Designing Embedded Hardware, 2nd Edition

A DSP56805-Based Computer

The DSP56805 has nine power pins. Each of these must be decoupled to ground using 100 nF ceramic capacitors. Each capacitor should be placed as close as possible

Figure 19-2. DSP56805 block diagram

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 the preferred option for construction. (See Chapter 6 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 80 MHz (giving 40 MIPS) or as slow as a few MHz to save power. The processor may even have its clock completely stopped (so-called "DC operation," meaning 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 an inbuilt oscillator circuit, requiring only an external crystal in the range of 4 MHz to 8 MHz and support components (Figure 19-3). From this low crystal frequency, the processor internally synthesizes a clock speed of between 40 MHz and 110 MHz under software control. Note that while the clock-generation circuit is able to produce 110 MHz, the processor isn't able to operate ...