Coming Soon: (Yet another) BeagleBone Display+CapTouch cape

LCD and CTP

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.

[ You can also follow the project at Hackaday.io ]

Thanks to the economy of scale, the market of lower-end tablets is flooded with N brands available at almost throwaway prices. Here one can buy a cheap one for less than ₹3000 ($45) and get a decent 7″ screen with capacitive touch. 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 🙂 . I decided to take on the challenge and build a cape for the BeagleBone out of these readily available parts.

This December during my winter break at Jakarta, I shopped at Glodok and Roxy – the electronics and mobile spare part shopping hubs respectively [compare that to HQB, Shenzhen but at a smaller scale though]. 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).

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). 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. Got home and tried to match the pinout on the panel with the AT070TN92. Bingo. Perfect match for almost every one of them. 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.

Tablet LCD - back

Here you can see the one which has KR070PM7T written on it. The giveaways – Pins 1 & 2 (VLED+) shorted, 3 & 4 (VLED-) shorted. Pins 45, 49 and 50 are not connected. If you refer the datasheets, the pin definitions seem to align.

I bought two panels and mix-matched them with available capacitive touch panels to see which one fitted the screen the best. I also bought from a nearby shop the ZIF FPC connectors for the display and the touchscreen. The display one is 50 pins, 0.5mm pitch and the touch panel has 6 pins at a 0.5mm pitch. Not exactly breadboard friendly but very PCB friendly. 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. No reverse engineering needed for the capacitive touch FPC as the connections for the 6 pins are already highlighted in the image!

Okay, now I had to find a good driver for the LCD. I was aware of the TI TFP401 DVI (HDMI) receiver and could get samples if I wanted. 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. Turns out we’re not done, yet.

The catch

Every LCD requires a high voltage to control the twist of individual liquid crystals. 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 LCD expects approximately 10.4V for AVDD, 16V for VGH and -7V for VGL to be supplied to it. Hmm, how do I generate these?

The answer was not very hard to find. 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. Looking at their gate voltage generation circuits, we get this:

lcd-driving-circuit

This app note from Maxim Integrated explains what we’re looking at [scroll down to the end of the appnote]. The AVDD rail draws the maximum current so it gets powered it by the boost converter. 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. Very cool.

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. Until then I would test with a TPS61061 which I have at hand.

The design

The schematic of a beta cape is almost done and I’m proceeding with routing of the PCB as at the time of writing. Here’s a peek on how the schematic looks right now, the final will be different from this one:

bb-disp-sch1

The beta version is to be a locally fabricated quick turnaround prototype so that I get something to work up the software side until the PCB for the first batch is manufactured. The production cape may include termination resistors or a 74LVC322245 buffer.

The capacitive touch side is simple. Two I2C pins and an interrupt pin to inform of touches. Turns out that the LCD uses only 47 out of the 50 signals and I can squeeze these three lines into the same FPC as the display using an extension cable and adapter PCB. So I’ve done it this way as can be seen above. The Linux kernel already contains a driver for the ft5x06 in drivers/input/touchscreen/edt_ft5x06.c . So getting the touch for the LCD should just be equivalent to writing some device tree code to invoke the module.

That was a long post. The next posts would feature testing of the capacitive touch panel and of the prototype.

An expansion board for the STM32F4Discovery

The STM32F4Discovery from STMicroelectronics is one of the mature, extremely affordable, and yet capable development boards available in the market [I say mature because it has been around for quite a while; since Q4 2011]. The board is equipped with a STM32F407VGT6 ARM Cortex-M4 Core with embedded FPU running at 168 MHz, 1 MB of Flash and 192 KB of SRAM, and adds an accelerometer, a CS43L22 Stereo Audio DAC (with a headphone jack), an MEMS digital microphone and USB OTG support. The demos provided by ST demonstrate its audio playback and recording capabilities, which I was quite impressed with when I tested it for the first time.

The STM32F407 Microcontroller also packs in support for seamless LCD interfacing (via FSMC), SDIO, DCMI and Ethernet, hence I built a circuit to augment the STM32F4Discovery board capabilities by adding support for attachment of:

  • TFT LCD module with a touchscreen. These are cheaply available via eBay.
  • a microSD card [power to the card controllable via a PMOS switch].
  • Camera module with a 8-bit HSYNC/VSYNC/PCLK interface.
  • [Planned] Ethernet support, either via ENC28J60 or a DP83848 Ethernet PHY

The circuit was initially wired in Summer 2013 and recently modified to support audio playback alongside SDIO [see article here]

Here’s a pic of the board up and running:

BoardCeption - a board within a board within a board :P
BoardCeption – a board within a board within a board :P

Hardware

A double sided PTH dot matrix board is used for construction. The board was just in size to accomodate the STM32F4Discovery board on one side and the LCD with its 40-pin connector on the other making it a stacked 3-layer design. 0.1mm Magnet wire is used for all electrical connections.

The LCD side also has a 3.3V LDO for power distribution, a connector and charging circuit for a 1-cell Li-Ion battery. It also has a connector for a 10-DOF IMU for expansion.

Two 2.5cm spacers and two 6 cm metal studs mounted on the 4 mounting holes on the PCB act as an inclined stand for the assembly to be kept on a table top.

LCD

The LCD used is a 3.5″ HVGA LCD display [320×480 pixels] with a 4-wire touchscreen, mounted on a breakout board. The module used in it is a TRULY TFT1P7134-E, which uses HFFS (High Fringe Field Switching) technology, which is somewhat similar to In-Plane switching (IPS) and gives true-to-life colors. There is no color distortion noticed in the image, even when viewed at almost 180degree angles from any direction. It uses the Renesas R61581 TFT controller, which is an enhancement of the ILITek ILI9481 originally used in such displays (the instruction set is nearly compatible).

3.5" HVGA, resistive touch
The LCD module – 3.5″ HVGA

It connects to the 16-bit FSMC bus on the STM32F407 Microcontroller, which allows the LCD to be accessible as simply external memory, and enables DMA usage for data transfer to save CPU utilization. The module reset pin is connected to the MCU reset pin. The backlight is connected through a PMOS to TIM5 on the STM32F407. The R61581 supports in built LED backlight control, but I have disabled it and gone for the direct control in order to support more compatible displays.

The LCD breakout board contains a XPT2046 (ADS7846 compatible) touchscreen controller that interfaces the 4-wire touch panel via the SPI bus to the microcontroller. The PENIRQ output provides feedback on screen touch.

Almost any LCD from eBay which comes with a 40-pin connector will be able to connect to this connector and can be supported with suitable changes to firmware.

The uGFX library is used to interface to the LCD and touchscreen. It also provides a widget toolkit for UI design. The R61581/ILI9481 drivers adapted for this board have been contributed by me to the uGFX project.

microSD Card

microSD Card socket
microSD Card socket
The microSD card connects to the SDIO bus on the STM32F407. 4-bit bus mode is supported, and performance is stable at 48 MHz operation after recent addition of 100 Ohm termination on SDIO_CK (the clock line).

FatFS is used on the software side for file operations on the card. Read speeds of up to 9MB/s (theoretical maximum is 12MB/s) have been achieved with large buffer sizes.

Card detection is currently not implemented, but will be taken care of in future hardware revisions.

Camera Module

The STM32F407 contains a Digital Camera Interface (DCMI) bus that captures data sent in by a digital camera in the 8-bit format with external/inframe synchronization. Here only the HW synchronization via HSYNC/VSYNC/PCLK is used. The connector on the board was designed to be compatible with a OV7670 camera module, like the one available here

To drive the camera module, the XCLK is fed from PA8, which is a master clock output of the STM32F407. The MCO1 is configured to output a 16MHz clock using the HSI.

The control bus for the camera is SCCB, which does work with standard 400kHz I2C signals, with modifications in the way data is read, and delay between subsequent transfers.

I did try interfacing with a OV7670 camera module but I was largely dissatisfied with the results, they were terribly off-color.

IMU

10DOF IMU
The 10DOF IMU
The board also has a connector for a 10 DOF IMU board available on eBay. It consists of an ADXL345 accelerometer, L3G4200D gyroscope, HMC5883L compass and BMP085 pressure sensor, everything accessible via the I2C port through pins PB6 and PB9.

Software

The board runs ChibiOS and uGFX.

ChibiOS is different from many other real time kernels in the fact that it offers a tightly integrated hardware abstraction layer (HAL), which is very well written and easy to use. The HAL provides, among other things, off-the-shelf support for SD cards via both SDIO and SPI and integration with the FatFS library, GPIO configuration and asynchronous transfers on SPI and I2C and serial ports, with built-in support for DMA usage to offload transfers and save the CPU for computation. ChibiOS 2.6.3 is currently being used.

uGFX offers support for rich graphics and touch input. It also offers a GUI toolkit which can be used with a Windows/Linux simulator to develop GUI on embedded devices.

Both ChibiOS and uGFX are available under a dual license for free non-commercial usage and commercial licenses once used in production purposes.

A custom board file was created off the reference STM32F4Discovery board.h and board.c .
Drivers have been written for the CS43L22 using the ChibiOS STM32 HAL. Since I2S support is currently not implemented in the STM32 HAL, I had to implement I2S configuration & circular transfer using DMA.

The software development takes place using an Eclipse-based development environment on Windows 8, with GNU Tools for ARM Embedded Processors . OpenOCD is used to debug in-circuit using the onboard ST-LINK/V2.

This summarizes the complete development platform.
Schematics would be coming soon.