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PICList Thread
'LCDs and brown out'
1998\01\26@090014 by mike

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Hi,

I am using a 16x1 lcd display with a 16C71. The C71 has a maxim
reset circuit (I think it is a 705, but can't remember exactly).

If the power to the circuit is removed and then returned quickly,
the display stops working. I guess this is because the 705 resets
the PIC, but the supply doesn't go low enough to properly reset
the LCD.

Is there a way round this?

Thanks for your help,


Mike Watson

1998\01\26@100909 by Steve Lawther

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    Mike,
   
    Sounds like the code doesn't implement a full initialisation, and
    instead relies on the LCD POR. See the data sheets for HD44780 or
    clone chips,
   
    or see
   
    http://www.hantronix.com/app-note.htm
   
    for initialisation codes


______________________________ Reply Separator _________________________________
Subject:      LCDs and brown out
Author:  PC:spam_OUTmikeTakeThisOuTspamD-M-G.DEMON.CO.UK at INTERNET-HUSKY
Date:    26/01/98 14:18


Hi,
   
I am using a 16x1 lcd display with a 16C71. The C71 has a maxim
reset circuit (I think it is a 705, but can't remember exactly).
   
If the power to the circuit is removed and then returned quickly,
the display stops working. I guess this is because the 705 resets
the PIC, but the supply doesn't go low enough to properly reset
the LCD.
   
Is there a way round this?
   
Thanks for your help,
   
   
Mike Watson
   

1998\01\26@113152 by Mike Watson

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In message  <.....38840KILLspamspam@spam@d-m-g.demon.co.uk> mikespamKILLspamd-m-g.demon.co.uk writes:
> Hi,
>
> I am using a 16x1 lcd display with a 16C71. The C71 has a maxim
> reset circuit (I think it is a 705, but can't remember exactly).
>
> If the power to the circuit is removed and then returned quickly,
> the display stops working. I guess this is because the 705 resets
> the PIC, but the supply doesn't go low enough to properly reset
> the LCD.
>
> Is there a way round this?
>

I should have added that I am using the LCD in 4-bit mode and
am not using the r/w pin.


Cheers,

Mike

1998\01\26@130344 by Kalle Pihlajasaari

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As mentioned you need to carefully follow the initialisation
procedure and assume nothing about the state of the LCD, it may
have survived the brownout and initially just have the nibble
phasing wrong.  The triple command sequence to force it into 8 bit
mode is used to reestablish the nibble sequence in 4 bit mode, this
is part of the recommended procedure.

If you think that the LCD is getting stuck due to the sloppy way
the supply dips you may have to find a spare PIC pin and power the
display from the PIC.  This is perfectly OK and can be used to save
power by removing the 3 to 10 mA that the display draws if you have
nothing to display when the unit is sleeping.  Otherwise you can use
it to reset the display during periodic program excecution phases
when you want to force all your I/O devices into known states.  Be a
shame for the display to get the OK message stuck on when you are
busy scrolling the FIRE-FIRE alarm message and nothing happens.
Reading from the display is a good idea in critical applications
when you need to know if the controller is there at least.

You could also perhaps use a 3 pin reset generator IC to power the
display if you find one that will source the 3 to 10 mA that a
typical display requires.  This will generate a clean and fast
supply switch on (if you don't have any caps on the LCD side)
that will survive most any brown out.

Cheers
--
Kalle Pihlajasaari   .....kalleKILLspamspam.....ip.co.za   http://www.ip.co.za/ip
Interface Products   P O Box 15775, DOORNFONTEIN, 2028, South Africa
+ 27 (11) 402-7750   Fax: 402-7751    http://www.ip.co.za/people/kalle

DonTronics, Silicon Studio and Wirz Electronics uP Product Dealer

1998\01\26@132646 by wwl

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On Mon, 26 Jan 1998 13:44:20 GMT, you wrote:

>Hi,
>
>I am using a 16x1 lcd display with a 16C71. The C71 has a maxim
>reset circuit (I think it is a 705, but can't remember exactly).
>
>If the power to the circuit is removed and then returned quickly,
>the display stops working. I guess this is because the 705 resets
>the PIC, but the supply doesn't go low enough to properly reset
>the LCD.
>
>Is there a way round this?
>
>Thanks for your help,
>
>
>Mike Watson
If the problem is that the LCD is locking up in such a way that the
normal initialisation procedure won't recover it, you could power the
LCD from a PIC pin to allow the power to be cycled. Alternatively, if
the reset chip has a standard CMOS output (rather than open-drain),
you could power the LCD from that, so power is cycled on a reset.
These displays only take a mA or two so there shoudn't be any loading
problems with either of the above.

First, though, check you are *exactly* following the LCD
initialisation procedure correctly - this is very important.
    ____                                                           ____
  _/ L_/  Mike Harrison / White Wing Logic / EraseMEwwlspam_OUTspamTakeThisOuTnetcomuk.co.uk  _/ L_/
_/ W_/  Hardware & Software design / PCB Design / Consultancy  _/ W_/
/_W_/  Industrial / Computer Peripherals / Hazardous Area      /_W_/

1998\01\27@220111 by Andrew Mayo

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Drive the power to the LCD from a PIC pin rather than +5V, then you can
pull it low and back high under program control. The current drain will
be minimal, I'd imagine.

{Quote hidden}

1998\01\28@072541 by Morgan Olsson

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At 16:00 1998-01-28 +1300, you wrote:
>Drive the power to the LCD from a PIC pin rather than +5V, then you can
>pull it low and back high under program control. The current drain will
>be minimal, I'd imagine.
>
The LCD is problably a strong capacitive load (decoupling caps at the drive
chips). Take care you don«t get a spike on PIC internal supply!
/Morgan
Morgan Olsson, MORGANS REGLERTEKNIK, Sweden, ph: +46 (0)414 70741; fax 70331
-

1998\01\28@102557 by Mike Watson

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In message  <spamBeGone9801261940.aa12350spamBeGonespamdevice.data.co.za> TakeThisOuTPICLISTEraseMEspamspam_OUTMITVMA.MIT.EDU
writes:
{Quote hidden}

I have convinced myself that the initialisation procedure is correct
by going through the code instruction by instruction, comparing it
with the data sheet and examples of code I have found elsewhere on
the web.

Adding some more info, this problem occurs with some manufacturers
displays and not with others. The worst appears to be an Hitachi
display, with some cheap Taiwanese displays never causing a
problem.

I had thought of using a PIC pin to power the LCD, but this will
require a new board layout (or at least some Kynar wire) which I
was hoping to avoid.

It seems to me, that if the display is not being reset properly
when there is a brown-out, that no amount of code trying to
re-configure it will work as the display will not accepting any
commands, so a hardware reset is required. Is this reasonable?
If so, the only solution is to ensure the supply is removed
when the PIC is reset. Right?

Regards,

Mike Watson

1998\01\29@153448 by Mike Keitz

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>It seems to me, that if the display is not being reset properly
>when there is a brown-out, that no amount of code trying to
>re-configure it will work as the display will not accepting any
>commands, so a hardware reset is required. Is this reasonable?

Are you waiting a suitably "long" time after the brownout (PIC reset)
before trying to do anything with the display?  If the display started a
lengthy internal operation because of the brownout, it may not respond to
attempts to re-initialize it.  The E pin should be certain to be low
(inactive) during this interval.

This should be easily tested by resetting only the PIC (keeping the power
to the display normal) during the failure mode to see if another attempt
at initialzing will work.  This test is not valid if the PIC software
constantly accesses the display.


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