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Project Help and Ideas » LCD checking

July 07, 2010
by Mujda
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I'm 90% complete on constructing a Salt Gulp meter (measures river flow) using a combination of the nerdkits thermometer project and an EC meter.

Now, my LCD (4x16 from nerdkit) has given up on me. I'm getting 1st and 3rd lines as blocks, which would indicate no data signal. It was working fine until I had to remove the connector in order to fit it into its project enclosure, and solder a ribbon cable directly onto the LCD baord. I did this v.carefully, there are no bridges terminals. There shouldn't be any heat damage as the IC's aren't close to the connector area.

I've very carefully checked the LCD pins when powered up and can confirm:

1 - 0V 2 - 5V 3 - Contrast, 0.4V, am getting blocks so should be ok 4 - CMD/DATA line, am getting a digital signal, AtMega168 pin 13 5 - 0V 6 - CLK, am getting a digital signal, ATMega168 pin 12 11 - Bit, digital signal, AtMega168 pin 4 12 - Bit, digital signal, AtMega168 pin 5 13 - Bit, digital signal, AtMega168 pin 6 14 - Bit, digital signal, AtMega168 pin 11

The firmware is currently only a slight modification of the Nerdkits thermometer project, and we previously working fine.

All the above seem fine - the MCU is definitely sending data (I can identify varying digital pulses on the scope). Could I have damaged the LCD driver with static? I've tried to be careful will it.

Are there any other checks I can do before throwing it away and ordering another one?

Thanks,

Mark

July 07, 2010
by Rick_S
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I apologize if this seems obvious but you didn't call pin 14 of the LCD the last pin right?? That's a common oops because sometimes the two backlight pins are forgotton and people hook their data lines starting at the end working backward.

Just a thought. Otherwise, your pinout seems right from the book. Could you maybe post a photo of the circuit at both the lcd and where it connects to the microcontroller? Sometimes a fresh pair of eyes (or in this case many pairs ;D ) can help.

Considering you have checked for bridged connections and lifted traces I do agree with you that it seem odd that you would have fried the LCD board and I would guess it's good.

Good luck and let us know,

Rick

August 03, 2010
by Mujda
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Thanks Rick_S for your input. Sorry for the very long delay in responding, but I've finally managed to upload some images of the project in question:

image1 image2 image3 image4

Pins 15 & 16 are the backlight pins, so are unconnected.

The unpleasant things I've done to the LCD are: 1)Notched the corners to get it to fit in its box (oh yes, I certainly should have got a bigger box). There are no tracks in the area of the notches, only the 0V plane. 2)Removed the connector. While doing this, 3 of the upper pads came loose, but all the other pads are in-place and connected to their respective tracks. Of the 3 effected pads, the contrast pin was the only one with a function, hence the red wire on the top. The contrast is working fine, as I get bars in lines 1 & 3.

Well, I guess it is probably beyond repair, but it would be good to have any tips on diagnosing problems for the future. Lesson learnt - get a big enough box...

Thanks, Mujda

August 03, 2010
by Rick_S
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1st thing I'd do, is remove the wires you currently have on there. There is a large amount of bare exposed wire just dying to short out with another piece.

The fact that you get bars on two lines is a good sign. That typically means the display is wanting to work just not getting the data.

After you have the wires removed, I'd do a good job cleaning up the pads. If you have a good magnifying glass or jewlers loupe, I'd then inspect the traces that lead up to the pads even the missing ones and determine if any of the traces were broken leading to a pad.

If you find any, repair them by scraping off the laquer finish and bridging the break with solder and if need be a small piece of bare wire.

Use of a good iron with the proper tip helps, but even a cheapie will work with proper care. The fact that pads were lifted says that they got too hot. However, this is easy to do as I'm well aware. Just try to be careful not to lift more.

The type of wire you are using while handy, I'll admit, is not the greatest. The shielding is not designed for heat, it's designed to be soft to allow for IDC connectors to easily pierce and contact the wire inside. The problem with this is that when heat is applied, it will shrink back quite a bit. If you are determined to use this type of wire, I'd strip and tin (apply solder to) each wire prior to soldering it into the board. Then once tinned, snip the wire short (about 1/16" - 1/8" (2-3mm)) long. This will minimise the shrinkage when installed and the amount of heat time needed to attach it to the display.

Most of all, take your time look it over real good, and have fun. The most you'll be out is the time spent trouble shooting, and who knows, you may just fix it in the process.

Rick

March 03, 2011
by leedawg
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Did your every get the display going? I'm having a similar issue routing the data through a cat 5 cable to a jack on either side and a cat 5 cable between the jacks. How far a distance from the mcu can the lcd be located? I'd imagine the signal dissipates the longer the cable is from the mcu to the lcd. I want to locate the lcd about 5 feet from the mcu.. Thanks for the help.

Lee

March 03, 2011
by Mujda
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Never got my LCD working again. The issue was almost certainly some internal mechanical damage to the display.

As it happens, right now I am using CAT5 cable and RJ45 jacks for a datalogging project. The sensors are on 5m of CAT5 cable. A round trip resistance is about 6 ohms for a single pair. From what you've said (assuming you have a Nerdkit type LCD - HD44780 compliant):

  • Any DC issues are very unlikely to be a problem, the currents you'll be pulling are very small (a few mA for the LCD, and less than a few uA for the control lines). Both the MCU and LCD won't notice.

  • High frequency issues. Long cables can be an issue due to their internal capacitance and inductance. Fortunately, the MCU controls the data rate, so you can slow things down if needed.

  • CAT5 cable. It's produced with 4x twisted pairs with the intention of equal and opposite current/voltage on each side of the pair. I guess you could get cross-talk between each side of the pair.

At 5 foot of cable my guess is all the above are negligible, and you have a bad connection somewhere. RJ45 plugs I've found tricky to put on, even with the right crimping tool. Have you put a scope on the LCD side to see you're getting clean digital signals on the control lines (E,RW & RS) and datalines (DB0 to DB3)?

Mujda

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