I'ts an interesting effect because the contrast usual must have nothing to
do with the communication ( and I'm not talking about sharing one cursor
at two different positions where multiplexation will decrease the contrast
) If all communication timing are ok then only Vlc must modify the
contrast. Vlc may be a positive or negative voltage depends on LCD
characteristics, tipically is somewhere between +2.5 to +4V or -3 to -5V.
But in your case I think is a 4bit mode error if the 8 bit looks great.

Vasile


On Fri, 16 Aug 2002, [iso-8859-9] =D6mer Yalh=FD wrote:

> I find that to be a programming error.  I had the similar problem,
> sometimes it would work just fine, sometimes the background would be so
> dark (like before init) it is hard to read the text.  I played with the
> timings a bit and that solved the problem (although not completely).  It
> is curious though, I never had this problem when using pure asm, I
> recently swithched to C and the problems began.  I cannot get the lcd to
> work with C commands, so I used assembly.
>
> IMHO,
>
> =D6mer YALHI
> oyalhi@teksan.com.tr
> http://www.teksan.com.tr
> Tel : +90 212 613 22 00
> Fax: +90 212 544 70 35
>
>
> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Peter L. Peres
> Sent: Thursday, August 15, 2002 9:27 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC]: LCD Contrast Variation
>
>
> On Wed, 14 Aug 2002, Thomas N wrote:
>
> >Hello Everyone,
> >
> >My LCD was working fine with 8-bit interface mode.  When I change it to
>
> >4-bit interface mode, the LCD contrast changes randomly each time I run
>
> >the program (I use ICD).  Sometimes, the display is clear with no dark
> >background, but sometimes the background is very dark and it's very
> >hard to read the text on the LCD.  Anyone know why it is intermittent
> >like this?  There is no improvement between floating the 4 un-used pins
>
> >and grounding them.  Please help!
>
> What is connected to Vlc ?
>
> Peter
>
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