MAC7200 datasheet(145/976 Pages) FREESCALE

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Clock Usage

MAC7200 Microcontroller Family Reference Manual, Rev. 2

Freescale Semiconductor

Figure 5-7. External Clock PLL Mode Clock Path

5.2.2

Self Clock Mode (SCM)

If the external clock frequency is not available due to a failure or due to long crystal start-up time, the

System Clock is derived from the VCO in the PLL, running at minimum operating frequency. This mode

of operation is called Self Clock Mode. Self Clock Mode may be entered at any time, upon failing the

Clock Quality Check. Self Clock mode may only be exited by passing a Clock Quality Check after a

system reset. When the system is in Self Clock Mode, the RTI, SWT and Oscillator Clock are all

automatically switched to PLLCLK.

5.2.3

Crystal Monitor

The MAC72xx provides an on-chip Crystal Monitor that detects a loss of Oscillator Clock. Please refer to

Section 23.4.2.3, “Clock Monitor (CM)” for more details. Note that the terms “Crystal Monitor” and

“Clock Monitor” are used interchangeably.

5.2.4

Clock Quality Checker

In addition to the Crystal Monitor, the MCU also provides a Clock Quality checker which performs a more

accurate check of the Oscillator Clock. Please refer to Section 23.4.2.4, “Clock Quality Checker” for more

details.

5.3

Clock Usage

Table 5-3 summarizes the usage of clocks on all MAC72xx modules.

OSC

EXTAL

OSCCLK

0 1

XTAL

PLL

PLLSEL

PLLCLK

System Clock

CRG

÷2

Peripheral

Bus Clock

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