Author Topic: LCD timing signals  (Read 2054 times)

v8dave

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LCD timing signals
« on: January 12, 2015, 09:13:52 PM »
I am using an NHD-7.0-800480EF-ATXV display with a Allwinner A20 Cortex A7 processor and I am seeing some flickering and whiteouts on the display when I run on battery power and the battery voltage gets low. The voltage to the LCD is still close to 3.3V but any current increase from the board causes small dips in the supply and this affects the stability in the LCD.

The battery is 6600mA capacity and I think the issue with the LCD is due to the fact the board design uses a buck regulator for the 3.3V instead of a buck/boost so that it can still run when the input is close to 3.3V

NOW.. What I did find during all this checking was that the LCD clock output from the CPU is not a square wave as I was expecting, based on the information in the datasheets.

This image shows that the clock is actually a sine wave. Now, when I posted about this on another group, they said to check the scope bandwidth etc. Measuring the R,G and B lines, which are clocked in at the same rate as the DCLK, they have a nice 0 to 3.3V swing and square edges so I am confident that the bandwidth is not the issue here.

http://www.axoninstruments.biz/download/lcd_clock_timing2.jpg

As you can see, the clock is not 0 to 3.3V but approx 1V to 1.8V swing. This clock is 33Mhz.

I also checked the output from an LPC1788 driving an LCD at 25Mhz and it shows the same type of output.

The issue I think is that the voltage levels are critical at lower voltages but not so sure until I can get confirmation that the clock output from the CPU is correct.

Does anyone know if what I am seeing is indeed correct? Even looking at the controller datasheet on the LCD shows a square type clock and the triggering voltages would not be met with this clock I am seeing.

Michael_L

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Re: LCD timing signals
« Reply #1 on: January 14, 2015, 10:03:29 AM »
Correct, for desirable and stable operation the clock should be a square wave and its voltage levels need to adhere to the display's voltage min/max requirements.  You must also ensure your power source can supply the display with the voltage/current it needs.

 

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