Beijer Electronics (formerly QSI Corporation)

Manufacturer of Mobile Data and Human Machine Interface Terminals.
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 Post subject: Power to backlight
PostPosted: Wed Sep 02, 2009 2:41 pm 
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Joined: Thu May 17, 2007 7:09 am
Posts: 47
Location: Cape Cod, MA
Hi,

I am trying to address a customer concern where there are some pretty stringent EMI requirements in their spec. They do not need the touch-screen display for their application, but they do need to interface to the G-70 via the serial port to use some Qlarity code I have running on it.

They say they are seeing EMI output that is too high when the backlight is on, but if they use my slider control (or the Power On Setup backlight control) to set the backlight to 0, the EMI disappears. However, even if they save the configuration with the backlight off, the EMI signature returns on the next boot. They have to tap my slider control to have it "re-seat" on 0 to get the EMI to stop.

My basic question: is there some sort of voltage that gets automatically applied to the backlight circuitry on power-up, even if the brightness is set to 0? It seems that programmatically setting to 0 will turn it off, but when booting up with a saved config it returns. Any thoughts on this?


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 Post subject: Re: Power to backlight
PostPosted: Wed Sep 02, 2009 3:00 pm 
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Joined: Thu Mar 02, 2006 2:12 pm
Posts: 487
Location: Salt Lake City, Utah
If your having to tap the slider again to set the backlight to 0, then it sounds like the backlight is not really saved. Here's what the code would look like to turn the backlight off and save it if that's what you wanted to do.

Code:
setsystemsetting(SYS_BACKLIGHT, 0, SYSACT_ALWAYS) 'set to 0 now, and on boot
setsystemsetting(SYS_SAVE, false, SYSACT_DONOW) 'call to save setting

_________________
Ron L.

http://www.beijerelectronicsinc.com/


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 Post subject: Re: Power to backlight
PostPosted: Wed Sep 02, 2009 3:11 pm 
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Joined: Thu May 17, 2007 7:09 am
Posts: 47
Location: Cape Cod, MA
Yes, this is exactly what I am doing. What's more, it was also attempted on the Power On Setup screen, setting to 0 and then saving everything to flash. I should note that the backlight does appear to be off when the unit boots back up. But what I can't tell is if it's truly off (0 V applied) or just too dim to see.

I am at a bit of a disadvantage since I am relying on a customer to quantify to me this EMI that he is seeing, unfortunately. Apparently on boot there is some sort of electrical activity that happens, even if the backlight was saved at 0, and that can be made to stop by setting the backlight to 0 explicitly.


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 Post subject: Re: Power to backlight
PostPosted: Thu Sep 03, 2009 6:50 am 
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Joined: Thu May 17, 2007 7:09 am
Posts: 47
Location: Cape Cod, MA
Hello again,

We have run some more tests, and duplicated our customer's test rig so as to see the EMI issue ourselves. here is what we found:

When the backlight is stored in flash as 0 (either through power on setup screen, or through SetSystemSetting) and power is cycled to the unit, the backlight DOES come on again albeit very dim. So dim that I do not think you can actually set it to that level with SetBacklight()... you have to cover the screen to make it dark before you can tell the light is actually still on.

If we go directly into the Power On Setup screen after boot, and we look at the backlight level parameter, it reads 0. However the light IS on, just extremely dim. If you tap the left arrow once, even though it already reads 0, the backlight then completely turns off, which is what we want.

We can verify the EMI phenomenon ourselves now... there is a noise spike at 94.5 kHz that is present when the backlight is on, regardless of brightness, and disappears when the light is completely off.

I am wondering if there is a subtle distinction between "backlight level = 0" and "backlight off", where the 0 case really means "on but very very dim". If there is a difference, how can I achieve keeping the backlight completely OFF while booting? We are now looking into shielding options or even disabling the light altogether inside the unit if we can't find a way to keep it off on power-up. Any help appreciated

--Rob


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