Designing Embedded Hardware

A DSP56805-Based Computer

The DSP56805 has nine power pins. Each of these must be decoupled to ground using 100nF ceramic capacitors. Each capacitor should be placed as close as possible to its respective power pin. Since this processor can operate at a relatively high speed, and can therefore generate a lot of noise, a four-layer circuit board is preferred for construction. (See Chapter 4 for more information.) As with any design, any unused inputs must be tied inactive.

Oscillator

Like all processors, the DSP56805 requires a clock signal. The processor can operate from an oscillator frequency of up to 80MHz (giving 40MIPS) or as slow as a few MHz to save power. The processor may even have its clock completely stopped (so-called DC operation, meaning that the clock is no longer an AC signal) to further save power. (This processor’s sibling, the DSP56801, has a complete internal oscillator and so requires no external clock generation circuit.)

The processor has a built-in oscillator circuit, requiring only an external crystal in the range of 4MHz to 8MHz and support components (Figure 8-3). From this low crystal frequency, the processor internally synthesizes a clock speed of between 40MHz and 110MHz under software control. Note that while the clock generation circuit is able to produce 110MHz, the processor isn’t able to operate at the speed. So keep the speed below 80MHz, and the processor, your software, and you will all be happy.

Figure 8-3. Crystal oscillator circuit

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