Post by thread:[OT] Voltage and Polarity of an LCD Display

I guess this is my night to post to the PICLIST! I have a clock project that needs to display hh:mm:ss on a largish LCD display. I have found a nice one at a good price (surplus -- no docs or markings of any kind) and got a sample to prototype. It is a bare (no driver chips) LCD in a kind of huge DIP. It would be perfect for my purposes. I sat down tonight to begin probing it for the pinouts when I realized that, while I have done any number of HD44780-based projects, I had no idea about driving a "raw" LCD. 1) What voltage range is typical? Is 5VDC sufficient? 2) What about polarity? Can these things be damaged by reverse polarity? 3) What about series resistance? I know these are *very* low amperage devices, but do I need a series resistor with each segment for some reason? If so, what size? 4) Is there going to be a chip-wide contrast pin on this thing, or is that up to the hardware that drives it? I did a search with a couple of the search engines, but could not find any info on any 'raw" LCD's. Any help or advice is appreciated. Michael REMOVE THE .NS (NO SPAM) FROM MY ADDRESS TO REPLY *************************************************************************When th e way of the Tao is forgotten, kindness and ethics must be taught. Men must learn to pretend to be wise and good. -- Lao Tzu *************************************************************************

The LCD backplane is typically driven with a 5 Vpp square wave (alternating between 0V and 5V). If a segment is driven with the same voltage (in phase) the segment is the same as the background color. If the segment is driven out of phase, the segment is the opposite of the background color. The frequency is typically 30 Hz or so. The LCDs are VERY sensitive to DC. If there is a DC component on the drive signals (that is, a DC component between the segment and the backplane), the life of the LCD will drop substantially. You can possibly drive the LCD directly with a PIC, using an interrupt routine to generate the backplane and segment signals. As I recall, Microchip has a chip with an LCD driver in it. You can also use exclusive OR gates to do the phase inversion for driving segments, but that takes a lot of chips to do many segments. Finally, there is the ICL7211, which is a 4 digit LCD driver in a 40 pin chip. You feed it BCD and digit select signals. It generates backplane and segment drive signals. On Sat, 6 Jun 1998 21:53:45 -0700 Michael Ghormley <spam_OUTmikegTakeThisOuTBLACKSAND.NS.COM> writes: {Quote hidden}>I have a clock project that needs to display hh:mm:ss on a largish LCD >display. I have found a nice one at a good price (surplus -- no docs >or >markings of any kind) and got a sample to prototype. It is a bare (no >driver chips) LCD in a kind of huge DIP. It would be perfect for my >purposes. > >I sat down tonight to begin probing it for the pinouts when I realized >that, while I have done any number of HD44780-based projects, I had no >idea about driving a "raw" LCD. > > >3) What about series resistance? I know these are *very* low >amperage >devices, but do I need a series resistor with each segment for some >reason? If so, what size? You can connect the segments and backplane directly to the driver with no series resistor. > >4) Is there going to be a chip-wide contrast pin on this thing, or is >that up to the hardware that drives it? The contrast or viewing angle control is only used on multiplexed LCDs. A seven segment display is (typically) not multiplexed. Further, the contrast or viewing angle control drives the display driver chip of muxed displays, not the display directly. > >I did a search with a couple of the search engines, but could not find >any info on any 'raw" LCD's. Any help or advice is appreciated. Check for application notes on the ICL7211. Also, check LCD manufacturers, such as AND, Hamlin, and others I can't think of right now... _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866]

Harold M Hallikainen wrote: You can possibly drive the LCD directly with a PIC, using an > interrupt routine to generate the backplane and segment signals. As I > recall, Microchip has a chip with an LCD driver in it. You can also use > exclusive OR gates to do the phase inversion for driving segments, but > that takes a lot of chips to do many segments. Finally, there is the > ICL7211, which is a 4 digit LCD driver in a 40 pin chip. You feed it BCD > and digit select signals. It generates backplane and segment drive > signals. According to the new Digi-Key catalog, the PIC 16C92X is an 8 bit Micro with an LCD driver built-in. Has anyone had any experience with this device? I am working on a project with a 16F84 that will hopefully output to an LCD. Would I be better off to use the 16C92X and avoid all the extra hardware to do the same thing with the 16F84? I'm just a newbie at this, so the answer may be somewhat obvious to others... Thanks for any help. Fred. .....fred.hatfieldKILLspam@spam@sstar.com

I also have a simular project that I am working which requires HH:MM:SS in 24 hour format. Both of the replies that I have seen to your post refer to the LCD driver PIC chip. If one works out the needed segments for a 6 digit clock, 32 segments is not enough and have yet to find a 6 digit LCD display that was multiplexed. Need about 36 segments. I have found quite a few 32 bit LCD drivers which is not enough unless you want to cascade two of them together. I did, however, come accoss something last week that gives me hope. My 1992 Signetics I2C data book shows a 40 seg driver. Part number is PCF8576. As far as the rest of your questions, just let the PCF8576 handle it for you. Thats what it's there for. Michael Ghormley wrote: {Quote hidden}> I guess this is my night to post to the PICLIST! > > I have a clock project that needs to display hh:mm:ss on a largish LCD > display. I have found a nice one at a good price (surplus -- no docs or > markings of any kind) and got a sample to prototype. It is a bare (no > driver chips) LCD in a kind of huge DIP. It would be perfect for my > purposes. > > I sat down tonight to begin probing it for the pinouts when I realized > that, while I have done any number of HD44780-based projects, I had no > idea about driving a "raw" LCD. > > 1) What voltage range is typical? Is 5VDC sufficient? > > 2) What about polarity? Can these things be damaged by reverse > polarity? > > 3) What about series resistance? I know these are *very* low amperage > devices, but do I need a series resistor with each segment for some > reason? If so, what size? > > 4) Is there going to be a chip-wide contrast pin on this thing, or is > that up to the hardware that drives it?

The PIC16C923/924 can drive up to 116 segments when used with a 1:4 multiplexed LCD pannel. >From owner-piclistKILLspammitvma.mit.edu Sun Jun 7 22:42:52 1998 >Received: from PEAR.EASE.LSOFT.COM (206.241.12.19) by vms.dc.lsoft.com (LSMTP for OpenVMS v1.1a) with SMTP id <.....4.5C812277KILLspam.....vms.dc.lsoft.com>; Mon, 8 Jun 1998 1:29:36 -0500 >Received: from MITVMA.MIT.EDU by MITVMA.MIT.EDU (LISTSERV release 1.8c) with > NJE id 3959 for EraseMEPICLISTspam_OUTTakeThisOuTMITVMA.MIT.EDU; Mon, 8 Jun 1998 01:30:33 -0400 >Received: from MITVMA (NJE origin SMTP@MITVMA) by MITVMA.MIT.EDU (LMail > V1.2b/1.8b) with BSMTP id 1963; Mon, 8 Jun 1998 01:30:08 -0400 >Received: from mail.eskimo.com (204.122.16.4) by mitvma.mit.edu (IBM VM SMTP > V2R3) with TCP; Mon, 08 Jun 98 01:30:06 EDT >Received: from eskimo.com (ape.ndip.eskimo.net [204.122.28.61]) by > mail.eskimo.com (8.8.8/8.8.8) with ESMTP id WAA25617 for > <PICLISTspam_OUTMITVMA.MIT.EDU>; Sun, 7 Jun 1998 22:29:44 -0700 (PDT) >X-Mailer: Mozilla 4.04 [en] (Win95; U) >MIME-Version: 1.0 >References: <md5:C45AE4E656BBE51DD7EBF4FF6123B229> > <@spam@357A1CD9.563DKILLspamblacksand.NS.com> >Content-Type: text/plain; charset=us-ascii >Content-Transfer-Encoding: 7bit >Message-ID: <KILLspam357B76C9.DB25D6F3KILLspameskimo.com> >Date: Sun, 7 Jun 1998 22:29:46 -0700 >Reply-To: RemoveMEapeTakeThisOuTeskimo.com >Sender: pic microcontroller discussion list <spamBeGonePICLISTspamBeGoneMITVMA.MIT.EDU> >From: Mark Devin Newland <TakeThisOuTapeEraseMEspam_OUTESKIMO.COM> >Organization: Devin Electronics >Subject: Re: [OT] Voltage and Polarity of an LCD Display >To: RemoveMEPICLISTTakeThisOuTMITVMA.MIT.EDU > >I also have a simular project that I am working which requires HH:MM:SS >in 24 hour format. Both of the replies that I have seen to your post refer >to the LCD driver PIC chip. If one works out the needed segments for a 6 >digit clock, 32 segments is not enough and have yet to find a 6 digit LCD >display that was multiplexed. Need about 36 segments. I have found quite >a few 32 bit LCD drivers which is not enough unless you want to cascade >two of them together. I did, however, come accoss something last week >that gives me hope. My 1992 Signetics I2C data book shows a 40 seg >driver. Part number is PCF8576. > >As far as the rest of your questions, just let the PCF8576 handle it for >you. Thats what it's there for. > >Michael Ghormley wrote: > >> I guess this is my night to post to the PICLIST! >> >> I have a clock project that needs to display hh:mm:ss on a largish LCD >> display. I have found a nice one at a good price (surplus -- no docs or >> markings of any kind) and got a sample to prototype. It is a bare (no >> driver chips) LCD in a kind of huge DIP. It would be perfect for my >> purposes. >> >> I sat down tonight to begin probing it for the pinouts when I realized >> that, while I have done any number of HD44780-based projects, I had no >> idea about driving a "raw" LCD. >> >> 1) What voltage range is typical? Is 5VDC sufficient? >> >> 2) What about polarity? Can these things be damaged by reverse >> polarity? >> >> 3) What about series resistance? I know these are *very* low amperage >> devices, but do I need a series resistor with each segment for some >> reason? If so, what size? >> >> 4) Is there going to be a chip-wide contrast pin on this thing, or is >> that up to the hardware that drives it? > ______________________________________________________ Get Your Private, Free Email at http://www.hotmail.com

Michael, driving a raw LCD with a PIC is easy and fun! First, is your LCD multiplexed? and by how much? How many segments are there? and how many pins? The easiest is a 2:1 multiplexed scheme. if your doing HH:MM:SS I would guess you have 6x7=42 segments plus one for the colons = 43 plus 2 commons = 45, make that 46, divide by 2 ... Does your raw LCD have 23 or so pins? then it's 2:1. you need to provide 2 square waves to the commons that have 3 states to them. low, high (5v) and middle (2.5v). No sweat, put a resistor divider network on 2 pins, say 10k, that way for 0V, make the pin an output and low, for 5V, set the pin high and for 2.5V make the pin an input..... for the correct sequence and get a good explanation with diagrams look in a Hitachi data book for LCD chips.... You figure out the segments to light with a look up table, then you have to send that data, then the compliment to create an AC drive. Any DC component will degrade the LCD life.... do this from an interrupt to only change the LCD states at 30hz or so and your done. Works great.... good luck Jason Wolfson jasonEraseME.....lipidex.com > {Original Message removed}

Michael, you may want to concider the PIC16C92X family. It has an on board LCD controller. The controller has its own oscillator, so you can put the microprocessor into sleep mode and keep the display running. It also has an on-board charge pump so you can run the processor at 3V and the display at 5V. regards, Paul Beyerl Technical Sales Mgr.- Future Electronics, Milwaukee Jason Wolfson wrote: {Quote hidden}> Michael, > driving a raw LCD with a PIC is easy and fun! > First, is your LCD multiplexed? and by how much? How many segments > are there? and how many pins? The easiest is a 2:1 multiplexed scheme. > if your doing HH:MM:SS I would guess you have 6x7=42 segments plus one > for the colons = 43 plus 2 commons = 45, make that 46, divide by 2 > ... Does your raw LCD have 23 or so pins? then it's 2:1. > you need to provide 2 square waves to the commons that have 3 states to > them. > low, high (5v) and middle (2.5v). No sweat, put a resistor divider network > on 2 pins, say 10k, that way for 0V, make the pin an output and low, for > 5V, set the pin high and for 2.5V make the pin an input..... > for the correct sequence and get a good explanation with diagrams > look in a Hitachi data book for LCD chips.... > You figure out the segments to light with a look up table, then > you have to send that data, then the compliment to create an AC drive. > Any DC component will degrade the LCD life.... > do this from an interrupt to only change the LCD states at 30hz or so and > your done. Works great.... > good luck > Jason Wolfson > EraseMEjasonlipidex.com > > > {Original Message removed}