Capacitive sensor takes buttons off camera phone

Handset OEMs typically differentiate their products by increasing the number of features. While this usually is a good thing, there is a downside—more features make the phone more difficult to use. To address this, Synaptics Inc. has developed a technology that simplified the user interface and made it easier to access all these features. In the Samsung SPH-V6800 Wi-Fi-enabled GSM camera phone, a slider-based handset, Synaptics has replaced a set of mechanical buttons with a capacitive sensor that controls the functions. The resulting thinner, sleeker phone is an added benefit.

In handset interface speak, the term "zero D" is used for scrolling up and down. (One-D is essentially some kind of closed-loop rotational scrolling. Two-D is more free-form, moving a cursor around.) "Zero-D is what most of the handset OEMs have implemented," said Clark Foy, Synaptics‘ VP of marketing. "Those products tend to be simpler [to use] but result in a more advanced industrial design."

Synaptics used a thin, capacitive sensor-based design that replaced the four- or five-way rocker buttons used for function selection. Without the capacitive sensor, the handset would have been much thicker. And it‘s already relatively thick, thanks to a rich feature set. In addition to the Wi-Fi and 1.3Mpixel camera capabilities, the Samsung handset offers a 2-inch TFT display, MP3/AAC audio playback, voice recognition, MPEG-4 recording, video-on-demand, TV-output and EVDO.

A category of buttons has been created for MP3 players called MOD, or multimedia on demand. The typical three buttons offer pause/play, fast-forward and rewind functionality. Because "thin is in" these days, the MOD buttons must also be thin as they usually reside on a hinge or on the outside of a clamshell phone. In many cases, they‘re the only buttons that are exposed to the exterior environment.

In addition to driving the MP3 functionality, the MOD buttons can serve as the directional navigation keys (up, down, left, right) as the user navigates through a menu structure. On the SPH-V6800, the traditional keypad number buttons remain as mechanical buttons, but everything else is handled using the capacitive sensor. Scrolling through long lists of songs or phone numbers is accomplished in one of two ways—using the down button or the up-down toggle switch on the side of the phone. Samsung could have chosen to use the sensor to handle this feature but decided not to.

The Synaptics sensor includes a flex pad—a flexible sensor that sits under the mechanical keypad so the entire flat surface where the keys are located can operate as a 2-D input area, enabling gaming, Web browsing, data entry and so on.

While not implemented in this phone, the display area could be used as a sensor. Under a lens, a clear sensor provides direct access to input right on top of the display. The advantages of such an implementation include higher-resolution finger performance; better protection, because it can be mounted under plastic; better industrial design; and, the ability to slightly curve the sensor (impossible with resistive sensors).

Sleep and doze
One of the keys to the success of a handset is keeping power consumption low. To that end, Synaptics‘ designers ensured that the sensor-IC combo wasn‘t a power hog. Several power modes can be employed, including a deep-sleep mode (which consumes less than 15μA), a wake-up-enabled sleep mode (70μA) and a doze mode (100μA). In active mode, the sensor requires less than 1mA.

Samsung was tough on quality. "There‘s a very rigorous quality process you have to go through to be qualified," said Foy. "Their model is different in terms of the way they do development and design. They do more parallel designs, then decide which stays, which gets modified and which gets killed." Many other OEMs tend to be more explicit in their planning process, doing more upfront design work and then taking fewer handsets into development.

The multimedia operations in the SPH-V6800 are provided by Qualcomm‘s MSM6500 chipset, which offers EVDO functionality. The 6500 offers a direct connection to Qualcomm‘s radio chipset, which is part of its RadioOne architecture, a direct-conversion radio that potentially minimizes the radio‘s design complexity for multibands and multimodes.

From an imaging standpoint, the handset‘s camera offers a 1.3Mpixel resolution for still images and motion video, as well as decoding and encoding capability. The 6500 provides enough horsepower and hardware acceleration so designers didn‘t need a separate applications processor for video or gaming.

Rich in multimedia
"The SPH-V6800 was designed for the Korean market, where they like a high level of multimedia content embedded into the handsets," said Brian Rodriguez, senior director of product management at Qualcomm CDMA Technologies. "Hence, the handset uses all the features of the 6500 [chipset] from a multimedia perspective." To enable lower system costs, he said, the baseband "integrates all audio, video and graphics, together with dedicated modem circuitry, a dedicated ARM processor for both multimedia and modem, and even hardware acceleration for some key multimedia functions, like graphics, video, GPS or audio."

The 6500 chipset has the same gpsOne engine as Qualcomm‘s other products. It uses both the satellite and the network to provide fast and accurate fixes for both E911 and enhanced location-based services. The 6500 supports a dual-receive architecture on the radio, which results in the ability to provide simultaneous or standalone GPS.

One of the most difficult pieces of engineering in the SPH-V6800, Rodriguez said, concerned its thinness. The thin form factor of the handset increases the complexity of the radio and antenna subsystems, while making it more difficult to avoid interference. There‘s also increased pressure to move the external antenna. Radio complexity is eased somewhat thanks to the chipset‘s ability to handle multiple bands and modes.

Another key component in the SPH-V6800 is a 10-channel bus switch that‘s used to connect or isolate a particular path. Texas Instruments‘ SN74CBTLV3861DGVR is housed in a 6.4-by-5mm small-outline package. According to TI, the DGV packaging consumes about 35 percent less board space than QSOP or TSSOP packages. A second source to this part comes from IDT. (The TI device has a slightly lower leakage current, while the IDT part has slightly faster enable and disable times.)

Typically, such a switch is placed on a bus when you want to address a particular port (by enabling the switch). Otherwise, the switch is left in an open position, thereby isolating the bus. The 3861 integrates 10 FET switches. The same functions could be handled discretely using 10 individual devices and some additional logic circuitry. "We still ship this part, but other functions in our bus switch family have superseded these," said Prasad Dhond, a TI applications engineer. "For example, the CBT-LV family is optimized for 3.3V, and the CBT-Q bus switches are suited for higher bandwidths up to 500MHz."

- Richard Nass
Editor, www.mobilehandsetdesignline.com, part of EE Times