Functional Description; System Clock Modes - Motorola ColdFire MCF5281 User Manual

Functional Description

Table 9-5. SYNSR Field Descriptions (continued)
Bit(s) Name
2 LOCS
1–0 —
PLLMODE:PLLSEL:PLLREF
9.7 Functional Description
This subsection provides a functional description of the clock module.
9.7.1

System 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 9-7 shows the clockout frequency to clockin frequency relationships for the possible
system clock modes.
9-10
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.
Reserved, should be cleared.
Table 9-6. System Clock Modes
000 External clock mode
100 1:1 PLL mode
110 Normal PLL mode with external clock reference
111 Normal PLL mode with crystal reference
MCF5282 User's Manual
Description
Clock Mode
MOTOROLA