight is
> enabled.

I was unaware that the default was "active low", but that makes sense 
for parallel port interfaces - Most (all?) standard parallel ports are 
capable of sinking a lot more current than they can source.

> Do you know which circuit was originally used by the you/author of
> that part?

I would imagine circuit [1] was used - I never tested it myself as I do 
not have a display with a backlight.

> Otherwise I will recommend to use circuit [1] and modify the code to
> work with that. The usual backlight circuit from [2] must not be used
> with a Pi.

Agreed - With circuit [2], there is a very real risk that the GPIO pin 
could be destroyed.

> [1]
> http://lcdproc.sourceforge.net/docs/lcdproc-0-5-6-user.html#hd44780-f
>tdi-backlight [2]
> http://lcdproc.sourceforge.net/docs/lcdproc-0-5-6-user.html#hd44780-c
>onnections-backlight.circuit


Regards, Paul.