MAC7200 datasheet(506/976 Pages) FREESCALE

506 / 976 page

Functional Description

MAC7200 Microcontroller Family Reference Manual, Rev. 2

428

Freescale Semiconductor

minimum frequency, fSCM. The System Clock is twice the Peripheral Bus Clock as shown in Figure 23-14.

Note that a CPU cycle corresponds to one Peripheral Bus Clock.

Figure 23-14. System Clock and Peripheral Bus Clock Relationship

PLL clock mode is selected with the PLLSEL bit in the CLKSEL register. When selected, the PLL output

clock drives SYSCLK for the main system, including the CPU and peripherals. The PLL cannot be turned

off by clearing the PLLON bit if the PLL clock is selected. When PLLSEL is changed, it takes a maximum

of 4 OSCCLK plus 4 PLLCLK cycles to make the transition. During the transition, all clocks freeze and

CPU activity ceases.

23.4.2.3

Clock Monitor (CM)

If no OSCCLK edges are detected within a certain time, the clock monitor within the oscillator block

generates a clock monitor fail event. The CRG then asserts self clock mode or generates a system reset

depending on the state of the CMRD bit. If the clock monitor is disabled or the presence of clocks is

detected, no failure is indicated by the oscillator block. The clock monitor function is enabled/disabled by

the CME control bit.

23.4.2.4

Clock Quality Checker

The clock monitor performs a coarse check on the incoming clock signal. The clock quality checker

provides a more accurate check in addition to the clock monitor.

A clock quality check is triggered by any of the following events:

•

Power on reset (POR)

•

Low voltage reset (LVR)

•

Clock Monitor fail indication (CM fail)

A time window of 50000 VCO clock cycles1 is called check window.

A number greater than or equal to 4096 rising OSCCLK edges within a check window is called osc ok.

Note that osc ok immediately terminates the current check window. See Figure 23-15 as an example.

VCO clock cycles are generated by the PLL when running at minimum frequency fSCM.

System Clock:

Peripheral Bus Clock

Memory Clock

MAC7200 Datasheet(PDF) 506 Page - Freescale Semiconductor, Inc