Post by thread:LCD displays, a fine point

On Wed, 10 Feb 1999 17:49:24 -0500 dave vanhorn <spam_OUTdvanhornTakeThisOuTCEDAR.NET> writes: >When E is low, and R/W is low, isn't the display supposed to tri-state >from >the data lines? >Did I miss something? > The 44780 has pull-ups on them. Another fine point is when E is low, R/W is don't care. ___________________________________________________________________ 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/getjuno.html or call Juno at (800) 654-JUNO [654-5866]

>>When E is low, and R/W is low, isn't the display supposed to tri-state >>from >>the data lines? >>Did I miss something? > >The 44780 has pull-ups on them. Another fine point is when E is low, R/W >is don't care. > I didn't think R/W was essential, but the docs were less than clear. How strong a pullup are we talking? What I'm seeing is that my processor can only pull them down about 0.5V. There may be some hardware fault, but I'm trying to make sure that it OUGHT to work, before I start hacking up our one and only proto.

dave vanhorn wrote: > How strong a pullup are we talking? What I'm seeing is that my processor > can only pull them down about 0.5V. > There may be some hardware fault, but I'm trying to make sure that it OUGHT > to work, before I start hacking up our one and only proto. Are you sure the LCD pin #1 (ground) has the same ground from the processor? The pull up is around 60k Ohms, it draws 73uA to drop a data pin to around 0.5Vdc. Wagner Lipnharski UST Research Inc http://ustr.net

>> There may be some hardware fault, but I'm trying to make sure that it OUGHT >> to work, before I start hacking up our one and only proto. > >Are you sure the LCD pin #1 (ground) has the same ground >from the processor? Yes, solid connection. I'm having trouble talking to other devices while the LCD is supposed to be sleeping. Data 6 seems to be pulling hard high from somewhere. Since the LCD works, it is my most likely culprit. This pin is "data" to three other devices, and I can't talk to them. >The pull up is around 60k Ohms, it draws 73uA to drop a data pin to around 0.5Vdc. One would think that a micro ought to be able to yank such a pin to ground :) In fact the adjacent pin (data 7) is used in the same manner, as clock for the other devices, with no problem.

Do a simple test, change your program to make all lines down to the LCD and free zes that way in an infinite loop or something, except the Data 6 pin, then using a micro/milliampermeter short circuit that pin to ground and measure the current. A low level at the /CE LCD pin would make it almost disconnected from the bus, only the internal "pullup resistors" would be active. Make sure no other device is selected and d riving the Data 6 pin, for sure without your consentment.... :) Wagner Lipnharski. dave vanhorn wrote: {Quote hidden}> > >> There may be some hardware fault, but I'm trying to make sure that it OUGHT > >> to work, before I start hacking up our one and only proto. > > > >Are you sure the LCD pin #1 (ground) has the same ground > >from the processor? > > Yes, solid connection. I'm having trouble talking to other devices while > the LCD is supposed to be sleeping. > Data 6 seems to be pulling hard high from somewhere. Since the LCD works, > it is my most likely culprit. > This pin is "data" to three other devices, and I can't talk to them. > > >The pull up is around 60k Ohms, it draws 73uA to drop a data pin to around > 0.5Vdc. > > One would think that a micro ought to be able to yank such a pin to ground > :) In fact the adjacent pin (data 7) is used in the same manner, as clock > for the other devices, with no problem.

If the current is high (more than 2mA) when shorting Data 6 to ground, while all other LCD pins are low, try to cut and isolate that Data 6 from LCD to board an d repeat the current measurement, now from the board to ground, and then from the LCD to ground... th en you found it. Wagner. dave vanhorn wrote: {Quote hidden}> > >> There may be some hardware fault, but I'm trying to make sure that it OUGHT > >> to work, before I start hacking up our one and only proto. > > > >Are you sure the LCD pin #1 (ground) has the same ground > >from the processor? > > Yes, solid connection. I'm having trouble talking to other devices while > the LCD is supposed to be sleeping. > Data 6 seems to be pulling hard high from somewhere. Since the LCD works, > it is my most likely culprit. > This pin is "data" to three other devices, and I can't talk to them. > > >The pull up is around 60k Ohms, it draws 73uA to drop a data pin to around > 0.5Vdc. > > One would think that a micro ought to be able to yank such a pin to ground > :) In fact the adjacent pin (data 7) is used in the same manner, as clock > for the other devices, with no problem.

At 12:41 AM 2/11/99 -0500, Wagner Lipnharski wrote: >If the current is high (more than 2mA) when shorting Data 6 to ground, while >all other LCD pins are low, try to cut and isolate that Data 6 from LCD to >board and repeat the current >measurement, now from the board to ground, and then from the LCD to >ground... then you found it. That's about where I'm at.. :( I hate to start cutting, but I'm not seeing a lot of options. It's all SMD on both sides of the board. Thanks for the sanity check though, at least I know it ought to work.

I just got done building my second prototype with a 44780 driver, and I swear the first one tri-states only when E and R/W are BOTH low (I tested this by measuring the pins as I switched R/W) and the second one doesn't tri-state even when they are both low. The boards and the chips on them are exactly the same, except one has a Phillips 22V10 and the other an AMD. Anyone have an explanation? I'll be working on these boards for the next few days, so I'll keep you posted on my efforts. I'm hoping to find I've gone brain dead somewhere, but right now it seems unlikely. Don

Donald L Burdette wrote: >I just got done building my second prototype with a 44780 driver, and I >swear the first one tri-states only when E and R/W are BOTH low (I tested >this by measuring the pins as I switched R/W) and the second one doesn't >tri-state even when they are both low. The boards and the chips on them >are exactly the same, except one has a Phillips 22V10 and the other an >AMD. > >Anyone have an explanation? I'll be working on these boards for the next >few days, so I'll keep you posted on my efforts. I'm hoping to find I've >gone brain dead somewhere, but right now it seems unlikely. > > Don Don, you do know the HD44780 has a MOS weak pull-up on almost all the I/O lines? This means if the input floats (tristated) it will be pulled high. MOSFET pull-up current is specified at 50 to 250uA with a typical value of 125uA. Try adding a 10K resistor to ground as a test and you should see the pin "float" about in the middle of the voltage range. Other thoughts: the data sheet wants R/W to be stable 25 nsec before E goes high and to stay stable for 25 nsec after E goes low again. The data lines should go "tristate" (with MOS weak pull-ups always ON) if E OR R/W is low. E is the only microprocessor side pin which does not have a weak pull-up on it. Good Luck, Chip

Donald L Burdette wrote: >I just got done building my second prototype with a 44780 driver, and I >swear the first one tri-states only when E and R/W are BOTH low (I tested >this by measuring the pins as I switched R/W) and the second one doesn't >tri-state even when they are both low. The boards and the chips on them >are exactly the same, except one has a Phillips 22V10 and the other an >AMD. > Chip Weller wrote: >Don, you do know the HD44780 has a MOS weak pull-up on almost all the I/O >lines? This means if the input floats (tristated) it will be pulled high. >MOSFET pull-up current is specified at 50 to 250uA with a typical value of >125uA. Try adding a 10K resistor to ground as a test and you should see the >pin "float" about in the middle of the voltage range. There are only three I/O other than the data lines. I was not aware that R/W or RS had pullups, but it shouldn't matter as they are driven by strong enough drivers that are never tri-stated. Still, that would be rather a PITA for low-power applications (for which LCD's are otherwise perfect). >Other thoughts: the data sheet wants R/W to be stable 25 nsec before E goes >high and to stay stable for 25 nsec after E goes low again. The data lines >should go "tristate" (with MOS weak pull-ups always ON) if E OR R/W is low. >E is the only microprocessor side pin which does not have a weak pull-up on >it. The data book from Hitachi shows the pullups disabled when "output enable" is inactive. Of course, there's no indication of what "output enable" is or when it's active. I assumed it would be disabled when E was low, but it turns out that R/W must be low too (I think). I verified this on my first prototype, but it doesn't seem to work the same on the second copy. Don

>The data book from Hitachi shows the pullups disabled when "output >enable" is inactive. Of course, there's no indication of what "output >enable" is or when it's active. I assumed it would be disabled when E >was low, but it turns out that R/W must be low too (I think). I verified >this on my first prototype, but it doesn't seem to work the same on the >second copy. > > Don I'm the sod who started this thread.. I determined that mine definitely does tristate when the E and R/W are low. I have not tested it wirh R/W high yet (it's working, don't muck with it!) My problem turned out to be a bad output on the CPU, (SMD, replacement was a real pain).