Motorola ColdFire MCF5281 - Manual (Page 171)

Freescale Semiconductor 9-1
Chapter 9
Clock Module
The clock module configures the device for one of several clocking methods. Clocking modes include
internal phase-locked loop (PLL) clocking with either an external clock reference or an external crystal
reference supported by an internal crystal amplifier. The PLL can also be disabled and an external
oscillator can be used to clock the device directly. The clock module contains:
• Crystal amplifier and oscillator (OSC)
• Phase-locked loop (PLL)
• Reduced frequency divider (RFD)
• Status and control registers
• Control logic
9.1 Features
Features of the clock module include:
• 2- to 10-MHz reference crystal oscillator
• Support for low-power modes
• Separate clock out signal
9.2 Modes of Operation
The clock module can be operated in normal PLL mode (default), 1:1 PLL mode, or external clock mode.
9.2.1 Normal PLL Mode
In normal PLL mode, the PLL is fully programmable. It can synthesize frequencies ranging from 2x to 9x
the reference frequency and has a post divider capable of reducing this synthesized frequency without
disturbing the PLL. The PLL reference can be either a crystal oscillator or an external clock.
9.2.2 1:1 PLL Mode
In 1:1 PLL mode, the PLL synthesizes a frequency equal to the external clock input reference frequency.
The post divider is not active.
9.2.3 External Clock Mode
In external clock mode, the PLL is bypassed, and the external clock is applied to EXTAL. The resulting
operating frequency is equal to the external clock frequency.
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3