Off-the-shelf MCU tweaked for ASIC-like duty

Off-the-shelf MCU tweaked for ASIC-like duty
By Ron Wilson, EE Times
January 6, 2004 (6:49 p.m. EST)
URL: http://www.eetimes.com/story/OEG20040106S0011

SAN MATEO, Calif. — As the business models of standard-product IC companies evolve, a gray area is forming between the once clearly defined territories of microcontrollers and ASICs. A case in point is a controller that Sharp Microelectronics of the Americas developed for a new version of the Pixter digital drawing pad from Fisher-Price.

Pixter Color is an interactive tablet on which children (and unobserved adults) can draw images on a touch screen, edit and animate them, and record audio tracks to create complete animated cartoons.

When the Pixter family was launched several years ago, the products used an 8-bit MCU, as was almost universally the case for low-end consumer products at the time. But with the addition of color and more sophisticated image-processing and audio-processing capabilities, the platform outgrew the 8-bit chip, said Christophe Chene, senor director of IC marketing at Sharp Microelectronics (Camas, Wash.). Fu ture plans in the Pixter line, such as an ability to establish data connections between units, would create even greater demands.

An obvious move for a vendor of very high-volume, low-cost products would be a modest system-on-chip design with a 32-bit CPU core. At least, that would have been the case two years ago. But in the present era of high initial design costs and uncertain markets, Fisher-Price went looking for alternatives.

The company, a subsidiary of Mattel Inc., went shopping not for an ASIC vendor or a design house but for an MCU vendor that would partner with it on a design. Sharp said it was willing to jointly develop an ARM-based standard-product; Fisher-Price would get an SoC-level controller with just the memory and peripherals it needed, and Sharp would get a family of MCUs suited to an entire category of toys.

Sharp was drawn to the deal, Chene said, in part because the Pixter family was rapidly converging on the functionality and form factor of another major MCU ap plication area: personal digital assistants. "The Pixter can capture graphics and audio and perform some quite sophisticated image-processing functions. The line is really starting to blur between toys and PDAs here," Chene said.

The resulting LH75411 controller was developed to be a perfect fit to the Pixter's hardware and memory needs, Chene said. Based on a 70-MHz ARM7TDMI core, the chip includes color LCD controller circuitry; a 10-bit, eight-channel A/D converter, an integral touchscreen controller; and the usual assortment of counter/timers, programmed I/O pins and SRAM. Pivotal to the design were the the 32 kbytes of SRAM, which make it possible to execute the Pixter's algorithms locally, and outputs that can be configured to implement a 12-bit pulse-width-modulated audio output. The addition of an inexpensive external capacitor gave the Pixter the functionality of a moderate-quality audio D/A converter, Sharp said.

Sharp's MCU designers worked closely with Fisher-Price's system design ers and with a contract manufacturer in China; the latter provided input on allocation of the chip's interrupts and other matters. Sharp also worked closely with the contractor on the final phase of code optimization to make sure that the chip's features would all fit without incurring additional assembly costs.

Is an MCU, then, an alternative to an ASIC design? In this case, Fisher-Price walked away with the chip it needed, and Sharp retains the rights to sell the part to other customers.