Programmable logic: The key to effective interface design

Ideally, OEMs should be able to get exactly the interfaces they need, without the cost and drawbacks of buying a fully-featured DSP.

On the topic of shrinking designs using programmable logic, Doug walks us through how complex systems like media gateways are improved using FPGAs to incorporate faster, and sometimes missing, interfaces to DSPs. Now that telecommunications OEMs are seeing sustained growth in the VoIP arena, margins are being squeezed and equipment designers are being asked to do more with less. For example, IP PBX and multiservice access nodes (MSANs) need to be price competitive at many different channel densities, and yet, the engineering cost of designing many different platforms is exorbitant.

Now that telecommunications OEMs are seeing sustained growth in the VoIP arena, margins are being squeezed and equipment designers are being asked to do more with less. For example, IP PBX and multiservice access nodes (MSANs) need to be price competitive at many different channel densities, and yet, the engineering cost of designing many different platforms is exorbitant.

Media gateway functionalities are best offered today using DSP technology. A few select vendors offer field-proven products that include a complete hardware and software solution. But in order to properly take advantage of these solutions, hardware designers are asked to make compromises.

The main problem is that DSPs don’t always offer the right combination of DSP performance and interfaces. For example, very often it is only the largest DSP in a family that supports high-end interfaces such as SGMII Ethernet or . When it comes time to interconnect many DSPs in larger systems, an is often used to glue devices together and perform functions such as back-pressure on packet transmission or packet classification. Hardware designers have already understood the value of using in these scenarios.

Same DSP, but different interfaces

While different types of equipment can use the same underlying media processing technology, they each have specific requirements and constraints. Very often, the same DSP devices are targeted at all of the following applications:

  • Large-scale media gateways
  • IP PBX
  • HMP media server off-loading
  • MSAN/Access equipment

While the basic feature set may be the same, all applications require VoIP processing with a list of codecs and voice quality features; the interfaces required for each may be quite different. For example, a PCI card designed for host-based media processing (HMP) off-loading would require a PCI Express interface, whereas a MSAN may require a lightweight interface to an Ethernet PHY and to T1/E1 framers, as well as a lot of glue logic to the various components on the board. Sometimes the ideal solution is "off the beaten track." Designers may sometimes find a more cost-effective way of doing things, but find that they are limited by what the DSPs support. In the IP PBX space, certain vendors have discovered that USB is the ideal interconnect for low-cost systems, instead of Ethernet.

USB provides many advantages. First, it is hot-pluggable by design. More importantly, it offers a cost structure that is more advantageous than Ethernet. A USB hub device is less expensive and less complex than an Ethernet switch, which means that the base cost for a system is lower. Furthermore, the incremental cost is lower, because for each processing blade added to the system, very simple, low-cost RISC processors are available with USB interfaces. Most DSPs offered into the VoIP gateway market do not support USB, whereas FPGA vendors provide IP cores for such an interface. Leveraging allows for this kind of innovation.

Combining DSPs with FPGAs

Ideally, OEMs should be able to get exactly the interfaces they need, without the cost and drawbacks of buying a fully-featured DSP. Typically, the DSPs that contain high-speed interfaces tend to have a large number of different interfaces, and be in a very large package with a very high pin count. These large packages are dense BGAs that are expensive and require a PCB with a high number of layers, which can all contribute to design delays.

By combining an FPGA with a DSP, the hardware designer can get the best of both worlds. In cases where the DSP interfaces are judged inadequate, or customization was required, DSPs are often front-ended with an FPGA. Adding an FPGA to an already fully-featured DSP is overkill. It adds unnecessary cost and board real estate, only to correct short comings in the DSP.

A different approach can be taken when combining a DSP and FPGA. In this method, the DSP serves only as a DSP. The physical package contains a high-performance, device and uses a single memory interface to communicate with the FPGA. In this case, there is no carrying cost for the interfaces inside the DSP, keeping area and power to an absolute minimum. The DSP vendor provides turnkey FPGA designs for each major application, which can then be customized by the OEM if desired. The FPGAs used can be high-end devices or very small, low-cost devices, depending on the target application (Figure 1).

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Figure 1
(Click graphic to zoom by 2.0x)

The main advantage to this approach is the flexibility that it offers. FPGAs can be reprogrammed in the field if necessary. Furthermore, FPGAs are always among the first devices to be released at each new process node. Therefore, they always support the latest I/O technologies with optimal power performance. As interconnect standards continue to evolve, it is possible to retarget a design very quickly to the latest fabric, whether it’sPCI Express, RapidIO, or a proprietary interface.

The combined DSP and FPGA approach offers many cost sav-ings. It is important to look at the total solution when evaluating cost. For example, there is a plurality of Ethernet standards available: MII, RMII, GMII, RGMII, SGMII, and so on. When deciding how to interconnect many VoIP DSPs together, hard-ware designers can choose from a large number of Ethernet switch ICs, or consider a FPGA solution. If a certain Ethernet switch offers the ideal price point, but does not support the interfaces provided by the DSP, then a more expensive switch must be used.

More savings possible

The cost savings can be further extended beyond the electrical interface. For example, many large OEMs use proprietary headers or flags to help classify packet traffic. With an FPGA front-end, this becomes trivial and can greatly increase performance by off-loading certain tasks from the DSP. This type of header manipulation is best done in hardware by an FPGA. This offers deterministic performance, and keeps control of the design in the hands of the OEMs.

A media gateway DSP often incorporates a TDM interface that is connected to timeslot interchangers (TSIs) or T1/E1 framers. Very often, an FPGA is already present between the DSP and these TDM-based devices, or the TDM backplane. FPGAs are currently used because custom logic is required, or because the FPGAs offer more reliable I/Os that can sustain the high voltage spikes often seen on backplanes. These FPGA tasks can now be incorporated into the front-end FPGA saving more money and board area.

Another major advantage of this FPGA front-end is to make life easier for designers who again find themselves "off the beaten track." DSP manufacturers release products with feature sets and interfaces for specific market segments, usually the largest ones. It is easy to find a DSP that can serve as a generic TDM to VoIP gateway, but if the job is to design a high-performance telepresence video over IP conferencing unit, then the choice of DSPs is quite slim. The amount of bandwidth required for passing or video streams between DSPs is immense. Finding a DSP with the right interface for this or any other emerging market is quite difficult and costly.

Instead, using an FPGA front-end allows access to the latest I/O technology, as well as IP blocks for the latest standards. The designer can now get the right interfaces, without making changes to the . In organizations with large FPGA teams, the designer may choose to develop a proprietary interface that meets his or her needs perfectly and provides a competitive edge.

Media gateway designers are faced with many challenges in-cluding increased price pressure and shortened design cycles. By using a DSP that is tightly coupled to an FPGA, designers are able to get exactly the interfaces they need, without the added cost or debugging of useless I/O banks. This greatly simplifies the design and allows OEMs to add their own unique features and differentiate their system. This kind of innovation is what allows OEMs to address current and future customer requirements while maintaining a common DSP software platform.

Doug Morrissey is Vice President and Chief Technology Officer at Octasic and has over 10 years of experience in the definition and marketing of semiconductor devices. JoiningOctasic in 1999, he strategically focuses on issues within the Voice over Packet market. Previously, he worked as Marketing Manager for ATM and DSL products for Agere (formerly Lucent Technologies, Microelectronics Group), and was Senior Systems Architect at Unisys Corporation. Morrissey holds a BSc from Rochester Institute of Technology.

Octasic Inc
514-282-8858
www.octasic.com