HD44780 problem

Aug 14, 2008 7 Replies
M

Hello



I'm puzzling over my own HD44780 library for Luminary ARM. I ported the code from a working library I use with AVR.



I see some activity with the oscilloscope and the signals seems ok. The LCD replies as expected. But I can see nothing on the display.



I remember I should see the pixels on the LCD uninitialized after power up ii the contrast voltage is good. Actually, I can't see this.



I'm sure the display is working because I removed from another (working) circuit some time ago.



Do you confirm I should see the pixels when the LCD is powered and not yet initialized? If so, where should I search the problem? Note I'm powering the LCD at 3.3V but the datasheet of HD44780 says it's ok.


Thanks! Marco / iw2nzm


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R

The HD44780 may be okay with 3.3 V but the LCD module itself may not be. What voltage is required for (Vdd - Vo) for the module you're using? It's typically around 4.2 V, so you'd need -0.9 V at Vo to get sufficient contrast.

Rich Webb Norfolk, VA
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M

Rich Webb ha scritto:

Oh! Thank you for the note. I have a lot of unbranded LCD modules so I haven't their datasheet. Anyway I try to set a negative voltage on Vo (because all other stuff is powered at +3.3v).

On the board I have no +5V at all. Do you know any LCD module that works at +3.3V?

Thanks! Marco / iw2nzm

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R

I'm sure there are some but all the ones I've worked with (purpose-bought for work and "whatevers" surplus units for play) have been with 5 V on the board.

You might try over at

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Rich Webb Norfolk, VA
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M

Rich Webb ha scritto:

Ok, thank you again.

Marco / iw2nzm

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C

Your question doesnt make sence. Power up will produce a row of blocks, initialisation blanks the display, anything else is undefined.

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M

snipped-for-privacy@aol.com ha scritto:

Actually, my question asked that. Maybe my English is not so good, but I was wondering just if the power up produces the row of blocks.

Anyway, Rich Webb found out the problem.

Marco / iw2nzm

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A

If you have a clock source or spare microcontroller pin you can make a simple negative bias charge pump with a BAV99 dual diode and two ceramic caps. The LCD drive is about is -4.5V relative to Vdd.

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