High resolutions for HMIs

Human machine interfaces (HMIs) supporting multi-touch interaction are now being adopted in an increasing number of modern electronic hardware, covering a broadening cross-section of industry sectors. They offer the benefits and convenience of an all-encompassing control method where otherwise a less attractive and more cumbersome electromechanical HMI (such as a keypad) would be called for. Through this technology HMI designs can be realised that keep in line with the overall aesthetics of the product into which they are being integrated, and enhance the user experience too. Many OEMs will now be looking at how to create a more effective capacitive touch HMI that fully addresses the numerous technical and logistical challenges currently being faced by engineering teams. 

The multi-touch functionality of capacitive touch screens have already seen widespread deployment in both the consumer electronics and computing sectors. There are now numerous opportunities opening up for them outside those sectors too. Among them is the highly lucrative domestic appliance market. Touch interaction is still relatively new to this particular sector, but in recent times it has started to be featured in goods including refrigerators, cookers and washing machines.
Capacitive touch can distinguish multiple touch points at the same time and it is thus able to accurately detect all sorts of intricate user gestures. In addition to the support of multi-touch operation, touch screens that have capacitive sensing exhibit greater operational robustness than conventional resistive touch screens, as no direct pressure needs to be imparted onto it to initiate a touch event (and there is the option for them to be placed behind protective glass if deemed necessary). This makes them suited to domestic appliances that often have to deal with fairly demanding working environments where it would almost certainly be impossible to use resistive touch screens.

Menu support
The schematic shown in Figure 1 describes a high resolution HMI as implemented in a washing machine unit, where the user is able to access a series of different easy-to-follow menus. Through its multi-touch operation various gestures can be detected and supported, such as two finger taps, page flicks, cut-and-paste, resizing, swipes, and two finger long presses. The design consists of an MM900A1A module, which has an FT900 microcontroller unit (MCU) on it. This is complemented by a demo board for the newly released FT813 graphics controller, which uses FTDI Chip’s innovative Embedded Video Engine (EVE) technology.
In addition to the graphic controller, the FT813 demo board incorporates a WVGA resolution LCD with a 24-bit RGB parallel interface and a FocalTech touch controller device with an I2C serial interface. The MM900A1A module has an SD card to store bitmap image content and a QSPI interface through which instructions can be passed to the FT813 for controlling the HMI system. At the heart of this module is the FT900 high performance MCU, which acts as host controller. This chip takes care of transferring all the image data to the LCD so that it can be displayed. Conversely the FT900 also captures all the touch data being sent to it from the LCD’s touch controller via the FT813, with master-in slave-out (MISO) and master-out slave-in (MOSI) signals being passed between the two ICs.
The FT900 controls the HMI by tracking the user’s touch inputs and subsequently connecting them to the relevant menus. Among these menus are the main menu, the setting menu and the wash progress menu. The main menu consists of scrolling selection, properties of the wash type, selection of settings/wash/lock, etc. The setting menu allows the levels of brightness and volume to be adjusted by the user to suit their particular preferences. The wash menu consists of a progress bar to indicate the different stages of the wash cycle (wash, rinse, spin, etc.). All related image content is stored on the MM900A1A’s SD card and subsequently downloaded onto the FT813 over the QSPI interface. Screenshots can be taken using FT813 graphics and widget instructions.
The FT813 has the capacity to track up to 5 touch points simultaneously and distinguish a total of 255 different objects. This means that more than adequate support for multi-touch operation is provided. As with all devices in the EVE series, the FT813 has the ability to streamline HMI implementations through its innovative object-oriented architecture. This treats all audio (chirps, beeps, etc.) and visual (overlays, fonts, bitmap images, templates, etc.) outputs as simple objects. The upshot of this is that image content, for example, can be rendered line-by-line (at 1/16th pixel resolution), as opposed to pixel-by-pixel (which is markedly more data intensive). As a result of this, rather than a large Flash memory for storing image data and the need for a capacious frame buffer for the HMI system, these items can be eliminated. Furthermore, the specification of a less expensive 8-bit or 16-bit MCU, as opposed to a more costly 32-bit device, is permissible. The wide parallel buses that are also normally mandatory to take care of communication between the various ICs are no longer necessary.
The component count for the HMI system detailed here is far less than what would normally be expected. It takes up less space on the PCB, shortens development times and requires a smaller engineering resource to complete. It is therefore much better suited to cost sensitive, space-constrained implementations than conventional multi-touch HMIs solutions. Another valuable feature is the opporutunity that an object-oriented graphics architecture presents for animation; as all the displayed images are regarded as objects animation is relatively easily achieved by just updating the coordinates of any object placed in the screen area. A single object may also be repeated in multiple locations, without having to create multiple objects.

Second generation
The FT813 is part of FTDI Chip’s second generation EVE devices. It has a higher resolution than the previous product offering - with support for 800 x 600 pixels compared to 512 x 512 pixels. This means that it is able to address larger display formats (7-inch diagonals and above), such as those likely to be needed for domestic appliances. In addition, the memory has been expanded from 256kbytes up to 1Mbytes. The FT813 is supplied in a compact 56-pin VQFN package.
As multi-touch is able to make the way people engage with technology less frustrating and more intuitive, there is a great deal of demand to broaden the scope of where capacitive touch technology can be utilised. The application example shown in this article allows the construction of a highly sophisticated but cost effective and also streamlined HMI system, where menus and widgets are utilised in order to form a graphically rich and highly compelling solution using only simple instructions. Through implementing an EVE-based HMI system, supported by an MCU that is specifically optimised for applications of this kind, dramatic reductions can be made in the board real estate allocated and the bill of materials costs involved.