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Initial Tests

A project log for (Yet another) BeagleBone Display+CapTouch cape

A cape for the BeagleBone Black using readily available replacement spare TFT Panels and capacitive touchscreen originally used in tablets

Kumar, Abhishek 05/27/2015 at 11:060 Comments

First try, and we've got a shaky display. Very shaky. I can make out the Beagle dog logo, see screens changing but the display isn't stable at all.

Soldering problem? Ran the soldering iron across the FPC connector to make sure all joints are tight and good. Did a continuity check, all connections look good, no bridges when viewed under a magnifier.

Continuity loss? Used a multimeter to check connections to the FPC. All looks good, nothing strange. Somewhere in between this testing the LED backlight wasn't lighting up when I connected the LCD. Whoops! I burn 4 LED drivers in the process (but they seem to have open LED "protection"?). Add a 16V Zener across the LED lines to prevent re-occurrence. The LEDs are in a 3-series (and currently unknown parallel lines) configuration and draw 9.5V.

Maybe some signal integrity issues? It's Scope time.

On testing with a multimeter, the LCD (with backlight) draws ~400mA from the power rail.

Okay, now that we know the issue, we should need to just add some bypass caps to fix it? Sprinkle a few 10uF 0805 ceramic caps, one before the LED driver and the other across the LED driver output still the display appears to be as unstable as it was before.

At this point I asked in the HaD channel, and with feedback that maybe the onboard 3.3V of the BBB is not supplying enough current to power everything, try to power the entire cape with a 1117-3.3 fed from the 5V of the BBB. An unstable display, still.

I look at existing cape reference designs (CircuitCo BB-VIEW7 and 4DSystems 70T) and find that they power their LED drivers off the 5V rail, and not the 3V3 VDD rail. Maybe they knew something about ripple from LED drivers affecting the LCD supply?

(To be continued)

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