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Date: Sat, 20 Oct 2012 15:02:27 +0100
Message-ID: <CALc5WvMP2Lfjh9kEZEwtYXwSXGNpXJwOqsZCLJQnxTJYWWy...@mail.gmail.com>
Subject: Re: [RDG-Hack] Request for a small bit of help
From: Matthew Daubney <m...@daubers.co.uk>
To: reading-hackspace@googlegroups.com
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On 20 October 2012 09:24, graham <g...@coastelect.com> wrote:
> On 20/10/2012 01:21, Nigel Worsley wrote:
>>
>> Is the buffer chip between the processor and the LCD really neccesary?
>> Most 77480 compatible
>> LCD controllers have TTL input thresholds which should work fine with 3.3V
>> inputs.
>
> Agreed...also watch out for another couple of "gotchas"...most
> Hitachi/Densitron compatible character LCDs have quite low value (1K or
> less) pull-downs on their inputs...this can spoil your day if you use...for
> example..the ICSP pins to talk to yr LCD  (yes...I've done it!!) or
> multiplex them in some other way... leaving you in the position where you
> can either have ICSP...or your LCD...making the board a pig to debug.
> Clearly you can't sort that problem by fitting resistor buffers...'cos
> everything then gets too low impedance for the ICSP to operate...and UR
> screwed.

I'd read about that somewhere, hence avoiding the ICSP pins with the LCD :)

<snip>

> Absolutely...a lot of designs use a 10K pot 'cos they begrudge the extra
> cost of an ickle resistor....V false economy if you ask me. Second
> gotcha....may need to fit 100nF between pot wiper and ground...some LCDs
> suffer from noise feeding through the contrast pin and ending up as a
> ghostly "snow" on the display.

The pot is a 10k pot (it came with another LCD at some point), but I
can see the logic in using a smaller pot and a resistor. Will add the
cap.

<snip>

Thanks :)

-Matt Daubney