National Semiconductor Corp. is using its X86-like embedded-CPU product line to explore the new world of network computers.

"We looked at the network computer (NC) reference design from Oracle and came away with some ideas," said National applications manager Mark Roberts. "We felt that the idea of the NC was good but that there was an opportunity to implement an NC with less raw computing horsepower, with access to low-cost silicon from the PC world and with an entry bill-of-materials cost under $200."

National felt that these targets could be achieved if, instead of starting the NC design with a RISC core, they started with their NS486SXF chip. That processor includes a nearly-X86 CPU core, PC-style core logic and a selection of peripheral controllers on one die.

By including hardware such as the DRAM controller, ISA bus controller, PCMCIA controller and serial communications on the chip, National hoped to take a big byte out of the support chip cost of the NC. And by providing an ISA bus and PCMCIA slot instead of a proprietary bus or PCI bus, National hoped to exploit the rapidly declining costs of legacy peripherals for those busses.

Software support

The NC design could also benefit from the extensive missionary work National has done in attracting software for its proprietary core. While the minor differences between the NS486 and Intel X86 instruction sets preclude use of Windows CE or 95 in their present forms, National has been able to land an array of software directly applicable to the NC concept. That portfolio now includes the Spyglass web browser from QNX, a Java OS and Hot Java port, Pharlap's cam file server and other pieces.

To hit the $200 materials point, National recognized that it would have to choose peripherals carefully. Shopping for devices that would play on the NS486SXF's unbuffered ISA bus, the company came up with a Trident TVG9470 graphics controller, a Rockwell RWA010 game and audio chip and Rockwell's RCV336ACF/SP v.34 modem chip, with the alternative of National's own AT/LANTIC II Ethernet controller. DRAM for main memory and frame buffer, buffer or interface chips for PCMCIA, audio and video, 10 Base T and telephone, a keyboard/mouse controller and so forth fill out the bill of materials.

But even with care, the $200 target requires some compromises. Roberts said that the Rockwell 28.8-kbit/second modem chip doesn't actually fit in the $200 limit. "At this price point, a more likely solution would be to use just the Ethernet chip, or to add a less-expensive 14.4-kbit/s modem," he said.

In addition, $200 limits the reference design to 4 Mbytes of DRAM, 4 Mbytes of ROM and a 256 kbyte Bootblock flash chip. "Designers can get their local applications into the ROM by using data compression, and then decompressing the code as it's transferred to main memory," Roberts said.

Another compromise involved use of the Trident chip. The part was Trident's initial attempt at NTSC video out from a graphics controller. It uses a silicon- saving two-line flicker-reduction algorithm that Roberts called "acceptable, but not ideal." The resulting image on a TV set may be difficult to work with when the image involves fine detail, such as small fonts on a web page, or characters imposed on a complex background. The compromise may be necessary, however, as more-sophisticated NTSC converter designs are primarily going into chips with PCI interfaces, not ISA bus interfaces.

Raw speed is another issue that may concern designers. At 25 MHz with no caches, the NS486SXF is not in the same performance category as the ARM7 core used in Oracle's reference design. But National software manager Jeff Baer said that Mips was not the key issue in an NC design.

"The two primary bottlenecks in a network computer are in the network interface and the graphics," he said. Baer said that with the support of a good networking chip and graphics controller-such as the Trident and AT/LANTIC parts, the NS486 would hold its own.

National will provide full documentation of the reference design and design assistance free to developers who register with the company. The material is planned for release in the first quarter of 1997.

Source: Electronic Engineering Times