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The Cub Scouts hold an annual model car race for aspiring engineers and mechanics called the Pinewood Derby. Each scout is given a block of pine, 4 nails, 4 wheels, and a 5 oz. weight limit. This makes for an equal playing field, but there is always a winner, someone who gets the most out of what they are given.
The cellular handset business is not much different. Engineering teams are given chipsets, LCD displays, LEDs, switches, and a battery life limit--an equal playing field--but one phone will win when a customer exits the store having purchased it. Standard chipsets make handsets extremely cost competitive. The chipsets provide a standard set of features at a standard price with little diversity. Chipset manufactures offer multiple products for basic, mid-range, and smartphones. Most design groups are challenged with creating a mobile phone with a few extra features in the hopes of setting it apart from competitors.
An excellent example is the keyboard features. The basic mobile phone chipset has the connections for a standard 18 keys plus navigation. The chipset typically has 9-10 pins dedicated to a keyboard matrix decode function. Adding a QWERTY keyboard would bridge the gap between a basic mobile phone and a mid-range mobile phone and possibly be the key feature for the buying decision if competitively priced.
The economics of chipset ASSPs is that pins cost money. Too often the pin count of an ASSP dictates cost more than silicon complexity. Therefore the typical 14-16 pins required to decode a QWERTY keyboard matrix is not possible in the cost structure of the basic ASSP chipset. It's safe to assume that the basic chipset application processor has the brains and code space required to use a QWERTY keyboard and only lacks I/O.
CPLDs are a small ASIC that can add features and capabilities quickly and cost-effectively. The advantage of a CPLD is high I/O count at a low cost. Although a QWERTY keyboard can be done with a 7x7 matrix, a more efficient design would make some keys independent of the matrix, SHIFT, SYM, ENTER, etc. This allows a simultaneous pressing of SHIFT and A, a feature most pin-limited keyboards can't afford. Remember it's all about product differentiation. The I/O abundance of CPLDs provides an advantage by making a feature-rich keyboard. CPLD vendors offer application notes that address the decoding of keyboards.
The next decision is how to connect the keyboard to the chipset. The most common way is to use a serial interface to an application processor like IC, SPI, or LPC. The serial interfaces are added to chipsets to allow the addition of I/O features that chipset pin costs cannot allow. Unfortunately the serial bus on most application processors is already over used. Most handset serial busses do not have sufficient address space or bandwidth to add an additional feature.
The abundance of CPLD I/O offers an alternative to the crowded serial bus. CPLDs have enough I/O to use a parallel Flash address and data bus. CPLDs are excellent at address decode, even complex decodes that map a few addresses from the Flash to the registers in a CPLD. CPLDs tap the Flash address, data, and control signals while generating the Flash Chip-Select according to a modified memory map. Once addresses are decoded for the keyboard very little additional logic or effort is required to assign other addresses to additional I/O expansion functions such as PWMs or SPI masters.
Figure 1. A Basic Mobile Phone Chipset Using a CPLD to Expand I/O
Figure 1 shows options for a keyboard decode CPLD added to an application processor Flash interface. Since CPLDs are user programmable, it is easy to adapt to the requirements of different Flash architectures, memory maps, and keyboard configurations.
CPLDs can be the solution to the many potential problems faced by handset designers. CPLDs can provide the pins and logic necessary to efficiently add a QWERTY keyboard at a fraction of the cost differential between a basic chipset and a mid range chipset as well as last year's chipset and this year's version.
Power is always a concern in handsets. The newest CPLDs on the market today offer static power characteristics that will meet the requirements of most handset designs. Since CPLDs are instant-on, the option of turning off the CPLD when the keyboard is idle is an option. The typical key click is 100-300mS. The typical CPLD power-up configuration time is <300uS. CPLDs can essentially power-up, configure, and interpret a key stroke in <1mS, which is long before the key is released. Methods for implementing a self-timed keyboard power-down are available from various vendor websites.
Low cost cellular chipsets offer consumers many low end mobile phone choices. CPLDs offer a path to a richer feature set and product differentiation. CPLDs fit within the power and BOM budget of handset designs and are a less expensive alternative to upgrading to a more advanced chipset for differentiating features.
About the Author
Rafe Camarota is a Non-Volatile Product Line Manager in the Low Cost Products Group at Altera Corporation.
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