Motorola ColdFire MCF5281 - Manual (Page 172)

Clock Module
9-2 Freescale Semiconductor
9.3 Low-power Mode Operation
This subsection describes the operation of the clock module in low-power and halted modes of operation.
Low-power modes are described in
Chapter 7, “Power Management .” Table 9-1 shows the clock module
operation in low-power modes.
Table 9-1. Clock Module Operation in Low-power Modes
During wakeup from a low-power mode, the Flash clock always clocks through at least 16 cycles before
the CPU clocks are enabled. This allows the Flash module time to recover from the low-power mode, and
software can immediately resume fetching instructions from memory.
In wait and doze modes, the system clocks to the peripherals are enabled, and the clocks to the CPU, Flash,
and SRAM are stopped. Each module can disable its clock locally at the module level.
In stop mode, all system clocks are disabled. There are several options for enabling or disabling the PLL
or crystal oscillator in stop mode, compromising between stop mode current and wakeup recovery time.
The PLL can be disabled in stop mode, but requires a wakeup period before it can relock. The oscillator
can also be disabled during stop mode, but requires a wakeup period to restart.
When the PLL is enabled in stop mode (STPMD[1:0]), the external CLKOUT signal can support systems
using CLKOUT as the clock source.
There is also a fast wakeup option for quickly enabling the system clocks during stop recovery. This
eliminates the wakeup recovery time but at the risk of sending a potentially unstable clock to the system.
To prevent a non-locked PLL frequency overshoot when using the fast wakeup option, change the RFD
divisor to the current RFD value plus one before entering stop mode.
In external clock mode, there are no wakeup periods for oscillator startup or PLL lock.
9.4 Block Diagram
Figure shows a block diagram of the entire clock module. The PLL block in this diagram is expanded in
detail in Figure 9-2 .
Low-power Mode Clock Operation Mode Exit
Wait Clocks sent to peripheral modules only Exit not caused by clock module, but normal
clocking resumes upon mode exit
Doze Clocks sent to peripheral modules only Exit not caused by clock module, but normal
clocking resumes upon mode exit
Stop All system clocks disabled Exit not caused by clock module, but clock
sources are re-enabled and normal clocking
resumes upon mode exit
Halted Normal Exit not caused by clock module
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3