MCU Internal Clock Generator (ICG)
MC1321x Reference Manual, Rev. 1.6
Freescale Semiconductor
14-9
LOLS is cleared by reading ICGS1 then writing 1 to ICGIF (LOLRE = 0), or by a loss-of-lock induced
reset (LOLRE = 1), or by any MCU reset.
If the ICG enters the off state due to Stop Mode when ENBDM = OSCSTEN = 0, the FLL loses locked
status (LOCK is cleared), but LOLS remains unchanged because this is not an unexpected loss-of-lock
condition. Though it would be unusual, if ENBDM is cleared to 0 while the MCU is in stop, the ICG enters
the off state. Because this is an unexpected stopping of clocks, LOLS will be set when the MCU wakes up
from stop.
Expected loss of lock occurs when the MFD or CLKS bits are changed or in FEI Mode only, when the
TRIM bits are changed. In these cases, the LOCK bit will be cleared until the FLL regains lock, but the
LOLS will not be set.
14.3.7
FLL Loss-of-Clock Detection
The reference clock and the DCO clock are monitored under different conditions (see
Provided the reference frequency is being monitored, ERCS = 1 indicates that the reference clock meets
minimum frequency requirements. When the reference and/or DCO clock(s) are being monitored, if either
one falls below a certain frequency, f
LOR
and f
LOD
, respectively, the LOCS status bit will be set to indicate
the error. LOCS will remain set until it is cleared by software or until the MCU is reset. LOCS is cleared
by reading ICGS1 then writing 1 to ICGIF (LOCRE = 0), or by a loss-of-clock induced reset (LOCRE =
1), or by any MCU reset.
If the ICG is in FEE, a loss of reference clock causes the ICG to enter SCM, and a loss of DCO clock causes
the ICG to enter FBE Mode. If the ICG is in FBE Mode, a loss of reference clock will cause the ICG to
enter SCM. In each case, the CLKST and CLKS bits will be automatically changed to reflect the new state.
A loss of clock will also cause a loss of lock when in FEE or FEI Modes. Because the method of clearing
the LOCS and LOLS bits is the same, this would only be an issue in the unlikely case that LOLRE = 1 and
LOCRE = 0. In this case, the interrupt would be overridden by the reset for the loss of lock.
Table 14-1. Clock Monitoring (When LOCD = 0)
Mode
CLKS
REFST
ERCS
External Reference
Clock
Monitored?
DCO Clock
Monitored?
Off
0X or 11
X
Forced Low
No
No
10
0
Forced Low
No
No
10
1
Real-Time
1
Yes
(1)
No
SCM
(CLKST = 00)
0X
X
Forced Low
No
Yes
2
10
0
Forced High
No
Yes
(2)
10
1
Real-Time
Yes
Yes
(2)
11
X
Real-Time
Yes
Yes
(2)
FEI
(CLKST = 01)
0X
X
Forced Low
No
Yes
11
X
Real-Time
Yes
Yes
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