Tear Down: Handset's touch sensor permits more functions in less spaceThe popular Chocolate handset has a unique design, with its slider, MP3 capabilities, and large display.
When you're designing a handset, it's all about how many features you can pack into a tiny space. The more features, the better, right? Not necessarily. Most of us are familiar with devices that are difficult to use because they have so many features packed into too small a space.
That's the fine line designers must walk—lots of features in a small space, but not too many features in too small of a space. One way to clear this hurdle, as evidenced by the popular LG Chocolate phone, is to employ a capacitive touch sensor to handle most of the handset's user input, like the one shown in Figure 1.
This particular version of the Chocolate that I "disassembled" is also know as the LG VX8500, which is the model manufactured for Verizon Wireless. LG Electronics developed parts of the firmware for this handset in conjunction with Cypress Semiconductor and then used that code in several iterations of the handset.
Most of the changes that LG made along the way had to do with the number of buttons and interfaces to be employed, so the overall technology and the details of how it's implemented have remained consistent.
The Cypress team first made contact with LG in May of 2005, extolling the virtues of the company's PSoC (Programmable System on a Chip) architecture. The two teams went back and forth, eventually coming up with the final design requirements. Actual development began around August 2005, with the goal of having the product released to manufacture in October 2005.
"The original requirements were somewhat unclear," says Greg Virge, a firmware and applications engineer at Cypress. Often when a customer employs capacitive sensing for the first time, it's all about understanding what they really need as opposed to what they think they need. That took a couple of months to sort out. But when we were done, we had something to work towards."
PSoC is a mixed-signal array with programmable logic. Hence, it can be configured for various functions, including capacitive sensing.
Cypress set out with a goal of creating a firmware platform that could be easily changed. This is done with a "user module," which are preconfigured analog and digital blocks that handle a specific function. For example, the digital blocks become an 8- or 16-bit pulse width modulator, and an analog and a digital block can combine to form a digital-to-analog converter. The blocks can be programmed to be communications interfaces, such as a SPI or half- and full-duplex UARTs. The capacitive sensing was handled in such a user module, which includes all the sensing, the baseline update, and the decision processes.
"At the same time we were working on the LG project, we were working on the CapSense user module," says Virge. "We were careful to put in all the requirements that LG would need to be able to do the application correctly into the user module. That same user module, and all the reference code that goes with it, has since been used in a few LG handsets."
The intellectual property with the user modules belongs to LG. Hence, they can make changes themselves and own those changes, making the module fit their specific needs. In the case of LG, they've actually used a modified version of this user module within other parts of the company, such as the group developing white goods.
The Cypress IC in this handset, shown in Figure 2, is the CY8C21434-24LFXI, housed in a 32-pin, 5- by 5-mm package. The sensor itself consists of nine copper pads on the opposite side of the board from the IC. Each pad has a hole in it where the light from the LED shines through. Each pad is connected to a PSoC I/O.
Because the PSoC is actually a microcontroller, it has the ability to perform other functions besides the CapSense. In the case of the Chocolate, LG integrated the LED drivers.
Because of where the IC is located on the board, it's became an obvious choice to integrate that control, or anything else related to the sensor. One such function is the ability to look for the priority of a button press. In other words, at different times, you want certain buttons to be easier to press than others, based on the button press that preceded it. This helps out when it appears that two buttons are pressed at the same time, possibly because the user has a big finger. As a result, the PSoC has a good chance to determine which button the user intended to press.
LG's designers were also concerned with electrostatic discharge (ESD) protection. Meeting the needs of the ESD requirements with the changing electrical and mechanical designs of each new model can be a challenge. When an ESD event occurs on the sensor, it impacts the integrity if the received data. The way to circumvent a negative impact is with algorithms that can differentiate between an ESD event and a valid button press.
Hence, it must recognize and ignore the ESD event, which tends to have a fairly specific signature when compared with data. The PSoC waits for a few cycles then checks the sensor again.
Another LG design change that had to be accommodated was a move from an FR4 board to a flex pcb. FR4 is thicker, which is a little easier to design with because you can better distance the signals from a ground plane on the back side of the board. The result is less native capacitance, therefore higher proportional change. FR4 also costs less. The flex pcb gives the system designers more mechanical options, which were needed on this tight design.
Like many of the CDMA handsets I take apart, this one contains lots of Qualcomm ICs, as you can see in Figure 3. The Chocolate is built with an MSM6500 baseband, an RFT6150 transceiver, and an RFR6500 receive diversity device.
Some of the other identifiable components include a Wolfson 8973G audio equalizer, a Toshiba multi-chip package memory device, and a Maxim power-management controller.
Richard Nass is editor in chief of Embedded Systems Design magazine. He can be reached at firstname.lastname@example.org.