Motorola MCF5281, MCF5282 user manual PLL Loss of Lock Conditions, PLL Loss of Lock Reset

Functional Description

		Loss of Lock Detected
		Set Tight Lock Criteria
Start		and Notify System of Loss
		of Lock Condition
with Tight Lock	Reference Count
		Reference Count
Criteria
	≠ Feedback Count	≠ Feedback Count

Count N		Count N + K
Reference Cycles		Reference Cycles
and Compare		and Compare Number
	Reference Count =
Number of Feedback		of Feedback Cycles
Cycles Elapsed	Feedback Count = N	Elapsed
	In Same Count/Compare Sequence
	Lock Detected.	Reference Count =
		Feedback Count = N + K
	Set Relaxed Lock
		IN Same Count/Compare Sequence
	Condition and Notify
	System of Lock
	Condition

Figure 9-6. Lock Detect Sequence

9.7.4.6PLL Loss of Lock Conditions

Once the PLL acquires lock after reset, the LOCK and LOCKS flags are set. If the MFD is changed, or if an unexpected loss of lock condition occurs, the LOCK and LOCKS flags are negated. While the PLL is in the non-locked condition, the system clocks continue to be sourced from the PLL as the PLL attempts to relock. Consequently, during the relocking process, the system clocks frequency is not well defined and may exceed the maximum system frequency, violating the system clock timing specifications.

However, once the PLL has relocked, the LOCK flag is set. The LOCKS flag remains cleared if the loss of lock is unexpected. The LOCKS flag is set when the loss of lock is caused by changing MFD. If the PLL is intentionally disabled during stop mode, then after exit from stop mode, the LOCKS flag reflects the value prior to entering stop mode once lock is regained.

9.7.4.7PLL Loss of Lock Reset

If the LOLRE bit in the SYNCR is set, a loss of lock condition asserts reset. Reset reinitializes the LOCK and LOCKS flags. Therefore, software must read the LOL bit in the reset status register (RSR) to determine if a loss of lock caused the reset. See Section 28.4.2, “Reset Status Register (RSR).”

To exit reset in PLL mode, the reference must be present, and the PLL must achieve lock.

MOTOROLA Chapter 9. Clock Module 9-15