Motorola MCF5282, MCF5281 user manual Functional Description, System Clock Modes

Functional Description

		Table 9-5. SYNSR Field Descriptions (continued)
Bit(s)	Name		Description
2	LOCS		Sticky indication of whether a loss-of-clock condition has occurred at any time since
			exiting reset in normal PLL and 1:1 PLL modes. LOCS = 0 when the system clocks are
			operating normally. LOCS = 1 when system clocks have failed due to a reference
			failure or PLL failure.
			After entering stop mode with FWKUP set and the PLL and oscillator intentionally
			disabled (STPMD[1:0] = 11), the PLL exits stop mode in the SCM while the oscillator
			starts up. During this time, LOCS is temporarily set regardless of LOCEN. It is cleared
			once the oscillator comes up and the PLL is attempting to lock.
			If a read of the LOCS flag and a loss-of-clock condition occur simultaneously, the flag
			does not reflect the current loss-of-clock condition.
			A loss-of-clock condition can be detected only if LOCEN = 1 or the oscillator has not
			yet returned from exit from stop mode with FWKUP = 1.
			1 Loss-of-clock detected since exiting reset or oscillator not yet recovered from exit
			from stop mode with FWKUP = 1
			0 Loss-of-clock not detected since exiting reset
			Note: The LOCS flag is always 0 in external clock mode.
1–0	—		Reserved, should be cleared.

Table 9-6. System Clock Modes

PLLMODE:PLLSEL:PLLREF	Clock Mode
000	External clock mode
100	1:1 PLL mode

110Normal PLL mode with external clock reference

111Normal PLL mode with crystal reference

9.7Functional Description

This subsection provides a functional description of the clock module.

9.7.1System Clock Modes

The system clock source is determined during reset (see Table 30-10). The values of CLKMOD[1:0] are latched during reset and are of no importance after reset is negated. If CLKMOD1 or CLKMOD0 is changed during a reset other than power-on reset, the internal clocks may glitch as the system clock source is changed between external clock mode and PLL clock mode. Whenever CLKMOD1 or CLKMOD0 is changed in reset, an immediate loss-of-lock condition occurs.

Table 9-7shows the clockout frequency to clockin frequency relationships for the possible system clock modes.

9-10 MCF5282 User’s Manual MOTOROLA