Topic: Flickerin' screen solution (?)

Ok, there are many people having issues with their LCDs flickering. Possible reasons might be the software hysteresis, fedback (not likely, but who knows...), and (it just ocured to me) the 60Hz light switching.
I was checking the schematics for chumby and it seems the easyest solution for this problem might be adding a cap to pins 3 and 4 of the light/bend sensors ADC.  Any cap from 10uF to 47uF (10V) should do the trick. Negative lead goes to pin 3 (Gnd) and positive lead goes to pin 4 (IN).
I don't own a chumby so there is no way I can test this. If anyone out there tries this out please post your results.
As a recomendation.... unless you know what you are doing don't solder directly to the IC pins. Follow the PCB tracks and scrape the mask just a little to expose the copper. Pin 3 already has a via next to it so that is a good spot to solder.
and btw, the IC is marked as component "U503".It's very close to the light sensor.
Don't be afraid to try this. In case it doesn't work just desolder the cap. After all it's only 2 wires you are soldering. smile

Let me know if it works!

2 (edited by jacubilloro 2006-08-30 03:17:50)

Re: Flickerin' screen solution (?)

BTW, how can I upload an image here? I even made a nice diagram of the connection to the PCB...

Edit (2 minutes later...)

From the Forum rules:

16. NO IMAGES. Chumby has decided to restrict the uploading of any images on our servers. However, you are free to post links to hosted images on your own servers. Please remember, no pornographic or obscene material of any kind.

Re: Flickerin' screen solution (?)

Yeah, the server running the wiki and forums isn't very beefy so we've had to restrict the hosting of images for the time being.  We may change this later.

We're putting together a new data center - this may free up some machines.

Re: Flickerin' screen solution (?)

hey, if you want some image upload space, just gimme a shout. its payment time for my hosting, so id upgrade it to the 2gb package if you wanted. i dont mind paying the extra

need upload space for the forums or a chumby blog? right here then
http://www.nophus.com/useru
username is chumby
password is chumby

5 (edited by jacubilloro 2006-09-05 22:23:10)

Re: Flickerin' screen solution (?)

Ok, here is the image I made of the posible flickerin' screen solution:
http://www.nophus.com/useru/users/chumby/Chumbymod.jpg
It should be very easy to solder the cap to the IC. Just be carefull not to overheat it.

Re: Flickerin' screen solution (?)

Having debugged a few units with the flickering screen problem, I'm not sure if it's a problem with the sensor being noisy. There is a hysetresis value setting that you can play with to try and debug the noise problem--see /proc/sys/sense1/dimhyster. You can adjust the value by issuing the command 'cat n > dimhyster', where n is a positive integer decimal in ascii.

The problem I've seen has been related to two issues:

1) at low light levels, the PWM dimmer control for the LCD backlight inverter has very coarse resolution. There is an attempt in the driver to do a linear interpolation based on intensity, and this is actually a bad idea based on the aforementioned fact. Basically, the resolution of control at low light levels is very coarse, so the intensity difference between two adjacent settings is noticeable and objectionably abrupt.
2) at low light levels, some backlight inverters exhibit some instability. It has been noted that at very low light levels, such as a setting below 16 or so, the backlight inverter has trouble sustaining the feedback loop because the PWM pulses are too sparse. This usually results in the backlight eventually being extinguished as the feedback loop collapses on itself. As a result, the default setting sticks the backlight at a minimum value of around 20. You may wish to play with the brightN settings (e.g., bright0, bright1 , bright2 ... etc) to try and tweak the minimum brightness up, to perhaps a value of around 30 or 40. The maximum value is 255, and remember that this is a linear control for a value that is perceived logarithmically, so values from around 100-255 will seem to be all about the same brightness to the human eye.

I think perhaps in future versions a different backlight inverter controller will be used because the current one exhibits these low-light level issues, which are definitely annoying.

7BAA 2E53 01C1 DCFF 497B  E7F0 9699 A303 78F0 D9B9

Re: Flickerin' screen solution (?)

w00t somebody used my webspace big_smile

need upload space for the forums or a chumby blog? right here then
http://www.nophus.com/useru
username is chumby
password is chumby