Electronic Design

Take Advantage Of Rider-Card Standards To Shrink PC Connectivity Costs

Riser cards provide new ways to deploy modem functions, helping conserve PCI slots, reduce part count, and speed time to market.

What's the fastest, easiest, and least expensive way to implement connectivity with the outside world on a PC? Just take advantage of recent riser-card standards that let manufacturers decouple and redeploy the various functions that make up modems and other communications subsystems. These standards continue a trend that began when external modem boxes turned into modem add-in cards, which then became modem software running on the PC's host CPU.

Specifically, riser cards let users move the most complex functions off the communications card so it can be shrunk and installed in a slot on the motherboard, rather than in a PCI slot of its own. Modems no longer need DSPs, modem control, and call-handling logic, leaving behind only the bare minimum hardware for a physical and electrical connection—the codec and telephone interface circuits.

A similar cost-savings strategy can be applied to other connectivity subsystems, such as DSL and Ethernet. Given the riser card's now-simplified interface requirements, it's possible to replace the card's PCI parallel interface with a less-expensive serial interface. To realize all these economies, there need to be standards for implementing the riser cards and the serial interface. These cards have arrived, and OEMs and their customers can begin reaping the benefits.

As with most PC standards, the initiatives surrounding riser cards have taken shape as a series of company- and industry-sponsored recommendations. Some of these overlap. On the surface, some even appear to compete with each other. The three primary standards OEMs need to be concerned with when implementing PC connectivity based on riser cards are Audio Mobile Riser and Mobile Daughter Card (AMR/MDC), Communication Network Riser (CNR), and Advanced Communication Riser (ACR). These standards attempt to make PC connectivity less expensive and more efficient to implement by reducing the amount of communications-specific hardware required and by simplifying the communications channel interface.

The key is to recognize that each type of communication subsystem is comprised of a consistent set of functional blocks, that there are optimum ways to deploy these blocks, and that these "rules of deployment" are similar whether designers are working on modems, audio, Ethernet, DSL, or HomePNA (which implements local area networks in the home using telephone jacks). One major benefit is that riser cards reduce the competition for PCI slots as the proliferation of add-on options grows, the size of PCs shrinks, and the number of available PCI slots is reduced.

Modems, a major focus of the standards, illustrate the various issues surrounding riser-card implementations. All modems, for example, must somehow implement the functions provided by the following parts:

  • Direct access arrangement (DAA): The DAA circuit provides the Public Switched Telephone Network (PSTN) isolation circuitry and two- to four-wire RJ-11 interface necessary for the PC to physically and electrically link to the telephone network.
  • Codec: This provides the digital-to-analog and analog-to-digital conversion that turn voltages on the phone line into discrete binary values (sines and cosines) processors can handle.
  • DSP: The DSP demodulates the sines and cosines from the codec into bytes of information the CPU can understand. It then modulates data from the CPU into sines and cosines. These real-time, computationally intensive operations not only modulate and demodulate the data stream, they also perform error checking and data compres-sion/decompression.
  • Modem controller: This device handles PCI and modem control.
  • EEPROM: The programmable memory is used to upgrade modem features. It's also important for assigning vendor identifications required for getting the Microsoft Windows Hardware Quality Labs (WQHL) "stamp of approval" for each function.
  • UART: The UART provides parallel-to-serial and serial-to-parallel conversion between the modem's data path and the CPU or system bus.

Over the past decade or so, the cost of implementing these functions has dropped dramatically. This is largely due to their migration out of the hardware-based modem which, as previously stated, has transitioned from a box to a PCI card to a riser card (Fig. 1). Such a migration path, where functions are implemented in software, results in the most dramatic cost reductions.

At the "less than $20" tier, all functions are implemented within the modem as hardware, including DSP and modem control. At "less than $15," those two functions are moved into software running on the CPU. This takes advantage of the "free" unused CPU cycles available on today's powerful Pentium-class machines while making the modem much easier to upgrade. Instead of reprogramming the EEPROM chip or buying another modem, designers can upgrade simply by downloading software over the Internet. At "less than $10," even greater cost savings are available by getting rid of the PCI control and migrating the entire modem controller to the motherboard chip set.

This strategy works for modems as well as for HomePNA, DSL, Ethernet, and audio. As with modems, both HomePNA and DSL have traditionally been implemented as PCI add-in cards, with each of these functions comprised of a digital controller and an analog interface. And just like modems, the opportunity exists to move the digital controller to the motherboard chip set. In fact, that's what the new class of Southbridge chips—commonly known as Super Southbridge—provide.

A Southbridge chip is half of the core logic in a traditional PC architecture (Fig. 2). The other half is the Northbridge chip. The Southbridge functions as the I/O controller running the PCI, IDE, and USB interfaces. It sends this I/O data to the Northbridge chip, which controls traffic between memory, graphics, and the CPU. In the traditional architecture, the only way the Southbridge chip sees the modem is through the PCI interface, hence the need to have a PCI controller on each modem card.

In the new core-logic chip sets, however, the Southbridge chip contains advanced controllers and interfaces to support audio, modem, and other functions. Because of this, instead of implementing audio and modem functions as PCI add-in cards, they can be implemented on a modem riser interfaced directly to the Super Southbridge chip (Fig. 3). These chips use AC '97's AC link as the serial interface standard to communicate with the audio and modem functions.

Taking the PCI controller off the card achieves other benefits besides eliminating redundant controllers. It also removes the need to use the PCI bus as a solution for interfacing the card to the motherboard so it can be replaced by a much less expensive, though just as functionally adequate, serial interface. Its performance can even be better than PCI. Direct chip-set communications avoid PCI bus congestion that might interfere with PC connectivity.

In a nutshell, riser cards trim PC connectivity costs. The various standards let multiple component sources execute on this strategy, producing economies of scale while forcing suppliers to compete on price, performance, and innovation. One way component makers can innovate is by replacing the modem's transformer-based telephony DAA with a solid-state capacitor-based design that takes advantage of IC technologies to reduce size, power, and cost. Another method combines functions on the riser card that reduce cost and increase value for the end user.

A prime example is combining HomePNA and V.90 (56k) on a single CNR or ACR card. An increasing trend in the United States is to network two or more home PCs using a HomePNA network. One PC acts as the server, providing an Internet connection. The other PCs are networked to the server and can share printers, peripherals, and the connection to the Internet.

Implementing a V.90/HomePNA combo riser is less expensive than implementing a separate card for each function. The combo riser eliminates redundant hardware and requires only one RJ-11 connector instead of two. End users also don't have to plug the server into two separate telephone jacks—one for the Internet connection and one for connecting to other PCs in the home.

Another valuable combination, modem and Ethernet, is useful for notebook owners who have an Ethernet LAN at work and an analog modem connection at home. A third desirable combination is a V.90/DSL/HomePNA riser card. Users who have DSL service in their areas can employ the DSL/HomePNA portion. Those who don't can use the V.90/HomePNA portion. PC OEMs can sell DSL-ready PCs that will work in areas where DSL service has yet to arrive.

An issue OEMs should consider when selecting riser-card chip sets and drivers is whether they will work with any or all implementations of the riser card standard and in any combination of function implementations. OEMs should ask the supplier how many environments the chip set supports and for how long.

In the new paradigm, increasing amounts of functionality will reside in software running in the host CPU. One significant challenge facing modem designers, therefore, is software support for the different Super Southbridge chip sets from various suppliers. Each Southbridge chip-set vendor's implementation of the serial link interface (between the riser card and the Southbridge chip set) is different.

The ideal is to develop a set of software drivers sufficiently robust so that they will run on all chip sets without requiring a different set of drivers. That's possible if the software designer has partitioned the driver's architecture so a clear separation exists between the actual hardware-dependent drivers and the rest of the modem stack (Fig. 4). This way, it only takes a relatively small and localized set of changes for the software provider to support various hardware platforms and chip sets. The driver can automatically detect the Super Southbridge chip set used at runtime and revert to the appropriate driver for that chip set.

Support for various modem and audio configurations is another design challenge. One common implementation in-cludes the Super Southbridge chip and a hardware audio codec directly on the motherboard while providing a riser slot (desktop) or a daughter card connector (notebook) for a soft modem riser card. That is, the primary (audio) codec is on the motherboard and the secondary (modem) codec is in the riser.

Other motherboard manufacturers may choose a completely different implementation, opting instead for a high-end PCI audio card. In this case, the software modem in the riser card will be the primary codec, a condition that must be accommodated by the software modem drivers. This can be done at runtime by detecting the system configuration (primary/secondary) in the Southbridge and reverting to the proper codec to support it.

These strategies for optimizing PC communications subsystems in riser-card implementations make a difference in cutting cost, reducing time-to-market, and enhancing end-user convenience. They illustrate how, even in a market increasingly defined by standards, OEMs should carefully consider the products they choose.

The standards themselves also have implications regarding design flexibility, functional richness, and breadth of industry support. Not all types of communications categories, such as DSL, are supported by all standards. OEMs should be aware of these differences, just as they should be alert to differences between component providers. They also should realize these standards do not support an aftermarket I/O expansion slot, which should be implemented by system manufacturers or system integrators at the factory. Standard I/O expansion slots, like those supported by the PCI bus architecture, are still intended to continue serving the retail channel upgrade market.

As stated earlier, there are three key standards for implementing riser-card-based connectivity solutions in PCs—AMR/MDC, CNR, and ACR (see "10 Key Facts About The Standards," p. 116).

The first riser-card form-factor connectivity standard, AMR/MDC, dealt with audio and modems. It defined the architecture, electrical characteristics, and mechanical requirements of the riser/daughter card. Proposed by Intel in 1997, it has been accepted by the PC industry.

AMR/MDC uses a serial interface, AC link, described in the AC '97 standard. It supports multiple codecs connected in series. One of the codecs, defined as the primary codec, provides the bit clock for the remaining codecs. These codecs are known as secondary codecs. This architecture facilitates the modem architecture in which the modem front end (codec and DAA) is connected via the serial AC-link bus directly to the Super Southbridge chip set (Fig. 3, again). The chip set also contains the audio and modem control required to drive the AC-link bus. Incorporating AC link directly in the core-logic chip set lets users directly connect audio codecs (AC '97) and modem codecs (MC '97) to the host CPU's core-logic chip set.

As a follow up to AMR/MDC, Intel recently announced the CNR standard. It supports AC link, so audio and modems can be implemented in the same manner as in AMR. Yet CNR adds new buses for networking devices such as HomePNA and Ethernet.

CNR-compliant Super Southbridge chips with integrated network controllers, in addition to modem and au-dio codecs, are now available. These network controllers, known as media access controllers (MACs), interface to PHY network analog interfaces. The PHY is the only component required to interface the LAN or HPNA directly to the Super Southbridge chip. It can be implemented directly on the motherboard or the riser card (Fig. 5).

In addition to the AC '97 and LAN interface, CNR supports two other interfaces, USB and SMBus. The SMBus is used to access EEPROM for the riser card. This makes it easier to assign the vendor IDs required for Microsoft WHQL certification.

An alternative to the Intel-sponsored CNR is the ACR, put forth by the ACR Special Interest Group (Fig. 6). ACR is similar to CNR, except its ACR connectors are backwards-compatible with AMR. Also, ACR supports a high-speed packet bus for integrating DSL modems. And, the ACR specification was designed by a large consortium of companies.

Like CNR, ACR supports AC link, Ethernet/HPNA interfaces, and an EE-PROM bus. But it also supports an integrated packet bus (IPB) for DSL. ACR (but not CNR), then, would support the integrated DSL/modem/HPNA riser.

Another significant difference is the PCI interface. According to the ACR Special Interest Group web site, www.acrsig.com/doc01.htm, the ACR standard is meant to "preserve choice in selection of discrete silicon components to promote industry innovation and product differentiation." ACR doesn't require interfacing to an Intel chip set. The PCI connector, as opposed to CNR's entirely new connector, also preserves AMR compatibility. ACR uses a low-cost PCI connector in a reverse offset fashion so existing AMR cards can be used in new ACR motherboards.

The standard selection—AMR/MDC, CNR, or ACR—depends on factors such as cost, parts availability, compatibility with existing designs, and future direction. For example, AMR/MDC specifies support for multiple channel audio, such as what's required for DVD audio. But today's Super Southbridge chips only support two-channel (stereo) audio. Likewise, the AMR spec calls for two-modem support, but the current Super Southbridge chips only support one modem.

OEMs need to consider how they will partition the audio, modem, and other subsystems to address various user models, and whether there are robust riser chip sets and driver software solutions available to support them. Fortunately, these are choices OEMs have now that they didn't have before. By migrating functionality away from the add-in card and onto the motherboard, the riser-card standards open new possibilities for reducing PC connectivity costs while tailoring designs to user environments. Depending on which choices OEMs make, exploiting these possibilities can be profitable.

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