LCD CAPE FOR BEAGLEBONE BLACK DRIVER DRIVER

Until then I would test with a TPS61061 which I have at hand. I’ve ordered some boost converters from AliExpress, the ones called SY7201 and XR1151 which are as of now stuck in the Chinese New Year holiday shutdown. Then the diode and the capacitors form a charge pump generating approximately +21 V and -10.4 V from that rail and the Zener regulates it down to the needed voltages. The AV DD rail draws the maximum current so it gets powered it by the boost converter. This app note from Maxim Integrated explains what we’re looking at. Looking at their gate voltage generation circuits, we get this: I was able to get Allwinner’s A13 based reference design for tablets that use a display (no points for guessing which one) with a 50 pin interface. The LCD expects approximately 10.4V for AV DD, 16V for V GH and -7V for V GL to be supplied to it.

This voltage is usually internally generated using charge pumps but turns out that this “dirty” LCD panel ( DirtyPCBs 😛 ) expects the voltages to be supplied externally to it. The catchĮvery LCD requires a high voltage to control the twist of individual liquid crystals.

LCD CAPE FOR BEAGLEBONE BLACK DRIVER DRIVER

But hey, the BeagleBone converts from TTL to HDMI and then I’m gonna convert HDMI to TTL, right? Why not just cut through the layers and wire the display directly? Should just be D0-D15, VSYNC, HSYNC, PCLK, 3V3, backlight, adjust LCD driver resolution, timings and done. I was aware of the TI TFP401 DVI (HDMI) receiver and could get samples if I wanted. Okay, now I had to find a good driver for the LCD. No reverse engineering needed for the capacitive touch FPC as the connections for the 6 pins are already highlighted in the image! As seen on the image at the top of the post (not this one) the LCD is sitting over the capacitive touch panel and you can see the 6-pin connections there. Not exactly breadboard friendly but very PCB friendly. The display one is 50 pins, 0.5mm pitch and the touch panel has 6 pins at a 0.5mm pitch.

I also bought from a nearby shop the ZIF FPC connectors for the display and the touchscreen. I bought two panels and mix-matched them with available capacitive touch panels to see which one fitted the screen the best. If you refer the datasheets, the pin definitions seem to align. The giveaways – Pins 1 & 2 (VLED+) shorted, 3 & 4 (VLED-) shorted. Here you can see the one which has KR070PM7T written on it. Even though the panels have slight dimensional differences in the bezel, they have the same pin layout and should (hopefully) be the same from the inside. Perfect match for almost every one of them. Got home and tried to match the pinout on the panel with the AT070TN92. But were the panels being sold in the markets the same AT070TN92 panels, their clones or something different? I decided to find out and requested at the shops to be able take a photo of a few models of these.

These panels are known by the name AT070TN9x (x=0,2,3,4) and are originally manufactured by Innolux and have 50-pin connectors with a TTL interface (datasheet here). What really caught my attention was 7″ TFT panels and capacitive touch digitizers in shops – the LCDs looked very close to the cheap 7″ LCD panels being sold on eBay and other places with a Realtek-based HDMI converter which I wanted to check out lately (like this). This December during my winter break at Jakarta, I shopped at Glodok and Roxy – the electronics and mobile spare part shopping hubs respectively.

I decided to take on the challenge and build a cape for the BeagleBone out of these readily available parts. But when it comes to the BeagleBone Black I either saw that most of touch screens available were resistive and panels with capacitive screens were out of my budget, especially when you know you could leveraging the same economy of scale build one 🙂. Here one can buy a cheap one for less than ₹3000 ($45) and get a decent 7″ screen with capacitive touch. Thanks to the economy of scale, the market of lower-end tablets is flooded with N brands available at almost throwaway prices. TL DR: This creates a cape for the BeagleBone Black using readily available replacement spare TFT Panels and a capacitive touchscreen originally found in cheap tablets.

Lcd cape for beaglebone black driver