section 7 - Index of

9.1 PLL CLOCK OSCILLATOR INTRODUCTIONThe DSP56K family of processors (with the exception of the DSP56000 and DSP56001)features a PLL (phase-locked loop) clock oscillator in its central processing module,shown in Figure 9-2. The PLL allows the processor to operate at a high internal clock frequencyusing a low frequency clock input, a feature which offers two immediate benefits.Lower frequency clock inputs reduce the overall electromagnetic interference generatedby a system, and the ability to oscillate at different frequencies reduces costs by eliminatingthe need to add additional qscillators to a system.The PLL performs frequency multiplication to allow the processor to use almost anyavailable external system clock for full speed operation, while also supplying an outputclock synchronized to a synthesized internal core clock. It also improves the synchronoustiming of the processor's external memory port, significantly reducing the timingskew between EXTAL and the internal chip phases. The PLL is unusual in that it providesa low power divider on its output, which can reduce or restore the chip operatingfrequency without losing the PLL lockA DSP56K processor uses a four-phase clock for instruction execution which runs at theinstruction execution rate. It can accept an external clock through the EXT AL input, or itcan run on an internal oscillator, bypassing the PLL function, when the user connects anexternal crystal between XTAL and EXTAL. (The PLL need not be disabled when theprocessor accepts an external clock.)9.2 PLL COMPONENTSThe PLL block diagram is shown below in Figure 9-1. The components of the PLL are describedin the following sections.EXTAL-'" Charge Voltage Low- PhasePump Controlled PowerDetector ~ ..... ..Divider ~--. Loop Oscillator -(PO)Filter (VCO) 2 0 to 2 15 DIVIDER OUTDFO-DF3..- VCOOUT-FrequencyMultiplierMultiplication r--Factor1 to 4096MFO-MF11Figure 9-1 PLL Block Diagram