Electronic Design

DSP Or A GPP? You Decide

It's tough to be all things to all people, but designers keep trying.

Designers starting with general-purpose processors (GPPs) attempt to add DSP registers and instructions, while digital-signal-processor (DSP) designers incorporate features that enable DSPs to handle chores like TCP/IP.

So are DSPs masquerading as GPPs or vice versa? Does it really matter? Probably not, but this class of processors that fits in between is becoming more important as single-chip solutions need to provide more aggressive analog processing as well as communication support. Solutions that mix a GPP and a DSP in one chip have many advantages, but they significantly increase the complexity of the underlying system. Interprocessor communication is more difficult to implement and debug compared to running multiple applications on a single processor.

I recently spoke with Ray Simar, a designer of Texas Instruments' (www.ti.com) high-performance C6000 DSP. Ray described how the very-long-instruction-word (VLIW) design took on RISC-like characteristics that made it possible to provide improved support for non-DSP applications such as TCP/IP communication or file system management. These characteristics included a large register file, a register-based load/store architecture, and good byte manipulation instructions.

A key to the C6000's success was an excellent C compiler. This hides any complexity in an underlying processor while delivering good code that's easier to write and maintain. It allowed companies like Softier (www.softier.com) to deliver a version of embedded Linux for the C6000.

One of the most difficult elements to deal with when mixing DSP and general applications on a DSP/GPP processor is timing. Here, a small, well designed real-time operating system will be invaluable (see "Making Reservations," p. 58). They can come in handy when those DSP algorithms really need to get done on time.

The bottom line is a mixed DSP/GPP architecture that can provide solid performance in both areas. This category offers lots of interest because a number of DSPs that have taken this mixed-mode approach, and many general processors, are acquiring DSP instructions.

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