转载：https://blog.csdn.net/hcx25909/article/details/7164650
飞思卡尔K60时钟系统如上图所示，可以发现器件的源时钟源一共有4个：
①内部参考时钟源，包括 Fast IRC和 slow IRC (IRC--Internal Reference Clock)
②外部参考时钟源，只一个EXTAL管脚作为时钟输入，这个可以使用有源晶体振荡器来实现
③外部晶体谐振器，使用EXTAL和XTAL两个管脚来输入
④外部32K RTC 谐振器，用于实时时钟的时钟输入
在图中可以看到，要为系统提供时钟信号，关键是要最终生成 MCGOUTCLK 输出。MCGOUTCLK 再经过分频便可以提供Core/system clocks、Bus clock、FlexBus clock和Flash clock。MCGOUTCLK 的产生有3个途径:
①由内部参考时钟源 Fast IRC 直接提供，这个时钟源集成在芯片的内部（包括Slow IRC），频率是2M
②由 FLL 或者 PLL 模块来提供
③由外部时钟来直接提供，包括外部参考时钟源（1个管脚输入）、外部晶体谐振器经内部OSC logic产生的XTAL_CLK 和 RTC OSC logic 的时钟输出。
一般情况下，MCGOUTCLK 是由PLL或者FLL倍频来产生的，飞思卡尔官方的例程最终是由PLL模块来产生。图中可以看到PLL模块的时钟输入是OSCCLK或者RTC OSC logic。我的板子以外部参考时钟源提供PLL时钟，最终经PLL倍频产生MCGOUTCLK。即 EXTAL-->PLL模块-->MCGOUTCLK.
2.关于时钟模式
从图中可以看到，该芯片一共包含8种工作时钟模式，外加Stop模式。系统在RESET后直接进入默认的FEI模式。图中，F--FLL、P--PLL、E--Enable或者EXTAL(外部时钟)、B--Bypass(旁路)、I--Internal(内部参考时钟)、L--Low Power.
·FLL 启用、内部参考时钟（FEI）, 内部参考时钟提供FLL的时钟，FLL驱动MCGOUT
·FLL 启用、外部参考时钟（FEE）, 外部参考时钟提供FLL的时钟，FLL驱动MCGOUT
·FLL 旁路、内部参考时钟（FBI），FLL虽然在运作但由内部时钟参考源驱动MCGOUT
·FLL 旁路、外部参考时钟（FBE），FLL虽然在运作但由外部时钟参考源驱动MCGOUT
·PLL 旁路、外部参考时钟（PBE），PLL虽然在运作但由外部时钟参考源驱动MCGOUT
·PLL 启用、外部参考时钟（PEE），外部参考时钟提供PLL的时钟，PLL驱动MCGOUT
·BLPI FLL和PLL都禁用，内部时钟参考源驱动MCGOUT
·BLPE FLL和PLL都禁用，外部时钟参考源驱动MCGOUT
由于系统在重启后默认进入FEI模式，我们的目标是要跳到PEE模式，所以要涉及到模式的转化。图中由FEI到PEE是不能直接跳转的，必须经由其他模式来转换。
3.官方具体的例子
来源于飞思卡尔官方\src\drivers\mcg\mcg.c
- unsigned char pll_init(unsigned char clk_option, unsigned char crystal_val)
{
unsigned char pll_freq;
if (clk_option > 3) {return 0;} //return 0 if one of the available options is not selected
if (crystal_val > 15) {return 1;} // return 1 if one of the available crystal options is not available
//This assumes that the MCG is in default FEI mode out of reset.
// First move to FBE mode
#if (defined(K60_CLK) || defined(ASB817))
MCG_C2 = 0;
#else
// Enable external oscillator, RANGE=2, HGO=1, EREFS=1, LP=0, IRCS=0
MCG_C2 = MCG_C2_RANGE(2) | MCG_C2_HGO_MASK | MCG_C2_EREFS_MASK;
#endif
// after initialization of oscillator release latched state of oscillator and GPIO
SIM_SCGC4 |= SIM_SCGC4_LLWU_MASK;
LLWU_CS |= LLWU_CS_ACKISO_MASK;
// Select external oscilator and Reference Divider and clear IREFS to start ext osc
// CLKS=2, FRDIV=3, IREFS=0, IRCLKEN=0, IREFSTEN=0
MCG_C1 = MCG_C1_CLKS(2) | MCG_C1_FRDIV(3);
/* if we aren't using an osc input we don't need to wait for the osc to init */
#if (!defined(K60_CLK) && !defined(ASB817))
while (!(MCG_S & MCG_S_OSCINIT_MASK)){}; // wait for oscillator to initialize
#endif
while (MCG_S & MCG_S_IREFST_MASK){}; // wait for Reference clock Status bit to clear
while (((MCG_S & MCG_S_CLKST_MASK) >> MCG_S_CLKST_SHIFT) != 0x2){}; // Wait for clock status bits to show clock source is ext ref clk
// Now in FBE
#if (defined(K60_CLK))
//MCG_C5 = MCG_C5_PRDIV(0x18);
MCG_C5 = MCG_C5_PRDIV(0x18); //基频2M 外部时钟源是50M时, 50/25=2M
#else
// Configure PLL Ref Divider, PLLCLKEN=0, PLLSTEN=0, PRDIV=5
// The crystal frequency is used to select the PRDIV value. Only even frequency crystals are supported
// that will produce a 2MHz reference clock to the PLL.
MCG_C5 = MCG_C5_PRDIV(crystal_val); // Set PLL ref divider to match the crystal used
#endif
// Ensure MCG_C6 is at the reset default of 0. LOLIE disabled, PLL disabled, clk monitor disabled, PLL VCO divider is clear
MCG_C6 = 0x0;
// Select the PLL VCO divider and system clock dividers depending on clocking option
switch (clk_option) {
case 0:
// Set system options dividers
//MCG=PLL, core = MCG, bus = MCG, FlexBus = MCG, Flash clock= MCG/2
set_sys_dividers(0,0,0,1);
// Set the VCO divider and enable the PLL for 50MHz, LOLIE=0, PLLS=1, CME=0, VDIV=1
MCG_C6 = MCG_C6_PLLS_MASK | MCG_C6_VDIV(1); //VDIV = 1 (x25)
pll_freq = 50;
break;
case 1:
// Set system options dividers
//MCG=PLL, core = MCG, bus = MCG/2, FlexBus = MCG/2, Flash clock= MCG/4
set_sys_dividers(0,1,1,3);
// Set the VCO divider and enable the PLL for 100MHz, LOLIE=0, PLLS=1, CME=0, VDIV=26
MCG_C6 = MCG_C6_PLLS_MASK | MCG_C6_VDIV(26); //VDIV = 26 (x50)
pll_freq = 100;
break;
case 2:
// Set system options dividers
//MCG=PLL, core = MCG, bus = MCG/2, FlexBus = MCG/2, Flash clock= MCG/4
set_sys_dividers(0,1,1,3);
// Set the VCO divider and enable the PLL for 96MHz, LOLIE=0, PLLS=1, CME=0, VDIV=24
MCG_C6 = MCG_C6_PLLS_MASK | MCG_C6_VDIV(24); //VDIV = 24 (x48)
pll_freq = 96;
break;
case 3:
// Set system options dividers
//MCG=PLL, core = MCG, bus = MCG, FlexBus = MCG, Flash clock= MCG/2
set_sys_dividers(0,0,0,1);
// Set the VCO divider and enable the PLL for 48MHz, LOLIE=0, PLLS=1, CME=0, VDIV=0
MCG_C6 = MCG_C6_PLLS_MASK; //VDIV = 0 (x24)
pll_freq = 48;
break;
}
while (!(MCG_S & MCG_S_PLLST_MASK)){}; // wait for PLL status bit to set
while (!(MCG_S & MCG_S_LOCK_MASK)){}; // Wait for LOCK bit to set
// Now running PBE Mode
// Transition into PEE by setting CLKS to 0
// CLKS=0, FRDIV=3, IREFS=0, IRCLKEN=0, IREFSTEN=0
MCG_C1 &= ~MCG_C1_CLKS_MASK;
// Wait for clock status bits to update
while (((MCG_S & MCG_S_CLKST_MASK) >> MCG_S_CLKST_SHIFT) != 0x3){};
// Now running PEE Mode
return pll_freq;
} //pll_init
