Screen re-calibration?


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canderson

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@Niranjan
Are you saying that when you press on the screen, it is selecting the wrong thing? If that is correct, you can try adjustment using a special calibration file in the root of your device. Can you tell us what is being selected when you press various places on the screen? Is everything too high or two low or left or right or ???

An adjustment can be made by placing a file called cal.txt in the root directory of your 720. All firmware versions since 8.051 require the sequence in this order:
MaxX MinX MaxY MinY
(where X = left/right and Y = up/down)

For Left/Right calibration:

MaxX affects the LEFT edge of the screen
Increase MaxX - touch point moves to the RIGHT of where you touched
Decrease MaxX - touch point moves to the LEFT of where you touched

MinX affects the RIGHT edge of the screen
Increase MinX - touch point moves to the RIGHT of where you touched
Decrease MinX - touch point moves to the LEFT of where you touched

For the Y axis, the process is the same only in the up/down axis.

I had this same problem with my old GO 740 and had to make some adjustment to access the bottom areas of the screen. My file looks like this:

926 77 830 200

If you press and hold the reset button in the bottom of your unit briefly. Now, press and hold the power button until the screen appears with white text on black background, then you can release the button. You will see a line on the screen that looks like this:

Touchscreen calib. data: 76 932 140 861

Beware! On THAT screen, the data is MinX MaxX MinY MaxY. Don't use that order when you create your cal.txt file! Anyway, this shows you where the unit calibration is by default if there is not any cal.txt file to change it., so you use this as a starting point for your adjustment. If your screen showed the numbers above, that would the same as if you had created the cal.txt file that looked like this: 932 76 861 140

You should create the file with Notepad or another very basic text editor that will allow you to create the file as BARE text and does not require line break at the end of lines.
 
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For info... post moved to it's own topic. OP informed by PM
 

canderson

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Thanks, Andy. Probably a good idea. Now it won't get lost.
 
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Yes, now the original source for that info has dissappeared, we might become the new global resourse!

I just hope Niranjan sees my PM and finds his post here!
 

canderson

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Yes, I saw that the website where this was fairly well described before is gone entirely.
 
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Sad, but inevitable. I'll tell you the story one day
 
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Mr. Canderson,

Still I couldn't operate my GO 720 Tomtom. Thanks for your great help indeed. My Divice Touchscreen calib. data: 85 930 100 821 @3300mV.How can I change This......Please tell me.

Thanks.
:sad:
 
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You create a text file and call it "cal.txt"
You put that file in the root directory of your TomTom (i.e. not inside any folder)
In that file you put four numbers separated by spaces.

Start off with the numbers you see on the info screen (but in the new order as described by canderson above).

Then slowly change the numbers (re-saving the file each time and restarting the TomTom each time) to move the screen calibration around, following the rules as described above.
By experimentsation, you should be able to improve it.
 

canderson

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Andy's advice, squared, but it would help to know what direction is the problem else we are unable to suggest a direction for number adjustment that will help.

Please review Post #2.
 
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As I recall the suggestion was to use a stylus to detect the calibration movement as fingers are a bit cumbersome for delicate adjustments.
 
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canderson

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I use the closed tip of a pen to get more precise locations, yes.
 
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Hi, My GO750 has started doing the same thing. When entering an address, if you press"W" you get an "E" always one key to the right.
I dont seem to have Touchscreen Calibration line anywhere.
Any assistance would be greatly appreciated
Cheers
Alan
 
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You make it yourself (i.e. you ADD it)

You create a text file and call it "cal.txt"
You put that file in the root directory of your TomTom (i.e. not inside any folder)
In that file you put four numbers separated by spaces.....
 

Arno

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My first firmware version was 8.002 on my 930.
Just now I looked at the root of the internal, as well as on the SD card, and see no cal.txt file.

I have no calibration problems, using HOME2 or the remote mostly anyway.
Do you suggest to make that file, just in case, and populate it with the data as per your description.

I could use something like this for the Via 1535 and the GO LIVE 1535 !
On either of those I have to almost touch the left line not to get the letter to the right.
 

canderson

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@Arno
No unit comes with the factory with a cal.txt file. If they exist, it is always because they are added by the user. The original calibration is burned in at the factory in an area of flash that isn't accessible to the user, so a cal.txt file is required to override that information in the event that the calibration slips.
 
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Thanks Andy and Canderson,
I created cal.txt in the root directory with the default value 827 224 656 330
(Tomtom file 224 827 330 656) the adjusted the X Max & X Min as per your instructions.
Final cal.txt = 730 180 656 330 which seems to be operating correctly again.

Just out of curiosity what causes the unit to need recalibrating as it never leaves the car?

Thanks again
 
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canderson

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Just out of curiosity what causes the unit to need recalibrating as it never leaves the car?
Good doggone question. After many years of fiddling with TT units, it was finally necessary to make a change to my GO 740 Live. My 'antique' GO 720 still retains its calibration without issues. The 740 had a bit of a vertical shift that was finally becoming frustrating enough to dive in and try a cal.txt file.

I can tell you WHAT happens, but I have never researched the technology closely enough to tell you WHY it happens.

Touch screens come in two primary flavors... resistive and capacitive. The resistive screen works with pressure on layers of the screen itself, and the capacitive screen works with the presence of an external 'object' (you).

The resistive version is made up of two VERY thin layers of material (one that is metallic, would you believe?) so thin that most of the light from the LDC display makes it through the layers -- else you wouldn't be able to see the display underneath. Where you press on the screen creates a different resistance to occur in both X and Y between the two layers on the touch screen, and that resistance in conjunction with a very carefully controlled source voltage is used to create different inputs to the processor. The voltages that the processor sees with a press determine where it thinks you pressed on the screen.

What happens to the resistive version is that the relative resistance of the material itself changes over time, causing a slight voltage shift in the output. Unfortunately, that voltage shift is sensed by the electronics as a difference in position for the touched point. Why the resistance changes is the mystery that I haven't tried to understand (yet). So what you are entering with the cal.txt file is offset information that 'shifts' how the processor interprets voltages from the touchscreen, correcting the drift in resistance that seems to occur over time for certain units like yours and like my GO 740. Seems only a fairly small percentage of units are impacted by this, and as I say, I have no idea what causes the resistance to drift over time.
 

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