Post by thread:[PIC][EE] Voltage Reference for HD44780 LCD displa

Hi all. I am trying to find a convenient way to (interactively) set the contrast on a "standard" character LCD display, and I wanted to avoid using a mechanical Pot., so I had the hair-brained idea that I could use the comparator's voltage reference to specify the contrast voltage for the LCD display. Basically, the contrast voltage is "guaranteed" to be between the Vss and Vdd, and so is Vref, so can I use the resistor ladder in the PIC to set the Vref (and allow it to be changed dynamically when the LCD is in sub-zero temps for e.g. ....). I have searched for specs on both "typical" LCD character displays, and on the PIC itself, but the LCD specs do not give the current requirements of Vo, and apart from the pin maximum sink/source current, I have not found a current max for the Vref. I am using the PIC18F4320, and there is the statement (page 228/229 of datasheet) that "The RA2 pin can be used as a simple D/A output with limited drive capability", but I can not find anywhere that indicates what the drive capability actually is. Will I need to create the Op-Amp "buffer" as described in that same section? Table 26.3 indicates the Voltage Reference Specifications, but nowhere does it indicate what the max supply current is.... Additionally, I can't seem to find the input current requirements for the LCD display. Initial investigation seems to indicate that it is only used for potential, and no current actually flows... based on the references: http://www.pacificdisplay.com/lcd_cont_adj.htm and http://www.microchip.com/stellent/groups/picmicro_sg/documents/devicedoc/en023491.pdf Essentially, the LCD voltage (Vo) is applied accross the LCD 'pixel', which acts as a capacitor, and thus no significant current flows. I guess I could just measure the current in to the pin, but I would like third-party confirmation that I could just plug the Vref of the PIC to the Vo of the LCD module, and get an easy "Dynamic" non-mechanical contrast adjuster. While I am on the subject, I also thought I could connect the backlight of the LCD module via a resistor and MOSFET to the PWM output of the CCP module, and thus effectively have dynamically adjustable "brightness" as well. More duty-cycle = more brightness. Is there anything in here I have got horribly (or even a little) wrong? Rolf

I just did a similar project last week. A PIC was driving a 120x64 graphic LCD. It needed an adjustable -12V for the contrast/viewing angle adjust. I took one of the PWM outputs and ran it through an inverting amplifier with a gain of 12/5 and put a capacitor across the feedback resistor to get rid of the PWM ripple. That handled the viewing angle. I took another PWM output and drove an N channel FET. The backlight LED went to +5V through a current limit resistor. The PWM varied the brigthness. Worked great! Harold {Quote hidden}> Hi all. > > I am trying to find a convenient way to (interactively) set the contrast > on a "standard" character LCD display, and I wanted to avoid using a > mechanical Pot., so I had the hair-brained idea that I could use the > comparator's voltage reference to specify the contrast voltage for the > LCD display. Basically, the contrast voltage is "guaranteed" to be > between the Vss and Vdd, and so is Vref, so can I use the resistor > ladder in the PIC to set the Vref (and allow it to be changed > dynamically when the LCD is in sub-zero temps for e.g. ....). > > I have searched for specs on both "typical" LCD character displays, and > on the PIC itself, but the LCD specs do not give the current > requirements of Vo, and apart from the pin maximum sink/source current, > I have not found a current max for the Vref. I am using the PIC18F4320, > and there is the statement (page 228/229 of datasheet) that "The RA2 pin > can be used as a simple D/A output with limited drive capability", but I > can not find anywhere that indicates what the drive capability actually > is. Will I need to create the Op-Amp "buffer" as described in that same > section? Table 26.3 indicates the Voltage Reference Specifications, but > nowhere does it indicate what the max supply current is.... > > Additionally, I can't seem to find the input current requirements for > the LCD display. Initial investigation seems to indicate that it is only > used for potential, and no current actually flows... based on the > references: http://www.pacificdisplay.com/lcd_cont_adj.htm and > www.microchip.com/stellent/groups/picmicro_sg/documents/devicedoc/en023491.pdf > Essentially, the LCD voltage (Vo) is applied accross the LCD 'pixel', > which acts as a capacitor, and thus no significant current flows. > > I guess I could just measure the current in to the pin, but I would like > third-party confirmation that I could just plug the Vref of the PIC to > the Vo of the LCD module, and get an easy "Dynamic" non-mechanical > contrast adjuster. > > While I am on the subject, I also thought I could connect the backlight > of the LCD module via a resistor and MOSFET to the PWM output of the CCP > module, and thus effectively have dynamically adjustable "brightness" as > well. More duty-cycle = more brightness. > > Is there anything in here I have got horribly (or even a little) wrong? > > Rolf > -

Hello again, all. Thought I would provide an update, and maybe it'll help someone, even me! After much thought, and gnashing of teeth, I put together the circuit I had conceived. I found that (as expected), the CVref was not able to "drive" the LCD contrast. I put together an op-amp voltage follower to try to accomplish a buffer, but still it did not work. Finally, I took some measurements, and realised that the voltage on the contrast "pin" was at 0.8V when the contrast was most "pleasing", and this was below the threshold for the op-amp (in the single-supply config I had), so it was actually driving the voltage to the 5V rail when the input was lower than about 1V. Anyways, I played with diodes to drop the voltage, and other "extreme" measures. Bottom line is that I made a "discovery", and I wish I had known it before. The Vo (contrast) pin on a "standard" LCD is not supposed to be "Driven", it is supposed to be grounded! All the documents that talk about a "voltage divider" 10K Pot to control contrast are misleading. In playing with my LCD module at least, I discovered that the contrast is proportional to the resistance between Vo and ground, not the voltage on that pin (or perhaps the LCD Module uses the resistance on Vo as part of an internal volt divider). Anyways, to make a long story short, you can not effectively use the CVRef to adjust the contrast of an LCD module because the contrast is not volt dependent, but resistance dependent. Additionally, the 10KPot commonly used for "contrast" adjustment could be simplified to a rheostat config, and that would save some energy from the power source. In my case, adding about 1K fixed resistance in series with a 1K Variable resistor, placed between Vo and Ground, gave me good coverage of the required contrast range. Additionally, there was about 0.3mA current from Vo to Ground, wheras with a 10Kpot there would be 0.8mA in a 5V system. Thus, unless someone points out something I have missed, I intend to investigate a system using 4 pins (I have them spare... I think) on the PIC to ground out different rungs on a resistor ladder in the following arrangement, thus giving me 5 contrast settings.... I may have to play with the actual resistor values a little... LCD Vo ---- | \ / 1K Ohm \ | Pin a ------| | \ / 250 Ohm \ | Pin b ------| | \ / 250 Ohm \ | Pin c ------| | \ / 250 Ohm \ | Pin d ------| | \ / 250 Ohm \ | Gnd --- Anyways, I learned something I though should be shared.... Rolf Rolf wrote: {Quote hidden}> Hi all. > > I am trying to find a convenient way to (interactively) set the > contrast on a "standard" character LCD display, and I wanted to avoid > using a mechanical Pot., so I had the hair-brained idea that I could > use the comparator's voltage reference to specify the contrast voltage > for the LCD display. Basically, the contrast voltage is "guaranteed" > to be between the Vss and Vdd, and so is Vref, so can I use the > resistor ladder in the PIC to set the Vref (and allow it to be changed > dynamically when the LCD is in sub-zero temps for e.g. ....). > > I have searched for specs on both "typical" LCD character displays, > and on the PIC itself, but the LCD specs do not give the current > requirements of Vo, and apart from the pin maximum sink/source > current, I have not found a current max for the Vref. I am using the > PIC18F4320, and there is the statement (page 228/229 of datasheet) > that "The RA2 pin can be used as a simple D/A output with limited > drive capability", but I can not find anywhere that indicates what the > drive capability actually is. Will I need to create the Op-Amp > "buffer" as described in that same section? Table 26.3 indicates the > Voltage Reference Specifications, but nowhere does it indicate what > the max supply current is.... > > Additionally, I can't seem to find the input current requirements for > the LCD display. Initial investigation seems to indicate that it is > only used for potential, and no current actually flows... based on the > references: http://www.pacificdisplay.com/lcd_cont_adj.htm and > http://www.microchip.com/stellent/groups/picmicro_sg/documents/devicedoc/en023491.pdf > Essentially, the LCD voltage (Vo) is applied accross the LCD 'pixel', > which acts as a capacitor, and thus no significant current flows. > > I guess I could just measure the current in to the pin, but I would > like third-party confirmation that I could just plug the Vref of the > PIC to the Vo of the LCD module, and get an easy "Dynamic" > non-mechanical contrast adjuster. > > While I am on the subject, I also thought I could connect the > backlight of the LCD module via a resistor and MOSFET to the PWM > output of the CCP module, and thus effectively have dynamically > adjustable "brightness" as well. More duty-cycle = more brightness. > > Is there anything in here I have got horribly (or even a little) wrong? > > Rolf

Rolf wrote: > Bottom line is that I made a "discovery", and I wish I had known it > before. The Vo (contrast) pin on a "standard" LCD is not supposed to be > "Driven", it is supposed to be grounded! But that would not allow for any adjustment, whether resistive or a voltage. > All the documents that talk > about a "voltage divider" 10K Pot to control contrast are misleading. In > playing with my LCD module at least, I discovered that the contrast is > proportional to the resistance between Vo and ground, not the voltage on > that pin And how did you tell the difference? Didn't the voltage vary as the resistance varied? How can you decide which is driving and which one is following along? > Thus, unless someone points out something I have missed, The data sheet apparently. Any one LCD at a particular temperature and other operating point characteristics probably exhibits a fixed relationship between contrast pin voltage and resistance to ground on that pin. However due to the way LCDs work, it's the voltage on that pin that is the important attribute. By providing only a resistance, you are counting on the effective impedence of the contrast pin to set the voltage. This is is not well specified and will probably vary over parts and conditions much more than the voltage to produce the same contrast. ****************************************************************** Embed Inc, Littleton Massachusetts, (978) 742-9014. #1 PIC consultant in 2004 program year. http://www.embedinc.com/products

See inline. Olin Lathrop wrote: > Rolf wrote: > >> Bottom line is that I made a "discovery", and I wish I had known it >> before. The Vo (contrast) pin on a "standard" LCD is not supposed to be >> "Driven", it is supposed to be grounded! >> > > But that would not allow for any adjustment, whether resistive or a voltage. > You are correct in the pedantic sense, but you missed the part where i describe a resistor ladder to implement adjustment. I thought it would be relatively obvious given the remainder of my mail. {Quote hidden}> >> All the documents that talk >> about a "voltage divider" 10K Pot to control contrast are misleading. In >> playing with my LCD module at least, I discovered that the contrast is >> proportional to the resistance between Vo and ground, not the voltage on >> that pin >> > > And how did you tell the difference? Didn't the voltage vary as the > resistance varied? How can you decide which is driving and which one is > following along? > Actually, I could tell the difference by lacing an LED in-line with Vo. Current flows FROM Vo to ground, and not the other way around. {Quote hidden}> >> Thus, unless someone points out something I have missed, >> > > The data sheet apparently. Any one LCD at a particular temperature and > other operating point characteristics probably exhibits a fixed relationship > between contrast pin voltage and resistance to ground on that pin. However > due to the way LCDs work, it's the voltage on that pin that is the important > attribute. By providing only a resistance, you are counting on the > effective impedence of the contrast pin to set the voltage. This is is not > well specified and will probably vary over parts and conditions much more > than the voltage to produce the same contrast. > > > Quoting your words: "However due to the way LCDs work, it's the voltage on that pin that is the important attribute.", it is exactly this that I have read elsewhere, and it is exactly this that is wrong (in my case)!. It is NOT *only* the voltage that counts. If this were the case, then you could put a Diode between the wiper of your POT, and the Vo, like so: Vdd ---- | \ 10K / \<------------->|--- Vo / | | Gnd ---- When in fact, the above will not work even if the Voltage at Vo is correct, the current still needs to flow (from out of Vo). In fact, I recall in my experimenting that the circuits: Gnd -------/\/\/\/\------Vo and Gnd --------/\/\/\/\-----|<--------Vo produced the same contrast for any given resistance, and thus, the Vo must have been different in each case. I will confirm this at home again. > ****************************************************************** > Embed Inc, Littleton Massachusetts, (978) 742-9014. #1 PIC > consultant in 2004 program year. http://www.embedinc.com/products > Imperial evidence (my testing) has revealed that there is more to the issue than just the voltage.... and in fact, the voltage (at least in my module) is not the determining variable, it is the dependant variable. In all cases, it is the resistance between Vo and Ground that influenced the Contrast. Thus, while what you say makes sense (and it did to me before I started this exercise...), the reality I have on the desk at home does not follow what I originally understood to be the case. Which is why I am raising this issue. Rolf

Rolf wrote: >> And how did you tell the difference? Didn't the voltage vary as the >> resistance varied? How can you decide which is driving and which one >> is following along? >> > Actually, I could tell the difference by lacing an LED in-line with Vo. > Current flows FROM Vo to ground, and not the other way around. So? That only shows the direction of current flow. It says nothing about whether voltage or resistance is a better means to control the contrast. You are completely missing the point I made. {Quote hidden}> Quoting your words: "However due to the way LCDs work, it's the voltage > on that pin that is the important attribute.", it is exactly this that I > have read elsewhere, and it is exactly this that is wrong (in my case)!. > > It is NOT *only* the voltage that counts. If this were the case, then > you could put a Diode between the wiper of your POT, and the Vo, like > so: > > Vdd ---- > | > \ 10K > / > \<------------->|--- Vo > / > | > | > Gnd ---- No. That only shows that current flows out of the Vo pin instead of into it, which has nothing to do with your premise that contrast is inherently adjusted by resistance. > Imperial evidence (my testing) has revealed that there is more to the > issue than just the voltage.... and in fact, the voltage (at least in my > module) is not the determining variable, it is the dependant variable. > In all cases, it is the resistance between Vo and Ground that influenced > the Contrast. But didn't voltage vary as you varied the resistance (I'm sure it did)? So how can you possibly say which was the direct cause of the contrast change when both varied? In other words this is a flawed experiment because there was no control for the voltage variable. Ideally you would want to hold the voltage contstant while changing the resistance. However, the LCD already dictates a relationship between voltage and current, and this is therefore impossible. About the only way you can do this experiment correctly is to find different operating points for the LCD where there are different relationships between the voltage and current. Then at each of those operating points apply a fixed voltage and separately a fixed resistance and measure if the resulting contrasts had less variation accross fixed voltages or fixed resistances. What the data sheet is trying to tell you (and which makes sense from how LCDs work) is that you will get a smaller variation with fixed voltage accross the range of operating points. ****************************************************************** Embed Inc, Littleton Massachusetts, (978) 742-9014. #1 PIC consultant in 2004 program year. http://www.embedinc.com/products

Olin Lathrop wrote: {Quote hidden}> Rolf wrote: > >>> And how did you tell the difference? Didn't the voltage vary as the >>> resistance varied? How can you decide which is driving and which one >>> is following along? >>> >>> >> Actually, I could tell the difference by lacing an LED in-line with Vo. >> Current flows FROM Vo to ground, and not the other way around. >> > > So? That only shows the direction of current flow. It says nothing about > whether voltage or resistance is a better means to control the contrast. > You are completely missing the point I made. > > >> Quoting your words: "However due to the way LCDs work, it's the voltage >> on that pin that is the important attribute.", it is exactly this that I >> have read elsewhere, and it is exactly this that is wrong (in my case)!. >> >> It is NOT *only* the voltage that counts. If this were the case, then >> you could put a Diode between the wiper of your POT, and the Vo, like >> so: >> >> Vdd ---- >> | >> \ 10K >> / >> \<------------->|--- Vo >> / >> | >> | >> Gnd ---- >> > > No. That only shows that current flows out of the Vo pin instead of into > it, which has nothing to do with your premise that contrast is inherently > adjusted by resistance. > > >> Imperial evidence (my testing) has revealed that there is more to the >> issue than just the voltage.... and in fact, the voltage (at least in my >> module) is not the determining variable, it is the dependant variable. >> In all cases, it is the resistance between Vo and Ground that influenced >> the Contrast. >> > > But didn't voltage vary as you varied the resistance (I'm sure it did)? So > how can you possibly say which was the direct cause of the contrast change > when both varied? In other words this is a flawed experiment because there > was no control for the voltage variable. > > Ideally you would want to hold the voltage contstant while changing the > resistance. However, the LCD already dictates a relationship between > voltage and current, and this is therefore impossible. About the only way > you can do this experiment correctly is to find different operating points > for the LCD where there are different relationships between the voltage and > current. Then at each of those operating points apply a fixed voltage and > separately a fixed resistance and measure if the resulting contrasts had > less variation accross fixed voltages or fixed resistances. What the data > sheet is trying to tell you (and which makes sense from how LCDs work) is > that you will get a smaller variation with fixed voltage accross the range > of operating points. > > > ****************************************************************** > Embed Inc, Littleton Massachusetts, (978) 742-9014. #1 PIC > consultant in 2004 program year. http://www.embedinc.com/products > Let's back it up a bit. Here's the rub. I thought, and based on previous mails: 1. I could use the CVref to create the necessary (adjustable) voltage to set the contrast. 2. If I could not do it directly, that I could use an Op-Amp as a Buffer / Voltage Follower to accomplish the same. Here's the problem... with 1. the resistor network in CVref is to Vdd, and thus, even though the voltage may be right, the current can not flow, and the LCD is "blank". with 2. putting the LCD Vo at the output of the OpAmp causes reverse currents through the OpAmp (I think), and it is not stable at the low voltages required for contrast. In my case, it would be too "dim", and then, as it approached "close", the op-amp would jump to the Vdd rail, and the display would "blank". Placing voltage-dropping Diodes after the Op-amp to get the right voltage range did not solve the problem, and screen is still blank. Bottom line for me.... Controlling *just* the voltage at Vo does not control contrast. Futher experiments using a voltage-divider after the buffer (and thus putting the buffer in to it's linear range) discovered that it did not matter what voltage I put at Vo, it was the resistance between Vo and ground that changed contrast.... in other words, I had the following: CVRef -------+|\ | \ | >----------/\/\/\/\/\-----Gnd | / | ^ |-|/ | | | | ---------Vo ------- Experimenting indicated that it did not matter what the Voltage at CVref was (from 0.0V through 3.8V), that it was only the position of the Pot that mattered (Admittedly, below the Min input voltage of the buffer, the op-amp would go to the +5V rail.....). So, I concluded that the Vo is not Voltage dependant, but resistance dependant. I then confirmed it by removing all components other than what became the rheostat, and behaviour was consistant with just a resistance (to ground) controlled device. Admittedly, I am new to most of this, op-amps included, and everything I know about them I learned in the past week. Additionally, my tools are crude, and methods are in-experienced.... yet, to me this is pretty conclusive.... Rolf

Rolf wrote: > Here's the rub. I thought, and based on previous mails: > 1. I could use the CVref to create the necessary (adjustable) voltage to > set the contrast. What is CVref? I don't remember you defining this. > 2. If I could not do it directly, that I could use an Op-Amp as a Buffer > / Voltage Follower to accomplish the same. Probably true. What is the useful range of Vo according to the data sheet? > Here's the problem... > with 1. the resistor network in CVref is to Vdd, I have no idea what that is supposed to mean. > and thus, even though > the voltage may be right, the current can not flow, and the LCD is > "blank". OK, you saw a blank LCD. I'm not convinced your assumed reasons for that are correct. I particularly don't get the part about the voltage being right but the current not flowing. This leads me to suspect a misunderstanding of the circuit, and perhaps some of electronics in general. > with 2. putting the LCD Vo at the output of the OpAmp causes > reverse currents through the OpAmp (I think), and it is not stable at > the low voltages required for contrast. In my case, it would be too > "dim", and then, as it approached "close", the op-amp would jump to the > Vdd rail, and the display would "blank". I'm not totally sure what the circuit was, but it sounds like a bad opamp circuit or the wrong opamp for the job. It sounds like you need an opamp with output that is "rail to rail" or at least goes down to the low rail. Unfortunately there is a lot gray area and sometimes outright misinformation in spec sheets pertaining to this. You have to know what kind of fish to wave and exactly how to wave it. In general, us a CMOS opamp. All else being equal, these are more likely to be able to drive their outputs close to the low rail without jumps or other unexpected behavior. > Placing voltage-dropping Diodes after the > Op-amp to get the right voltage range did not solve the problem, and > screen is still blank. That makes sense as the voltage drop on a diode will make the opamp output rail-to-rail problem worse. > Bottom line for me.... Controlling *just* the voltage at Vo does not > control contrast. Not really. I think the problem was that you weren't controlling the voltage as you thought. > Futher experiments using a voltage-divider after the buffer (and thus > putting the buffer in to it's linear range) discovered that it did not > matter what voltage I put at Vo, it was the resistance between Vo and > ground that changed contrast. I find this very hard to believe. Did you measure Vo as you changed the resistance? I'm sure it changed. The difference is that now you had a circuit that was able to change the voltage, even if it was only loosely controlled by controlling a resistance. > Experimenting indicated that it did not matter what the Voltage at CVref > was (from 0.0V through 3.8V), that it was only the position of the Pot > that mattered Again, what is CVref and what does it have to do with this? I thought the job was to control Vo. The load on Vo may be non-linear with voltage, maybe your driving impedence was wrong, I don't know. Measure Vo as you change the pot and you will see it change. > So, I concluded that the Vo is not Voltage dependant, but resistance > dependant. I then confirmed it by removing all components other than > what became the rheostat, and behaviour was consistant with just a > resistance (to ground) controlled device. Again, measure the Vo voltage as you are doing this. You will see it change. > yet, to me this is pretty conclusive.... Sorry, but it isn't since you haven't (can't) separate the Vo voltage from the resistive load applied to Vo. Another thing to consider. LCDs are inhernetly switched devices. The current coming out of Vo may be very pulsed. The voltage probably only matters during one phase of this pulse. All this means there should probably be a capacitor on Vo to keep it at a "constant" voltage. I'm just guessing so check the data sheet. Unless it says not to, try putting 100nF on Vo with respect to ground. ****************************************************************** Embed Inc, Littleton Massachusetts, (978) 742-9014. #1 PIC consultant in 2004 program year. http://www.embedinc.com/products

Spehro Pefhany wrote: {Quote hidden}> At 05:04 PM 12/21/2005 +0100, you wrote: > >>> Here's the problem... >>> (snip) >>> Bottom line for me.... Controlling *just* the voltage at Vo does not >>> control contrast. >>> >> If I read your post correctly you made a bad voltage buffer, it did not >> work, and now your conclusion is that voltage driving does not work???? >> >> >>> yet, to me this is pretty conclusive.... >>> >> It is, but conclusive of what excatly? That you need much more >> experience before you jump to conclusions? >> >> A free tip (as in free beer): Olin is not known for his subtle human >> interface, but I don't recall him being wrong on electrical matters. >> > > Yes. Ideally you feed Vo a voltage V(T) which varies with temperature. The > function of temperature depends on the LCD type. You can add a BF > for the user to fiddle with the contrast depending on viewing angle, lighting > conditions and personal preference. > > Sounds like he was using a bipolar op-amp for a buffer than would not swing > to the -ve rail, and further one that had the phase reversal "feature". > > > Best regards, > That's what I like about this list ..... I was using TL082 http://www.national.com/pf/TL/TL082.html Wouter, I have "lurked" around long enough to get a sense of the regular players here at piclist, and I take no offense with Olin. In fact, I am always happy to be corrected, and so on. In my professional capacity (as opposed to PICs which are a hobby) I find that it is sometimes refreshing for a "un-filtered" honest opinion. I am fully aware that Olin's and most other people's experience far exceeds mine (otherwise I would not have asked questions). As I expressed in a post a while ago, one of the values of experience in Electronics appears to be in being able to choose the right part for the job, and I guess the Op-Amp I chose was not the right one. I had based some of my experiments on http://www.maxim-ic.com/appnotes.cfm/appnote_number/3658/ln/en and that is why I was so confused by the lack of correlation between my expectations and "reality". I will get a "rail to rail" opamp and see if I get better results. While I have everyone's ear, what chip would be suggested in my situation... ;-) Narrowing the field down, MicroChip has OpAmps, and given the Maxim app-note above with the Op-Amp controlling the Backlight as well, I think a dual op-amp would be useful for me. Additionally, I am bread-boarding things... (so DIP is good). Finally samples are great, so, I would think that either of: MCP6002 or MCP6042 would work for me. There are a bunch of others to choose from at http://www.microchip.com/ParamChartSearch/chart.aspx?branchID=110153&mid=11&lang=en&pageId=79 I see that Olin has another response... I will respond to that too. Thanks all Rolf

Olin Lathrop wrote: > Rolf wrote: > >> Here's the rub. I thought, and based on previous mails: >> 1. I could use the CVref to create the necessary (adjustable) voltage to >> set the contrast. >> > > What is CVref? I don't remember you defining this. > It is the Comparator Voltage Reference output. > >> 2. If I could not do it directly, that I could use an Op-Amp as a Buffer >> / Voltage Follower to accomplish the same. >> > > Probably true. What is the useful range of Vo according to the data sheet? > Hmmm... got me there - no-name brand LCD module with real HD44780 chip - found in surplus store. No useful identifying features I could find. > >> Here's the problem... >> with 1. the resistor network in CVref is to Vdd, >> > > I have no idea what that is supposed to mean. > Looking at block-diagram of Comparator Voltage Reference module of PIC18F4320 datasheet (http://www.microchip.com/stellent/idcplgidcplg?IdcService=SS_GET_PAGE&nodeId=1335&dDocName=en010287) I see the resistor network may in fact be referenced to Vss, but the Mux is a "black box". Regardless, I can't seem to be able to sink any current through CVref pin when enabled. {Quote hidden}> >> and thus, even though >> the voltage may be right, the current can not flow, and the LCD is >> "blank". >> > > OK, you saw a blank LCD. I'm not convinced your assumed reasons for that > are correct. I particularly don't get the part about the voltage being > right but the current not flowing. This leads me to suspect a > misunderstanding of the circuit, and perhaps some of electronics in general. > > I'm getting a better understanding of my deficiencies with op-amps, and I am sure your last statements are accurate! {Quote hidden}>> with 2. putting the LCD Vo at the output of the OpAmp causes >> reverse currents through the OpAmp (I think), and it is not stable at >> the low voltages required for contrast. In my case, it would be too >> "dim", and then, as it approached "close", the op-amp would jump to the >> Vdd rail, and the display would "blank". >> > > I'm not totally sure what the circuit was, but it sounds like a bad opamp > circuit or the wrong opamp for the job. It sounds like you need an opamp > with output that is "rail to rail" or at least goes down to the low rail. > Unfortunately there is a lot gray area and sometimes outright misinformation > in spec sheets pertaining to this. You have to know what kind of fish to > wave and exactly how to wave it. In general, us a CMOS opamp. All else > being equal, these are more likely to be able to drive their outputs close > to the low rail without jumps or other unexpected behavior. > > I gather that (now). {Quote hidden}>> Placing voltage-dropping Diodes after the >> Op-amp to get the right voltage range did not solve the problem, and >> screen is still blank. >> > > That makes sense as the voltage drop on a diode will make the opamp output > rail-to-rail problem worse. > > >> Bottom line for me.... Controlling *just* the voltage at Vo does not >> control contrast. >> > > Not really. I think the problem was that you weren't controlling the > voltage as you thought. > > >> Futher experiments using a voltage-divider after the buffer (and thus >> putting the buffer in to it's linear range) discovered that it did not >> matter what voltage I put at Vo, it was the resistance between Vo and >> ground that changed contrast. >> > > I find this very hard to believe. Did you measure Vo as you changed the > resistance? I'm sure it changed. The difference is that now you had a > circuit that was able to change the voltage, even if it was only loosely > controlled by controlling a resistance. > > Well, I did a *lot* of things. And much of it is in the too-early hours of the morning. I certainly did measure Voltage at Vo, and I changed Vo by changing the voltage at the top of the voltage divider by changing the CVref output from the PIC. I did measure very similar voltages at the PIC Pin, and at the buffer output. The voltage divider 10Kpot did change the contrast, but if I kept the pot unchanged, then changing CVref did not *appear* to change contrast. But, perhaps my voltage divider was such that a change in the voltage at the top of the Pot was not suffient to change the Vo noticably (visible contrast wise). {Quote hidden}>> Experimenting indicated that it did not matter what the Voltage at CVref >> was (from 0.0V through 3.8V), that it was only the position of the Pot >> that mattered >> > > Again, what is CVref and what does it have to do with this? I thought the > job was to control Vo. The load on Vo may be non-linear with voltage, maybe > your driving impedence was wrong, I don't know. Measure Vo as you change > the pot and you will see it change. > > I did draw an ASCII circuit.... ;-) CVRef -------+|\ | \ | >----------/\/\/\/\/\-----Gnd | / | ^ |-|/ | | | | ---------Vo ------- Again, CVref is the Reference voltage generated by the Comparator Voltage Reference Module. I fed it in to the OpAmp (TL082) and I was able to measure (mostly) linear Vout from the buffer with a 1:1 gain. >> So, I concluded that the Vo is not Voltage dependant, but resistance >> dependant. I then confirmed it by removing all components other than >> what became the rheostat, and behaviour was consistant with just a >> resistance (to ground) controlled device. >> > > Again, measure the Vo voltage as you are doing this. You will see it > change. > > I will do more homework tonight. My recollection fuzzy with these details, but is that it may have changed, although the contrast "appearance" did not.... But I may be wrong with that as well. I now think that everything was happening so close to the low-end of the Op-amp's linear region that all the observations are meaningless. {Quote hidden}>> yet, to me this is pretty conclusive.... >> > > Sorry, but it isn't since you haven't (can't) separate the Vo voltage from > the resistive load applied to Vo. > > Another thing to consider. LCDs are inhernetly switched devices. The > current coming out of Vo may be very pulsed. The voltage probably only > matters during one phase of this pulse. All this means there should > probably be a capacitor on Vo to keep it at a "constant" voltage. I'm just > guessing so check the data sheet. Unless it says not to, try putting 100nF > on Vo with respect to ground. > > > ****************************************************************** > Embed Inc, Littleton Massachusetts, (978) 742-9014. #1 PIC > consultant in 2004 program year. http://www.embedinc.com/products > Thanks for the input... I will try things with better (more appropriate) parts (will probably be after Christmas....). I will have to try harder to find a datasheet for my particular LCD module, the one I have gives absolute-maxes and that's about it. It mostly regurgitates the HD44780 "standard" interface documentation (bad translations and all). I thknk I should halod off more experiments until I get a better buffer set up. Rolf