PCI bus needs DMA, PLD support

Electronic News, August 8, 1994 by Reinhardt Krause

SAN JOSE, CALIF.--Despite the Peripheral Component Interconnect (PCI) local bus growing in popularity among PC OEMs, not all the pieces are yet in place for the standard to extend its reach into multimedia, add-in boards and some other areas. At the Silicon Valley Personal Computer (SVPC) design conference here recently, systems engineers pointed to several areas where the PCI standard requires more evolution or support from chip vendors.

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Among the areas identified were more support from programmable logic device (PLD) suppliers and improvements for direct memory access (DMA) transfers, obstensively for PC game applications.

The lack of PLD support--which may soon change in the wake of Altera's planned acquisition of Intel's PLD business--was attributed by some at the conference to the PCI bus standard's demanding electrical specifications. In fact, some chip vendors have said previously that their selection of a foundry was impacted by the fab's record in producing PCI-compliant parts--for example, 3Dlabs said that its choice of IBM as a fab for its Glint devices was partly based on such considerations.

When asked recently about the apparent difficulty in conforming to the PCI electrical specifications, Intel's Ron Smith, general manager, PCI Components Division, noted that the MPU king is using several foundries to product components for its PCI chipsets. Sources indicate that Intel may have as many as seven fabs in all, including North America and Asia.

NEC, Hitachi and LSI Logic have also announced intentions to support PCI interfaces with their ASIC families (EN, May 9). Still, the unavailability of programmable logic to support PCI-based systems may be holding back some designs. At the SVPC design conference, Mr. Smith noted that the PCI bus standard was originally geared for motherboards; he added that it may also be difficult for some PLD vendors to support the specification in small volumes.

Still, the planned Altera acquisition has caused concern at competitors, because the PLD vendor will gain access to PCI bus technology. However, other sources indicate that Intel's PLD division was not supporting the local bus with parts prior to its planned sale.

Another area of work for PCI bus advocates may lie in making improvements that support DMA. One big player in the add-in board market is Creative Technology Ltd., whose Sound Blaster technology requires DMA capability and is currently tied to the ISA bus. Although Mr. Smith indicated that some DMA capability for the PCI bus may be on the way, he added that supporting all existing games may not be a legitimate goal.

Although the PCI special interest group (SIG) has already targeted a new specification for multimedia applications, the SVPC conference had much debate over its attributes compared with the Video Electronics Standards Association's media channel (VMC) and Advanced Feature Connector (VAFC).

While the VAFC is an electrical specification for video applications it still needs an I/O bus--leading some system engineers to speculate that it could in fact be combined with the PCI bus standard in some designs. An alternative pushed by Intel, however, is to use a secondary PCI bus, in a hierarchical bus structure.

According to Billy Garrett, Rambus applications manager for graphics, both the VMC and PCI solutions are attempting to produce "TV quality" displays by moving around uncompressed video in a system. He said the PCI guidelines, which use destination addressing, require windowing/scaling to be performed with a video device; the VMC, meanwhile, performs those functions with a bus master device. As a result, as a video window scales, more data has to be transferred across PCI--opening up the need for memory bandwidth.

Mr. Smith, in outlining the PCI bus standard's evolution, had separately indicated that among the coming changes would be more bus mastering capability. He said these capabilities would enhance videoconferencing and hard disk drive throughput.

Support for multimedia application using the PCI bus may also be coming from other areas. Reports surfaced at the show of new PCI-based graphics accelerators, with Toshiba's Vertex Semiconductor subsidiary expected to debut a chipset at the Hot Chips VI conference next week.

Other PCI-based accelerators are also expected to appear. Mr. Smith said the devices would accelerate data transfers between I/O and memory, with a data path unit allowing direct access to memory. To some degree, Intel has put some of these functions in its 82430 chipset, he added.

On other matters, Mr. Smith said that "beyond 1995" a 64-bit version of the PCI standard may come into play. Mr. Smith added that while the 64-bit specification would at some point see increased customer demand, he does not envision raising the PCI bus' frequency from the current 33MHz. The 64-bit specification would use the same connector, he added.

The PCI SIG, meanwhile, is moving ahead with a mobile computing design guide for bridging the PCI bus to the PCMCIA's new 20MHz "Card-bus" architecture. The design guide began circulating in late July (EN, July 4). Card-bus will be used as an expansion bus standard for PCI-based portable computers.