BELK-AN-004: Interfacing BoraEVB/BoraXEVB to TFT LCD display

Info Box Applies to Bora

History

Version	Date	BELK version	Notes
1.0.0	15:03, 4 September 2015 (CEST)	2.2.0	First release

Introduction

This application note describes the interfacing of Ampire AM-800480STMQW-TA1 display to BoraEVB. Main characteristics of this 7" TFT LCD panel are:

• 800x480 resolution
• color depth: 18 bpp
• electrical interface: LVDS (3 pairs).

This project is based on BELK 2.2.0.

Physical interfacing

To interface the display a small adapter board is needed. It interfaces J22 connector on BoraEVB side and provides a 20-pin connector to directly attach display cable.

The adapter board

• is also equipped with a linear regulator generating 2.5V. This voltage is used as power supply for the VDDIO_BANK13 rail. This voltage is required to implement LVDS differential pairs that drive display.
• integrates a pad (denoted as TP1) that is used to connect 5V power supply generated by MOD2 PSU of BoraEVB. This additional power rails is required by display backlight circuitry.

Ampire AM-800480STMQW-TA1 part integrates resistive touch too. This is directly connected to BoraEVB's J25 connector. Resistive touch is managed by Texas Instruments TSC2003 controller (U27).

Schematics of adapter board can be downloaded from this link.

Block diagram and Vivado project

The following picture shows simplified block diagram of the design.

LCD is driven by a controller implemented in PL that fetches pixel data from frame buffer and periodically refreshes physical screen. The LCD controller is composed of an AXI VDMA IP, the LCD controller itself and a parallel-to-LVDS serializer. AXI VDMA and the LCD controller provides configuration registers that are mapped in the following address range:

• AXI VDMA: 0x43000000 - 0x4300FFFF
• LCD Controller: 0x43C00000 - 0x43C0FFFF

The following picture shows the block diagram of the Vivado project with LCD controller:

To implement frame buffer, a portion of main SDRAM is used. This area is allocated at runtime by linux frame buffer driver. Even if LCD is 18 bpp, each pixel is represented by 32-bit word in memory. In fact each pixel is in RGB666 format, so for each colour only the six most significant bits of the frame buffer RGB888 are used to drive the display.

Here is the pinout assignment to drive the LCD:

LCD Signal	BORA SOM Signal
BackLight (*)	IO_L15P_T2_DQS_13
LVDS_CLK_P	IO_L22P_T3_13
LVDS_CLK_N	IO_L22N_T3_13
LVDS_D0_P	IO_L21P_T3_DQS_13
LVDS_D0_N	IO_L21N_T3_DQS_13
LVDS_D1_P	IO_L19P_T3_13
LVDS_D1_N	IO_L19N_T3_VREF_13
LVDS_D2_P	IO_L18P_T2_13
LVDS_D2_N	IO_L18N_T2_13

The Vivado project can be downloaded from this link.

(*) This signal is used to control backlight. It is usually driven by a PWM signal whose duty cycle is proportional to backlight intensity. For the sake of simplicity, in this project this signal is driven by a GPIO, thus only two intensity levels are supported (0% and 100%).

Enabling frame buffer driver in linux kernel

To enable frame buffer driver user needs to:

• get the pre-built binaries from here.

Kernel and device tree can also be built with the following procedure:

• update Bora kernel repository (as described here)
• checkout bora-feat-lcd-support branch (using git checkout bora-feat-lcd-support command)
• build the updated kernel source as usual.

Put the binaries on the first (FAT32) partition of your BELK 2.2.0 SD card, overwriting the original one if needed. Please note that you need the following files:

• boot.bin
• bora.dtb
• uImage
• fpga.bin
• uEnv.txt

Insert the SD card into BoraEVB and turn on the board.

During kernel bootstrap, the following messages are printed out on console, indicating framebuffer driver has been loaded succesfully:

[    0.600840] borafb borafb.0: fb0: Virtual frame buffer device, using 16384K of video memory @ phys 2d900000

You will also see two Tuxes on the top left corner of the LCD, indicating that this Linux system has two cores, as shown in the following picture:

Once the kernel has completed boot, frame buffer can be accessed from user space applications via /dev/fb0 device file (for more details please refer to https://www.kernel.org/doc/Documentation/fb/framebuffer.txt).

The following picture shows Qt 4.8.3 Affine Transformations demo application running on top of it.