[sdiy] LCD hot rodding

grant musictechnologiesgroup.com grant at musictechnologiesgroup.com
Mon Apr 8 20:40:13 CEST 2024


Many of these designs push the envelope on timing for the Hitachi chip (or clones), especially the Z-80 designs that attempt to memory map the LCD. Almost always it's the data and/or address setup time before E goes high. There are two common fixes (they are now anyway).

1. Add a 33pF to ground to the E (enable) pin. YMMV. This delays E slightly and sounds like a bad idea, but it works (example: Yamaha TX7).

2. Sometimes there are timing violations or logic level issues (or a combination thereof) due to the logic chips families used for the address decoding.  Example: Lexicon MRC - replace U118 KS74HCTLS00N with SN74LS00N.

Also don't overlook a defective LCD. I was getting 10% to 20% crap from BuyDisplay.com so I stopped buying from them.

GB

------ Original Message ------
From "Nathan Trites" <nathan at idmclassics.net<mailto:nathan at idmclassics.net>>
To "rburnett at richieburnett.co.uk<mailto:rburnett at richieburnett.co.uk>" <rburnett at richieburnett.co.uk<mailto:rburnett at richieburnett.co.uk>>
Cc "synth-diy at synth-diy.org<mailto:synth-diy at synth-diy.org>" <synth-diy at synth-diy.org<mailto:synth-diy at synth-diy.org>>
Date 4/8/2024 10:27:46 AM
Subject Re: [sdiy] LCD hot rodding

I'll second Richie - working on less common gear over the last few years I've had more and more experiences with 44780 compatible displays that won't initialize properly. Usually black boxes, or no display at all, and it makes me second guess my work every time.

Nathan

On Mon, Apr 8, 2024 at 9:25 AM <rburnett at richieburnett.co.uk<mailto:rburnett at richieburnett.co.uk>> wrote:
Both of these issues (top line only on the OLED display, and black
blocks only on the LCD) sound like initialisation issues with the
display controller IC.

In each case did you have the new display connected at the time when the
kit was powered up?  Or only connected the display after power up?  It
needs to be connected at boot-up to receive commands and get properly
initialised for the right number of rows & columns, etc.

Failing that it sounds like the replacement displays aren't entirely
compatible with the Hitachi 44780 initialisation sequence of the
original display.  I've had problems with this before when substituting
displays in various bits of kit.  Sometimes the firmware doesn't wait
long enough between commands, or it doesn't follow the exact recommended
command sequence but still manages to work with some screens (>.<)

-Richie,



On 2024-04-08 13:56, mark verbos via Synth-diy wrote:
> I added a trim and it only lightened or darkened the black boxes.
>
>
>
> Mark
>
>
>
>
>> On Apr 8, 2024, at 2:51 PM, Flügel, Werner <werner.fluegel at b-tu.de<mailto:werner.fluegel at b-tu.de>>
>> wrote:
>>
>> Am 08.04.2024 um 14:38 schrieb mark verbos via Synth-diy:
>>
>>> Second, I plugged in the LED backlighted one today. The backlighting
>>> looks great, but I only get solid black boxes all the way across the
>>> screen.
>>> Does anyone know about these things? What am I doing wrong here?
>>
>> Is there a trimmer for contrast? I had a similar effect building the
>> Ambika Polysynth, adjusting the contrast helped in my case.
>>
>> --
>> mfG.
>>
>> W.Flügel
>>
>> Wir sind IT. Das ist kein Beruf, das ist eine Diagnose.
>> <7123838.uCcSxbalbl at rotfl.franken.de<mailto:7123838.uCcSxbalbl at rotfl.franken.de>>
>>
>> ________________________________________________________
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