General Purpose 3.5" Screen

Current offer on general purpose screens for the LaunchPad eco-system is rather limited. Most of the screens available are highly specialised.

The Kentec 3.5" SPI screen doesn't include a touch controller, and is thus incompatible with the CC3200 LaunchPad.
The Sharp Memory Display only offers monochrome 128x128 pixels and contrast is rather hard to read.
The Pervasive Displays e-paper screens are perfect for ultra-low power applications, offer monochrome and Black-White-Red variants, but require almost all the pins available.

So I decided to build my own general purpose screen.

Requirements and Specifications

I set the following requirements...

Good quality 3.5" screen
Touch
As fewer used pins and as many shared buses as possible
Total target cost < USD30
Compatible with existing libraries.

...which translate into the following specifications:

320 x 480 pixels x 16-bit colours
Capacitive touch with haptic feed-back
Connection with shared SPI and I²C buses
Off-the-shelf components
Support of the LCD_screen Library Suite

Hardware

The screen module comes from East Rising and includes the capacitive touch with its I²C controller, a micro-SD slot and provisions for two Flash memories.

The controller of the screen is the ILI9488 from Ilitek, connected through SPI.

The ILI9488 was rather strange to deal with: some parameters for the interface are defined with hardware shunts, while other parameters are set through software commands.

As always, the Saleae logic analyser was of great help in debugging the communication.

The capacitive touch controller by FocalTech is embedded into the flat cable of the screen. It is connected through I²C.

I faced no special issues while developing the library. However, the data-sheet mentions features I was unable to implement, like pressure measurement and gesture recognition.

A second SPI port connects the micro-SD card, and the two EEPROMs.

The board provisions pads for the recommended Flash Winbond W25Q128FV with 128 Mb or 16 MB in SOIC package. The Flash U2 chip exposes write-protect and hold signals, contrary to the Font U1 chip.

I faced an unexpected issue while integrating the screen, the micro-SD card and the W25Q128FV Flash on the same SPI bus.

While each component worked as expected separately, that was no longer the case when combining them. The MISO output of the ILI9488 wasn't floating (hi-z) and was thus interfering with the MISO signal from the micro-SD card and the W25Q128FV Flash.

The haptic controller is the DRV2605L from Texas Instruments, connected through I²C. It features self-calibration, and drives either an eccentric rotating motor (ERM) or a linear resonant actuator (LRA). Surprisingly, LRAs were very difficult to find.

Soldering the controller with 0.5 mm pitch was made possible thanks to an electronic oscilloscope.

In order to solve the conflict on the MISO signals, I used an analog switch to select between the two lines.

Software

I faced no major issues adding support for this screen to the LCD_screen Library Suite, despite the rather confusing data-sheets for the screen controller and the touch controller.

Adding haptic feed-back was very easy to the LCD_GUI library. The DRV2605L includes a library of 123 different effects, with 5 variants for ERM and one variant for LRA.

On the right: the DRV2605L tested with an eccentric rotating motor (ERM).

After some search, I managed to get a linear resonant actuator (LRA). Although smaller, the LRA provides a stronger and cleaner haptic feed-back, as it operates on the Z-axis only.

On the right: the two haptic devices. Left, the linear resonant actuator (LRA); right, the eccentric rotating motor (ERM)

After just changing a couple of settings, the DRV2605L drove the LRA effortlessly. I also tried the self-calibration, but didn't notice a strong difference from the default settings.

On the right: the DRV2605L tested with an linear resonant actuator (LRA).

The tests use the standard panel with one slider, an on-off button, a bar-graph, a label and an exit button.

The haptic feed-back works as follow:

A short tick acknowledges the touch on all elements,
A strong click confirms the change of the on-off button after 100 ms,
A buzz varies in intensity according to the position of the slider.

Finalised and Working Screen

Total consumption remains under 100 mA, validating the use of the included 3.3V regulator, as some boards can't provide such power.

To be continued...