Reset problem with LCD display

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Hey

I've got this weird problem with a HD44780 compatible display. It's a
40x4 display from Batron. The displays is based on 2 HD44780
controllers.

When I power up the uC and display I get the 2 black bars on the
display. I then reset the display and it works fine.
BUT I got problems, when I download a new code in the uC and resets it
afterwards. Now the display writes a's instead of my text. The problem
is on both HD44780 controllers, so i figured that the controllers
shares the problem. The display works fine, if a reset the uC a few
times?!??

I tried with an older 40x2 HD44780 compatible display, but same
result.

The uC is a Philips LPC 2124. I've tried changing the delays in my
code, I had to use 1 us delay to get a satisfying length on the Enable
signal.

The contrast voltage is supplied thru a var. resistor, so the
Vcontrast is contemporart with the Vcc. Is this a problem?

I'm confused.... ANY SUGGESTIONS???

Regards

Jesper Kristensen
Denmark

Re: Reset problem with LCD display

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it
problem
Enable

Have you tried to power cycle the controllers after the microprocessor
reset?


Re: Reset problem with LCD display
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Hey

This could solve my problem..

How can I run a power cycle on the controllers. The datasheet
describes a power cycle on Vdd > 4.5 Volt. But I cant find a "manual"
power cycle??!??!

Jesper

Re: Reset problem with LCD display
On 15 May 2005 02:16:55 -0700, the renowned snipped-for-privacy@mail.dk (Jesper)

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If you float (or bring low) the pins connected to the display, then
interrupt the power (eg. by using a port pin and a PNP transistor or
p-channel MOSFET to control the display Vdd) for a suitable lenth of
time (maybe a couple hundred msec), then you can start up as from a
power-on.

The micro is probably putting the display into some undesirable state
as a side effect of the downloading process and you do not have a
power off/on cycle to reset it. Those parts do not have a reset pin.

You may also be able to recover by repeating the power-on sequence,
but it sounds like you've tried it. It might be worth double checking
that you have the sequence exactly right by the book.


Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
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Re: Reset problem with LCD display
Clearly the problem is with your code and not the controller. These
chips have a reset sequence that you must follow to get reliable
operation. Another thing to watchout for is writing too fast, these are
very slow chips by todays standards, you must also check the busy flag
before writing data.


Re: Reset problem with LCD display
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Are you using the 4 bit interface? If you are initialising the display twice
will get the upper/lower nibbles out of line between sender and reciever.
Two solutions, make use of the read/status instructions to establish whether
the display is correctly responding, if not issue the init commands to go
into 4 bit mode and then try again.

Richard



Re: Reset problem with LCD display

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twice
whether

There is a EPE magazine article online re using these modules. IIRC the
article they recommend initialising 2 or three times in succession for
reliable start up. Have a look at the Everyday Popular Electronics website
for the 2 part article "How to use intelligent LCDs"
rob




Re: Reset problem with LCD display
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It is a long time since I used these beasts, but as I recall the
problem revolves around the fact that the busy signal is not valid
after the reset.  You have to impose a delay of something like 40
millisecs after reset before trusting anything.

--
 Some informative links:
   news:news.announce.newusers
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Re: Reset problem with LCD display
Hey

Thanks for all your replies. I've tried to add delays and do the
initialize the LCD multible times. But none if it helped.

Please notice that the error is on both controllers.

Jesper
Denmark

Re: Reset problem with LCD display
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Check that the enable pin is the correct sense (active high)

I once made a unit which drove the display with active low enable and it
worked very bizarrely, sometimes OK, sometimes not.
--
Tim Mitchell

Re: Reset problem with LCD display
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That means the problem is in your code and/or hardware. My guess - CODE.
Or maybe the logic level incompatibility between LPC2xxx and 5V LCD? How
do you handle the level translation? Do you read the statur from LCD?

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Yes. Forget and delete all the sample code you got from any source and
write your own init routine, strictly following the HD44780 data sheet.
It always works for me!

If you use 4 bit interface, remember to read two nibbles while querying
the busy flag! (discard the second nibble but toggle Enable for it).

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