K60P100M100SF2RM Datasheet, PDF (561/1809 Pages) Freescale Semiconductor, Inc – K60 Sub-Family Reference Manual

2. Write to C1 register to select the clock mode.
Chapter 24 Multipurpose Clock Generator (MCG)
• If entering FEE mode, set C1[FRDIV] appropriately, clear the C1[IREFS] bit to
switch to the external reference, and leave the C1[CLKS] bits at 2'b00 so that the
output of the FLL is selected as the system clock source.
• If entering FBE, clear the C1[IREFS] bit to switch to the external reference and
change the C1[CLKS] bits to 2'b10 so that the external reference clock is
selected as the system clock source. The C1[FRDIV] bits should also be set
appropriately here according to the external reference frequency to keep the FLL
reference clock in the range of 31.25 kHz to 39.0625 kHz. Although the FLL is
bypassed, it is still on in FBE mode.
• The internal reference can optionally be kept running by setting the
C1[IRCLKEN] bit. This is useful if the application will switch back and forth
between internal and external modes. For minimum power consumption, leave
the internal reference disabled while in an external clock mode.
3. Once the proper configuration bits have been set, wait for the affected bits in the
MCG status register to be changed appropriately, reflecting that the MCG has moved
into the proper mode.
• If the MCG is in FEE, FBE, PEE, PBE, or BLPE mode, and C2[EREFS] was
also set in step 1, wait here for S[OSCINIT] bit to become set indicating that the
external clock source has finished its initialization cycles and stabilized.
• If in FEE mode, check to make sure the S[IREFST] bit is cleared before moving
on.
• If in FBE mode, check to make sure the S[IREFST] bit is cleared and S[CLKST]
bits have changed to 2'b10 indicating the external reference clock has been
appropriately selected. Although the FLL is bypassed, it is still on in FBE mode.
4. Write to the C4 register to determine the DCO output (MCGFLLCLK) frequency
range.
• By default, with C4[DMX32] cleared to 0, the FLL multiplier for the DCO
output is 640. For greater flexibility, if a mid-low-range FLL multiplier of 1280
is desired instead, set C4[DRST_DRS] bits to 2'b01 for a DCO output frequency
of 40 MHz. If a mid high-range FLL multiplier of 1920 is desired instead, set the
C4[DRST_DRS] bits to 2'b10 for a DCO output frequency of 60 MHz. If a high-
range FLL multiplier of 2560 is desired instead, set the C4[DRST_DRS] bits to
2'b11 for a DCO output frequency of 80 MHz.
K60 Sub-Family Reference Manual, Rev. 6, Nov 2011
Freescale Semiconductor, Inc.
561