Motorola DSP56000 manual PLL Clock Oscillator Introduction, PLL Components, Vco

PLL CLOCK OSCILLATOR INTRODUCTION

9.1PLL CLOCK OSCILLATOR INTRODUCTION

The 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 fre- quency using a low frequency clock input, a feature which offers two immediate benefits. Lower frequency clock inputs reduce the overall electromagnetic interference generated by a system, and the ability to oscillate at different frequencies reduces costs by eliminat- ing the need to add additional oscillators to a system.

The PLL performs frequency multiplication to allow the processor to use almost any available external system clock for full speed operation, while also supplying an output clock synchronized to a synthesized internal core clock. It also improves the synchro- nous timing of the processor’s external memory port, significantly reducing the timing skew between EXTAL and the internal chip phases. The PLL is unusual in that it pro- vides a low power divider on its output, which can reduce or restore the chip operating frequency without losing the PLL lock

A DSP56K processor uses a four-phase clock for instruction execution which runs at the instruction execution rate. It can accept an external clock through the EXTAL input, or it can run on an internal oscillator, bypassing the PLL function, when the user connects an external crystal between XTAL and EXTAL. (The PLL need not be disabled when the processor accepts an external clock.)

9.2PLL COMPONENTS

The PLL block diagram is shown below in Figure 9-1.The components of the PLL are de- scribed in the following sections.

EXTAL

Phase

Detector

(PD)

Charge	Voltage
Pump	Controlled
Loop	Oscillator
Filter	(VCO)

Frequency

Multiplier

Multiplication

Factor

1 to 4096

Low

Power

Divider DIVIDER OUT

20 to 215

DF0-DF3

VCO OUT

MF0-MF11

Figure 9-1 PLL Block Diagram

MOTOROLA PLL CLOCK OSCILLATOR 9 - 3