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Successful DDR2 Design

The introduction of the first SDRAM interface, in 1997, marked the dawn of the high-speed memory interface age. Since then, designs have migrated through SDR (single data rate), DDR (double data rate), and now DDR2 memory interfaces to sustain increasing bandwidth needs in products such as graphics accelerators and high-speed routers.
As a result of its highbandwidth capabilities, DDR and DDR2 technology is used in nearly every sector of the electronics design industry from computers and networking to consumer electronics and military applications. DDR technology introduced the concept of “clocking” data in on both a rising and falling edge of a strobe signal in a memory interface. This provided a 2x bandwidth improvement over an SDR interface with the same clock speed. This, in addition to faster clock frequencies, allowed a single-channel DDR400 interface with a 200 MHz clock to support up to 3.2 GB/s, a 3x improvement over the fastest SDR interface. DDR2 also provided an additional 2x improvement in bandwidth over its DDR predecessor by doubling the maximum clock frequency to 400 MHz. Table 1 shows how the progression from SDR to DDR and DDR2 has allowed today’s systems to maintain their upward growth path.



Table 1 – The progression from SDR to DDR and DDR2 has allowed today’s systems to maintain theirupward growth path. Speed grades and bit rates are shown for each memory interface.

C and C++ languages and hardware description

C and C++ have been a popular starting point for developing hardware and systems for many years; the languages are widely known, quick to write, and give an executable specification, which allows for very fast simulation. C or C++ versions of standard algorithms are widely available, so you can easily reuse legacy and publicly available code. For system level design, C and C++ allow you to describe hardware and software descriptions in a single framework.

Two drawbacks exist, however. First, C and C++ do not support the description of some important hardware concepts, such as timing and concurrency. This has led to the development of proprietary C-like languages, which aren’t popular because they tie users to a single software supplier. Second, you must manually translate C and C++ to a hardware description language such as VHDL or Verilog for hardware implementation. This time-consuming step requires hardware experts and often introduces errors that are difficult to find.

The first problem has been solved by the development of SystemC, which now a widely accepted industry standard that adds hardware concepts to C++. The second problem has been solved by the development of tools like SystemCrafter SC, which allows SystemC descriptions to be automatically translated to VHDL.

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