Information

5.5.1 Clock divider values after reset
Each clock divider is programmed via the SIM module’s CLKDIVn registers. The flash
memory's FTFL_FOPT[LPBOOT] bit controls the reset value of the core clock, system
clock, bus clock, and flash clock dividers as shown below:
FTFL_FOPT
[LPBOOT]
Core/system
clock
Bus clock FlexBus clock Flash clock Description
0 0x7 (divide by 8) 0x7 (divide by 8) 0xF (divide by 16) 0xF (divide by 16) Low power boot
1 0x0 (divide by 1) 0x0 (divide by 1) 0x1 (divide by 2) 0x1 (divide by 2) Fast clock boot
This gives the user flexibility for a lower frequency, low-power boot option. The flash
erased state defaults to fast clocking mode, since where the low power boot
(FTFL_FOPT[LPBOOT]) bit resides in flash is logic 1 in the flash erased state.
To enable the low power boot option program FTFL_FOPT[LPBOOT] to zero. During
the reset sequence, if LPBOOT is cleared, the system is in a slow clock configuration.
Upon any system reset, the clock dividers return to this configurable reset state.
5.5.2 VLPR mode clocking
The clock dividers cannot be changed while in VLPR mode. They must be programmed
prior to entering VLPR mode to guarantee:
• the core/system, FlexBus, and bus clocks are less than or equal to 2 MHz, and
• the flash memory clock is less than or equal to 1 MHz
5.6 Clock Gating
The clock to each module can be individually gated on and off using the SIM module's
SCGCx registers. These bits are cleared after any reset, which disables the clock to the
corresponding module to conserve power. Prior to initializing a module, set the
corresponding bit in SCGCx register to enable the clock. Before turning off the clock,
make sure to disable the module.
Any bus access to a peripheral that has its clock disabled generates an error termination.
Clock Gating
K10 Sub-Family Reference Manual, Rev. 6, Nov 2011
168 Freescale Semiconductor, Inc.