Information

• C1[CLKS] set to 2'b10 to select external reference clock as system clock
source
• C1[FRDIV] set to 3'b010, or divide-by-128 because 4 MHz / 128 = 31.25
kHz which is in the 31.25 kHz to 39.0625 kHz range required by the FLL
• C1[IREFS] cleared to 0, selecting the external reference clock and enabling
the external oscillator.
c. Loop until S[OSCINIT0] is 1, indicating the crystal selected by C2[EREFS0] has
been initialized.
d. Loop until S[IREFST] is 0, indicating the external reference is the current source
for the reference clock.
e. Loop until S[CLKST] is 2'b10, indicating that the external reference clock is
selected to feed MCGOUTCLK.
2. Then configure C5[PRDIV0] to generate correct PLL reference frequency.
a. C5 = 0x01
• C5[PRDIV0] set to 5'b001, or divide-by-2 resulting in a pll reference
frequency of 4 MHz/2 = 2 MHz.
3. Then, FBE must transition either directly to PBE mode or first through BLPE mode
and then to PBE mode:
a. BLPE: If a transition through BLPE mode is desired, first set C2[LP] to 1.
b. BLPE/PBE: C6 = 0x40
• C6[PLLS] set to 1, selects the PLL. At this time, with a C1[PRDIV] value of
2'b001, the PLL reference divider is 2 (see PLL External Reference Divide
Factor table), resulting in a reference frequency of 4 MHz/ 2 = 2 MHz. In
BLPE mode, changing the C6[PLLS] bit only prepares the MCG for PLL
usage in PBE mode.
• C6[VDIV0] set to 5'b0000, or multiply-by-24 because 2 MHz reference * 24
= 48 MHz. In BLPE mode, the configuration of the VDIV bits does not
matter because the PLL is disabled. Changing them only sets up the multiply
value for PLL usage in PBE mode.
c. BLPE: If transitioning through BLPE mode, clear C2[LP] to 0 here to switch to
PBE mode.
d. PBE: Loop until S[PLLST] is set, indicating that the current source for the PLLS
clock is the PLL.
Chapter 24 Multipurpose Clock Generator (MCG)
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
Freescale Semiconductor, Inc. 459