Post by thread:[PIC] How to control a dumb LCD

Hi People of the list I'm trying to control an LCD 3 1/2 digit display, this one has no controller, the main reason for use this, is the cost, about 3$ each. the data sheets says that it works with 30 - 150 Hz AC 5V ps, so I build an small oscilator and test the display, all Ok, It works, but how can I control it with the pic without a dedicated chip 4 it. If I connect the oscilator to the common lcd pin and all the segments to a PIC, all segments will lite on, doesn't matter if the ouputs are high or low because it is AC. So, the question is: It's possible to turn on/off the segments setting the tris registers???, so if the pin is input the segment should turn off. I don't know if the small leakeage of the pic's input is enough to turn on the LCD segment because they need almost no current to lite on. Also I was reading, (don't remember where) that an intermediate voltage on a pic's digital input will cause an increment of the pic current. is it true?, if almost all ports are connected to the LCD, it will be dangerous to turn all as inputs?. I know that what I am trying to do is quite weird but if it works will be nice :-). Thanks. Leandro J. Laporta (LU2AOQ) mail: spam_OUTlu2aoqTakeThisOuTyahoo.com wrk: Arg. Assoc. for Space Tech. ham: TCP/IP high speed group HSG __________________________________________________ Do You Yahoo!? Talk to your friends online with Yahoo! Messenger. http://im.yahoo.com

Direct LCD drive with a PIC is real easy. Attached one output pin to the LCD common. Drive the LCD segment inputs with more PIC outputs. Setup an interrupt to run at about 50 hz. At each interrupt switch (invert) the common state and the segment states. Make the segments you want on, the opposite of the common state. The segments you don't want on, make them the same as the common state. This will give you AC across all the LCD segments, lighting the ones you want. For 2:1 or 3:1 multiplexing put a resistor divider across the commons. For a high, drive it high, for a low, drive low, for a mid voltage (2.5 typ), make the pin an input. With a 16C62 and 22 I/O's, you can direct drive a 3:1 mux LCD with up to 57 segments! Jason Wolfson ___________________________________________ Lipidex Corp 50 Franklin Terrace Duxbury MA 02332 781-834-1600 781-834-1601 FAX .....jasonKILLspam@spam@lipidex.com ____________________________________________ > {Original Message removed}

These LCDs require you drive the segments either in-phase (segment off) or out of phase (segment-on) with the back plane signal (5 Vpp square wave, 30 Hz or so). Years ago I did this with an ICL7211. It's a 4 digit BCD to 7 segment decoder/LCD driver in a 40 pin package. The BCD is multiplexed (4 bits plus digit select lines). The other standard approach is to use exclusive or gates on each segment, though this takes a lot of parts! As I recall, Microchip has a chip with an LCD driver in it, though I have not used it. Using your TRIS trick would work for a while, but when you enable the output driver on the pin and then hold the line high or low, you are putting 2.5Vdc across the LCD, and it doesn't like DC (they turn permanently black). If you don't want to use an LCD driver, I think you'll have to go to an interrupt routine that generates the backplane and segment drive. Hoepfully the interrupt response time is regular enough that you will not have a DC component in the drive so the LCD will last. Harold On Mon, 5 Jun 2000 01:02:46 -0300 Lea <lu2aoqKILLspamYAHOO.COM> writes: {Quote hidden}> Hi People of the list > > I'm trying to control an LCD 3 1/2 digit display, this one has no > controller, > the main reason for use this, is the cost, about 3$ each. > the data sheets says that it works with 30 - 150 Hz AC 5V ps, so I > build > an small oscilator and test the display, all Ok, It works, but how > can I > control it with the pic without a dedicated chip 4 it. > If I connect the oscilator to the common lcd pin and all the > segments to a PIC, all segments will lite on, doesn't matter if the > ouputs > are high or low because it is AC. > So, the question is: It's possible to turn on/off the segments > setting the > tris registers???, so if the pin is input the segment should turn > off. > I don't know if the small leakeage of the pic's input is enough to > turn on > the LCD segment because they need almost no current to lite on. > > Also I was reading, (don't remember where) that an intermediate > voltage > on a pic's digital input will cause an increment of the pic current. > is it > true?, if almost all ports are connected to the LCD, it will be > dangerous to > turn all as inputs?. > > I know that what I am trying to do is quite weird but if it works > will be > nice :-). > > Thanks. > Leandro J. Laporta (LU2AOQ) mail: .....lu2aoqKILLspam.....yahoo.com > wrk: Arg. Assoc. for Space Tech. ham: TCP/IP high speed group HSG > > > __________________________________________________ > Do You Yahoo!? > Talk to your friends online with Yahoo! Messenger. > http://im.yahoo.com FCC Rules Online at http://hallikainen.com/FccRules Lighting control for theatre and television at http://www.dovesystems.com ________________________________________________________________ YOU'RE PAYING TOO MUCH FOR THE INTERNET! Juno now offers FREE Internet Access! Try it today - there's no risk! For your FREE software, visit: dl.http://www.juno.com/get/tagj.

LCD's require an AC waveform. There is no reason that the PIC cannot directly generate this waveform. Just tie one PIC output pin the common, and other pins to the segments. You toggle the common pin periodically, and each time you do so you also toggle all the segment pins (try COMF PORTx,F for example to do 8 at a time). To turn a segment on you just make sure that it is toggled 180 degrees out of phase with the common. To turn it off you toggle it in phase with the common. Alternately, you can run each segment line thru an external XOR gate, the other input of which is connected to an oscillator. This is much simpler from a code standpoint, but requires more hardware. Bob Ammerman RAm Systems (high function, high performance, low-level software) {Original Message removed}

There is a Microchip app note on driving raw LCD displays. Don't have the number offhand. I can look it up for you if you can't find it. Lea wrote: {Quote hidden}> Hi People of the list > > I'm trying to control an LCD 3 1/2 digit display, this one has no controller, > the main reason for use this, is the cost, about 3$ each. > the data sheets says that it works with 30 - 150 Hz AC 5V ps, so I build > an small oscilator and test the display, all Ok, It works, but how can I > control it with the pic without a dedicated chip 4 it. > If I connect the oscilator to the common lcd pin and all the > segments to a PIC, all segments will lite on, doesn't matter if the ouputs > are high or low because it is AC. > So, the question is: It's possible to turn on/off the segments setting the > tris registers???, so if the pin is input the segment should turn off. > I don't know if the small leakeage of the pic's input is enough to turn on > the LCD segment because they need almost no current to lite on. > > Also I was reading, (don't remember where) that an intermediate voltage > on a pic's digital input will cause an increment of the pic current. is it > true?, if almost all ports are connected to the LCD, it will be dangerous to > turn all as inputs?. > > I know that what I am trying to do is quite weird but if it works will be > nice :-). > > Thanks. > Leandro J. Laporta (LU2AOQ) mail: EraseMElu2aoqspam_OUTTakeThisOuTyahoo.com > wrk: Arg. Assoc. for Space Tech. ham: TCP/IP high speed group HSG > > __________________________________________________ > Do You Yahoo!? > Talk to your friends online with Yahoo! Messenger. > http://im.yahoo.com