new bus architectures - how cardbus fits with ieee 1394, usb, pci and others

8/8/2019 New Bus Architectures - How CardBus Fits With IEEE 1394, USB, PCI and Others

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New Bus Architecture: http://www.pcmcia .org/papers/new_

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(c)1997 PCMCIA .Ail rights reserved. This document may not copied, reproduced, transmitted;electronically or mechanically, stored in a retrieval system, or translated into anycomputer in any form or an y means w ithout th e written permissionThe Personal Computer Memory Card I n ternationa l Association

Claude A. CruzMobile Technology LaboratoryIntel Corporation

P ropr ie tary In format ionInformation contained in th is document is proprietary to PCMCIA.

DisclaimersPCMCIA makes no representations or warranties with respect to the anddocumentation described in this document. PCMCIA., especially disclaims an y impliedwarranties of merchant ability or fitness fo r any particular purpose. Further, thisdocumentation is subject to change without notice.

TrademarksPCMCIA is a registered trademark of Personal Computer Memory Card InternationalAssociation.Microsoft, MS, MS-DOS, and Windows are registered trademarks of MicrosoftCorporation.

and Pentium are registered trademarks of Inte l Corporation.AI! other trademarks are recognized as the property of th e ir respective holders.

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Figure 2 : Evolution o f Mobile-Platform Archi tec ture

PCI r idges

Traditiona!!y, PCI-based PCs are organized around tw o "bridge" functions: a "North"bridge, which links the central processing unit (CPU) and it s cache memory to mainmemory, and to th e level-O PCI bus; and a "South" bridge, which provides variousperipheral connections onto th e PCI bus. (These ma y include a PCI-to-ISA bridgingfunction). The North bridge consists mainly of high-speed data paths, while the Southbridge is largely a collection of peripheral devices (e.g., timers, direct memory access[DMA] controller, etc.) and peripheral-bus connections to the system. The integrationpressures of th e mobile e nvironment are forcing a rejuggling of this partition.

In the future, even th e high throughput of th e PCI bus wili be inadequate to support theneeds of multiple high-speed subsystems. Thus, th e major const ituent blocks of acomputer ma y connect through one or more higher-speed links, and th e PCI ma y serve

as a secondary bus attached to one of th e system blocks.

All notebook computers being designed today are built around th e PCI architecture. ISA,th e dominant architecture of the 1980's, is swiftly being replaced. As a result, th erelevance and importance of ISA-" Iegacy" features such as dedicated inte r rupt request(IRQ)-style interrupts are decreasing. Some of these legacy mechanisms such as8259-style DMA are ar chitecturally mismatched to PCI, which provides analogousservices through native bus-mastering by individual PCI devices.

functions will continue to exist duwill

user-transition period,th e contents of the South

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fo r PCL This allows Card Busto PCI devices, BIOS and powerand Socket Servi ces . These similarities make it

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Figure 3 : CardBus Relationship to PCI

Mechanical Form-Factor

CardBus share PC C a r d ~ 1 6 ' ssmall, thin mechanical card design, as well as asimila r 68-pin connector. Because of th e h igh clock-rate of CardBus, th e CardBusconnector design en cl oses th e standard 68-pin PC Card connector in a grounded shield ..When used with the controlled buffers, this shield ensures

eve n at bus as high as 33 MHz.

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Figure 4 : A d d - i n / Add-on At tachment Locat ions

Below is a brief description of these add-in and add-on technologies, followed by a

discussion positioning them relative to CardBus.

Small Form-fac to r PCI (SPCI)

Small form-factor PC I (SPCI), is simply a miniature version of th e standard PCI cardformat. This version makes SPCI appropriate for use in space-constrained platforms,potentially including mobile PCs. SPCI was developed as a compact and cost-effectivemeans fo r instailing add-in functions such as LAN ada pt e rs permanently orsemi-permanently into a host system.

Like full-size PCI cards, SPCI cards attach directly to a PCI bus; that is, they do no t needto connect to th e bus through a bridge. This attachment reduces th e cost and complex it y

of these cards relative to a bridge technology, such as CardBus. Note, though that SPCIcannot be "hot-plugged", or inserted into a running system. In addition, SPCI cards

cannot normally be installed without opening a system's covers. These drawbacks, plusth e lack of obvious over what CardBus offers, have restricted the adoption ofSPCI in the PC industry.

' s Relat ionship rdBus

As described small form-factor PCI is best suited as a compact add-in mechanismopen-covers installation. In of their sma SPCI ca rds ma y have

in mobile fact that such realize

around a standardconstraint. Since most mobile

non-standard

SPCI pre-allocation of additional

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includes bus masteringwithout assistance

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which enables it to drive its own dataCPU or DMA engine. These features enable

video data over th e PC l thusfor other uses . CPU

interconnect mechanisminto the

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The root of each device tree includes one"to-hub" which the communicates with that

tree's USB devices. All of th e USB devices are on th estandard 4-conductor cables.

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which is excessive20-25 Watts.

ma y exist on a 1394node can read th e power-source fo r each eachrequ irements. bus manager can

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CardBus possesses severa ! which suit it we!! fo r use in a mobilecomputing environment. First, CardBus shares PCl's an d protoco/ to such anextent that CardBus can be considered to a hot-pluggable mobile version o f PCl. Thissimilarity eases the implementation of CardBus in a PCl-based system . Like Pel,CardBus is a general-purpose bus which supports multiple bus maste rs. In furthersimilarity to PCI, CardBus lacks dedicated isochronous-processing (Le., guaranteed

bandwidth) s uppo rt, depending instead on bus-arbiter mechanisms if isochrony isneeded. CardBus also includes native support for dynamic PC Card attachment to an ddetachment from a functioning base platform. CardBus bridges and card-side interfacesinclude st andard mechanisms fo r handling card/socket status-change interrupts (e.g., oncard attach and detachL and fo r isolating a card from the CardBus bridge's host ( PCl )bus . The PC Card standard also defines software mechanisms which support theoperation of these hardware mechanisms.

Mobile computing demands more than just high performance and architectural flexibil ity ;mobile platfo rms and t heir assoc ia ted peripheral devices must also be power-efficient.Again, CardBus rises to th e chal lenge, with it s CCLKRUN# card-clock control signal andprotocol. An analogous signa!, CLKRUN#, is defined in th e Mobile PCl standard; butwhereas implementation of CardB us CCLKRUN is c o mmonplace, implementation of PClCLKRUN is not. This gives Card Bus cards an advantage in control of on-card dynam icpower dissipation. This advantage is further en trenche d in Card Bus cards which supportthe several power-management states defined in th e ACPI specification.

These "mobile-friendly" attributes of CardBus are dilute d somewhat by th e expense ofthe miniature cards and small Ie packages required in PC Cards. theseminiaturization costs are also incurred SPCI. The cost Card Bus' nearest

competitor, IEEE 1 presently exceeds that Card Bus. Onlyit s much lower levels and device volumes, haslower cost than mobile are

more cost ofCard Bus is no t inordinate

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9/201004:24



New Bus Architecture: http://www pcmcia .orgipapers/n ew

Committee is also well-placed to groom an eventual successor to CardBus. Add-in andadd-on capability will continue to be essential as a means fo r upgrading and enhancingmobile platforms. Since this is th e role presently filled by PC Cards, th e PCMCIACommittee is an ideal candidate to do th e strategic planning and subsequentspecif ication of tomor row'S prefer red standar d mobile add-i n and add-on mechanism s.Such an un d erta king is a logical and attai nab le extension of th e PCMCI A's presentmission .

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new bus architectures - how cardbus fits with ieee 1394, usb, pci and others

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