FPGA configuration is often a last-minute design decision, because engineers view FPGA configuration as an easy, no-brainer step in the design cycle. That is true when customers use the Xilinx “recipe” – a Xilinx® FPGA, Platform Flash PROM, ISE™ software, and platform cable USB. However, FPGA configuration becomes increasingly complex if you use a non-Xilinx solution. In this article, I’ll discuss the differences between Platform Flash and commodity Flash (see Table 1 for a summary).

Added Flexibility in Configuration Solutions
Before the introduction of the Spartan™-3E FPGA family, customers who configured Xilinx FPGAs with commodity Flash would use a three-chip solution: an FPGA, a commodity Flash PROM, and a CPLD. Spartan-3E FPGAs eliminate the need for a CPLD (or other controller) by providing a direct interface for leading commodity Flash devices, thus reducing the chip count to two (FPGA and PROM).

Because Xilinx gives you complete flexibility to use multiple memory sources for FPGA configuration, you should consider the following factors at the start of the design process: total cost of ownership, board space, configuration speed, source of supply, valueadded features, and ease of use. If these factors are not thoroughly thought out from the beginning, then the final design can incur additional costs and possible board redesign.

Total Cost of Ownership
On a per-unit basis, commodity Flash might appear to be attractively priced; however, you need to consider the total cost of ownership. Total cost of ownership is the summation of per-unit cost, design and prototyping cost, and manufacturing and test cost.

1. The cost difference between a commodity Flash PROM versus a Platform Flash PROM is negligible when compared to the overall board cost. In fact, Xilinx Platform Flash PROMs are competitively priced with all non-volatile memories in the market (commodity Flash PROM and competing PROMs).
2. The prototyping phase of a design significantly favors Platform Flash over commodity Flash because Xilinx offers one of the lowest cost in-system programming (ISP) solutions:
   • Platform cable USB: $150
   • Programming software – iMPACT: $0
   • Xilinx award-winning support: $0 (included)
3. Once in production, you can significantly reduce costs by utilizing the Boundary Scan (JTAG) capability of Xilinx FPGAs and Platform Flash PROMs (along with other JTAG devices on the board) for low-cost Boundary Scan testing and programming. Commodity Flash devices do not offer JTAG interfaces; therefore, customers cannot take advantage of the low cost of Boundary Scan testing. In most cases, expensive Automatic Test Equipment (ATE) is required to test and perform in-system programming of commodity Flash memories.

• Standard SPI PROMs are typically offered in the smallest form factor. The 1 Mb to 4 Mb SPI PROMs are usually offered in a SOIC-8L (5 x 6 mm) package and 8 Mb (and larger) devices are usually offered in a SOIC-16L (10 x 6 mm) package.
• Platform Flash PROMs are a close second with 1, 2, and 4 Mb PROMs offered in a TSOP-20L (6.5 x 6.4 mm) package and 8, 16, and 32 Mb PROMs in a TFBGA-48 (8 x 9 mm) package.
• Parallel commodity Flash devices have large packages to address the additional control, address, and I/O pins.

Configuration Speed
Parallel commodity Flash devices are typically the fastest memory on the market. They are offered in either x8 or x16 I/O configurations. The theoretical data transfer rate can be as fast as 50 MHz x 16 I/O, but there are limitations when configuring a Xilinx FPGA.

1. At this time, Xilinx FPGAs can only be configured in x8 mode.
2. Before Spartan-3E devices, configuring a Xilinx FPGA with a commodity Flash PROM would require using a CPLD device to translate memory into the FPGA bitstream (refer to XAPP058, “Xilinx In-System Programming Using an Embedded Microcontroller”). Resulting data transfer rates can degrade based on the translation logic.
3. If using a Spartan-3E device with a parallel commodity Flash, the configuration mode is limited to 6 MHz. Platform Flash features a maximum transfer rate of 40 MHz x 8 I/O (or 320 bps) with Spartan-3E devices.

Source Supply
Although there are many commodity Flash vendors, you should be aware of two potential pitfalls. First, every vendor provides similar commodity Flash PROMs, but there are nuances with each vendor that can limit their interoperability. For example, a STMicro SPI is not fully compatible with an Atmel SPI PROM. Second, during a period of tight source supply, customers might find themselves paying more for expedites or end up with very long lead times. Xilinx answers the source supply conundrum by holding a large inventory of Platform Flash at finished goods, which allows Xilinx to quickly react to increased demand.

Ease of Use
Platform Flash was designed to work seamlessly with all Xilinx FPGAs and is also supported by an award-winning support team. Xilinx provides a total configuration solution that includes software and hardware. No other configuration memory solution offers this type of support.

Value-Added Features
Finally, Platform Flash offers the following value-added features that are not found in commodity Flash:

1. Compression. The higher density Platform Flash PROM devices have built-in de-compressors, which, on average, can fit 50% more configuration data into the same memory space. Xilinx patented compression technology can help you reduce costs in two ways:
   • Reduce component costs by fitting a large bitstream into a lower density Platform Flash PROM device. For example, a Virtex™-4 LX60 design requiring more than 17 Mb of configuration bits can fit into a XCF16P instead of a 32 Mb PROM.
   • Reduce component count by fitting a design into one PROM as opposed to two or more. For example, a Virtex-4 LX160 device requires more than 40 Mb of configuration bits, which normally would require a 32 Mb and 8 Mb PROM, but compression enables the design to fit into a single XCF32P.
2. JTAG. Allows low-cost board-level Boundary Scan testing for opens and shorts, as well as programmability during prototyping and in the production environment.
3. Design Revisioning. Allows one board to have many functions. Platform Flash PROMs (XCF08P, XCF16P, and XCF32P) have blocks of memories that can be written and read independently of one another. The logic to switch between each block is already built into Platform Flash, thus reducing design time and cost. Although commodity Flash devices have a similar feature called “sectors,” you would need glue logic and software to access the various sectors.
4. Access to unused memory. Most FPGA bitstreams will not use all of the memory of a PROM. Thus, any unused memory can be used for processor “scratch pad” or “boot code.” You can access unused memory within a Platform Flash PROM through JTAG (refer to XAPP544, “Using Xilinx XCF02S/XCF04S JTAG PROMs for Data Storage Applications”). Note that you can still access unused memory within commodity Flash PROM, but you might need to design additional logic and software to access the unused memory.

Platform Flash PROMs provide you with a system-level drop-in solution that allows you to maximize the flexibility of Virtex and Spartan FPGAbased systems to significantly reduce your design effort and accelerate time to market. Platform Flash PROMs are competitively priced, reduce the amount of board space required for configuration, and offer a complete 1 to 32 Mb PROM density solution (Table 2).

For more information, visit www.xilinx.com/products/silicon_solutions/proms/pfp/index.htm.

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