Searching \ for '[PIC]: PICs and LCDs - Open Drain?' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/io/lcd/pic.htm?key=lcd
Search entire site for: 'PICs and LCDs - Open Drain?'.

Exact match. Not showing close matches.
PICList Thread
'[PIC]: PICs and LCDs - Open Drain?'
2001\03\29@120117 by Chris Pringle

flavicon
picon face
Hi All,
Please correct me if I'm wrong anywhere here...don't want another duff
circuit board.

From my understanding of LCDs, you have to switch the polarity several times
a second in order for them to work. So, BP goes negative, and segments which
you want on go positive, the rest go negative...then the BP goes positive,
segments which you want on go negative, and the rest go positive. Is that
correct?

Finally, what is an open drain type output. Is this type of output still
safe to use on LCDs?

---
Regards,
Chris Pringle
Administrator of "The Linux Site"
http://thelinuxsite.sourceforge.net

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics


2001\03\29@122622 by Spehro Pefhany

picon face
e,At 02:51 PM 3/29/01 +0100, you wrote:
>Hi All,
>Please correct me if I'm wrong anywhere here...don't want another duff
>circuit board.
>
>>From my understanding of LCDs, you have to switch the polarity several times
>a second in order for them to work. So, BP goes negative, and segments which
>you want on go positive, the rest go negative...then the BP goes positive,
>segments which you want on go negative, and the rest go positive. Is that
>correct?

Yes, so far as it goes. Assuming you are talking about a static drive
LCD (one backplane) then that's all correct, but you must also ensure that
there is *no* DC component to the voltage across the LCD display. In this
case "no DC component" means that it must be < 50mV DC. Otherwise,
unpleasant electrochemical things will happen in the display and it will
fail. So, with your PIC, you must ensure that each phase of the drive
is about exactly the same length, at least on the average.

The LCD looks (electrically) like a capacitor between the BP and each
segment. Your drive frequency should probably be 30-100Hz.

>Finally, what is an open drain type output. Is this type of output still
>safe to use on LCDs?

What is it? It's a single N-channel MOSFET with the drain brought to the
output pin, rather than a push-pull drive using complementary P and N
channel MOSFETs. Probably best avoided because the drive signals would
likely end up being asymmetrical.

Best regards,
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics


2001\03\29@144906 by Drew Vassallo

picon face
>From my understanding of LCDs, you have to switch the polarity several
>times
>a second in order for them to work. So, BP goes negative, and segments
>which
>you want on go positive, the rest go negative...then the BP goes positive,
>segments which you want on go negative, and the rest go positive. Is that
>correct?

Don't mean to get off the subject, but why not just use an LCD driver chip?
Or a PIC with an integrated LCD driver port?

--Andrew
_________________________________________________________________
Get your FREE download of MSN Explorer at http://explorer.msn.com

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics


2001\03\29@164507 by Dwayne Reid

flavicon
face
At 02:49 PM 3/29/01 -0500, Drew Vassallo wrote:
> From my understanding of LCDs, you have to switch the polarity several
>>times
>>a second in order for them to work. So, BP goes negative, and segments
>>which
>>you want on go positive, the rest go negative...then the BP goes positive,
>>segments which you want on go negative, and the rest go positive. Is that
>>correct?
>
>Don't mean to get off the subject, but why not just use an LCD driver chip?
>Or a PIC with an integrated LCD driver port?

Cost and availability are VERY good reasons.  One can drive a 4 digit
static LCD (31 segments plus backplane) with 4- 74hc595 shift registers
using only 3 pins from the processor.  All 3 of those pins can also be used
for other, non-time critical funtions.  Total cost is less than a dollar.

dwayne



Dwayne Reid   <.....dwaynerKILLspamspam@spam@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

Celebrating 17 years of Engineering Innovation (1984 - 2001)

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Do NOT send unsolicited commercial email to this email address.
This message neither grants consent to receive unsolicited
commercial email nor is intended to solicit commercial email.

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics


2001\03\29@164534 by mike

flavicon
face
On Thu, 29 Mar 2001 14:51:00 +0100, you wrote:

>Hi All,
>Please correct me if I'm wrong anywhere here...don't want another duff
>circuit board.
>
>>From my understanding of LCDs, you have to switch the polarity several times
>a second in order for them to work. .well not in order for them to work, but in order for them to live for
a sensible amount of time. They will work on DC, but not for long!
>So, BP goes negative, and segments which
>you want on go positive, the rest go negative...then the BP goes positive,
>segments which you want on go negative, and the rest go positive. Is that
>correct?
Yes. It is important that the dudty cycle is EXACTLY 50%, so the best
software is to 'flip' everything at a constant interval. If segments
are on more than one port you may have problems getting simultaneous
switching.
>Finally, what is an open drain type output. Is this type of output still
>safe to use on LCDs?
No - you'd need pullups, which would draw power, and there is a high
risk of asymmetrical drive, which can effectively act as a DC bias,
reducing life.
>---
>Regards,
>Chris Pringle
>Administrator of "The Linux Site"
>http://thelinuxsite.sourceforge.net

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics


2001\03\30@062029 by Chris Pringle

flavicon
picon face
Hi,
Thanks for that...that cleared that bit up.

I'm now not using any open drain I/O pins for the LCD.

Just out of interest how much current does an LCD draw? I know they are
voltage driven devices, but will a PIC object to having the BP connected to
one of its pins without a transistor there? I don't want to blow the PIC up
because I was trying to draw too much current from it. Assume all the
segments are lit up, whats the current on the BP likely to be?

---
Regards,
Chris Pringle
Administrator of "The Linux Site"
http://thelinuxsite.sourceforge.net


{Original Message removed}

2001\03\30@080338 by Bob Ammerman

picon face
> Just out of interest how much current does an LCD draw? I know they are
> voltage driven devices, but will a PIC object to having the BP connected
to
> one of its pins without a transistor there? I don't want to blow the PIC
up
> because I was trying to draw too much current from it. Assume all the
> segments are lit up, whats the current on the BP likely to be?

Near zero. The only current you should have is a pulse each time the pin
changes state to charge the capacitance of the LCD.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.


2001\03\30@084317 by mike

flavicon
face
On Fri, 30 Mar 2001 12:19:35 +0100, you wrote:

>Hi,
>Thanks for that...that cleared that bit up.
>
>I'm now not using any open drain I/O pins for the LCD.
>
>Just out of interest how much current does an LCD draw? Almost none (nanoamps)- it's mostly capacitive.
>I know they are
>voltage driven devices, but will a PIC object to having the BP connected to
>one of its pins without a transistor there? I don't want to blow the PIC up
>because I was trying to draw too much current from it. Assume all the
>segments are lit up, whats the current on the BP likely to be?
Just check that the capacitance of the panel doesn't affect rise/fall
time significantly, as if assymetrical this could cause an effective
DC offset. I don't think this would be an issue unless you have a
REALLY big panel!

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.


2001\03\30@090445 by Olin Lathrop

face picon face
> Just out of interest how much current does an LCD draw?

Very little.  They look mostly like capacitors.  One gotcha is that they
have to be run with AC, 20 to 100Hz usually.  An average DC component will
damage them.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinspamKILLspamembedinc.com, http://www.embedinc.com

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.


More... (looser matching)
- Last day of these posts
- In 2001 , 2002 only
- Today
- New search...