通俗点说，设置PLL锁相环就相当于超频，单片机超频的原因和PC机是一个道理。 分频的主要原因是外设需要的工作频率往往远低于CPU/MEMORY，这也和PC机南北桥的原理类似。
相对来说，PLL锁相环的设置还是比较简单的，因为东西很死，完全可以照搬。只是大家也不要太贪，设置太高相对来说不够稳定，进行过PC机超频的应该很有体会，一般我们现在用的XS128我觉得设置在80MHz是比较合适的，相比前几届比赛用的DG128，这个频率已经蛮高的了。还有就是SYNR,REFDV只有在CLKSEL_PLLSEL=0的情况下才能写入，不过这是系统默认状态。
多半大家可能还会有以下几点疑问：
1.PLL锁相环怎么设置？
答：通过写REFDV（CRG参考分频寄存器）和SYNR（CRG合成器寄存器）进行设置
2.代码里while(!CRGFLG_LOCK);这句是干什么的？
答：时钟校正同步
3.为什么代码中会有多多少少的几句空语句？
答：锁相环从设定到最后稳定还是需要一点点时间的，所以需要加几条空指令
/**************************************************************************************
------------------------------------
Code Warrior 5.0
Target : MC9S12XS128
Crystal: 16.000Mhz
============================================
本程序主要包括以下功能：
设定系统工作在xxMHZ bus clock时钟下;
by:庞辉
*****************************************************************************************/
#include <hidef.h> /* common defines and macros */
#include <MC9S12XS128.h> /* derivative information */
#pragma LINK_INFO DERIVATIVE "mc9s12xs128"
void SetBusCLK_16M(void)
{
CLKSEL=0X00; // disengage PLL to system
PLLCTL_PLLON=1; // turn on PLL
SYNR=0x00 | 0x01; // VCOFRQ[7:6];SYNDIV[5:0]
// fVCO= 2*fOSC*(SYNDIV + 1)/(REFDIV + 1)
// fPLL= fVCO/(2 × POSTDIV)
// fBUS= fPLL/2
// VCOCLK Frequency Ranges VCOFRQ[7:6]
// 32MHz <= fVCO <= 48MHz 00
// 48MHz < fVCO <= 80MHz 01
// Reserved 10
// 80MHz < fVCO <= 120MHz 11
REFDV=0x80 | 0x01; // REFFRQ[7:6];REFDIV[5:0]
// fREF=fOSC/(REFDIV + 1)
// REFCLK Frequency Ranges REFFRQ[7:6]
// 1MHz <= fREF <= 2MHz 00
// 2MHz < fREF <= 6MHz 01
// 6MHz < fREF <= 12MHz 10
// fREF > 12MHz 11
// pllclock=2*osc*(1+SYNR)/(1+REFDV)=32MHz;
POSTDIV=0x00; // 4:0, fPLL= fVCO/(2xPOSTDIV)
// If POSTDIV = $00 then fPLL is identical to fVCO (divide by one).
_asm(nop); // BUS CLOCK=16M
_asm(nop);
while(!(CRGFLG_LOCK==1)); //when pll is steady ,then use it;
CLKSEL_PLLSEL =1; //engage PLL to system;
}
void SetBusCLK_32M(void)
{
CLKSEL=0X00; // disengage PLL to system
PLLCTL_PLLON=1; // turn on PLL
SYNR =0x40 | 0x03; // pllclock=2*osc*(1+SYNR)/(1+REFDV)=64MHz;
REFDV=0x80 | 0x01;
POSTDIV=0x00;
_asm(nop); // BUS CLOCK=32M
_asm(nop);
while(!(CRGFLG_LOCK==1)); //when pll is steady ,then use it;
CLKSEL_PLLSEL =1; //engage PLL to system;
}
void SetBusCLK_40M(void)
{
CLKSEL=0X00; //disengage PLL to system
PLLCTL_PLLON=1; //turn on PLL
SYNR =0xc0 | 0x04;
REFDV=0x80 | 0x01;
POSTDIV=0x00; //pllclock=2*osc*(1+SYNR)/(1+REFDV)=80MHz;
_asm(nop); //BUS CLOCK=40M
_asm(nop);
while(!(CRGFLG_LOCK==1)); //when pll is steady ,then use it;
CLKSEL_PLLSEL =1; //engage PLL to system;
}
void SetBusCLK_48M(void)
{
CLKSEL=0X00; //disengage PLL to system
PLLCTL_PLLON=1; //turn on PLL
SYNR =0xc0 | 0x05;
REFDV=0x80 | 0x01;
POSTDIV=0x00; //pllclock=2*osc*(1+SYNR)/(1+REFDV)=96MHz;
_asm(nop); //BUS CLOCK=48M
_asm(nop);
while(!(CRGFLG_LOCK==1)); //when pll is steady ,then use it;
CLKSEL_PLLSEL =1; //engage PLL to system;
}
void SetBusCLK_64M(void)
{
CLKSEL=0X00; //disengage PLL to system
PLLCTL_PLLON=1; //turn on PLL
SYNR =0xc0 | 0x07;
REFDV=0x80 | 0x01;
POSTDIV=0x00; //pllclock=2*osc*(1+SYNR)/(1+REFDV)=128MHz;
_asm(nop); //BUS CLOCK=64M
_asm(nop);
while(!(CRGFLG_LOCK==1)); //when pll is steady ,then use it;
CLKSEL_PLLSEL =1; //engage PLL to system;
}
void SetBusCLK_80M(void)
{
CLKSEL=0X00; //disengage PLL to system
PLLCTL_PLLON=1; //turn on PLL
SYNR =0xc0 | 0x09;
REFDV=0x80 | 0x01;
POSTDIV=0x00; //pllclock=2*osc*(1+SYNR)/(1+REFDV)=160MHz;
_asm(nop); //BUS CLOCK=80M
_asm(nop);
while(!(CRGFLG_LOCK==1)); //when pll is steady ,then use it;
CLKSEL_PLLSEL =1; //engage PLL to system;
}
void SetBusCLK_88M(void)
{
CLKSEL=0X00; //disengage PLL to system
PLLCTL_PLLON=1; //turn on PLL
SYNR =0xc0 | 0x0a;
REFDV=0x80 | 0x01;
POSTDIV=0x00; //pllclock=2*osc*(1+SYNR)/(1+REFDV)=176MHz;
_asm(nop); //BUS CLOCK=88M
_asm(nop);
while(!(CRGFLG_LOCK==1)); //when pll is steady ,then use it;
CLKSEL_PLLSEL =1; //engage PLL to system;
}
void SetBusCLK_96M(void)
{
CLKSEL=0X00; //disengage PLL to system
PLLCTL_PLLON=1; //turn on PLL
SYNR =0xc0 | 0x0b;
REFDV=0x80 | 0x01;
POSTDIV=0x00; //pllclock=2*osc*(1+SYNR)/(1+REFDV)=192MHz;
_asm(nop); //BUS CLOCK=96M
_asm(nop);
while(!(CRGFLG_LOCK==1)); //when pll is steady ,then use it;
CLKSEL_PLLSEL =1; //engage PLL to system;
}
void SetBusCLK_104M(void)
{
CLKSEL=0X00; //disengage PLL to system
PLLCTL_PLLON=1; //turn on PLL
SYNR =0xc0 | 0x0c;
REFDV=0x80 | 0x01;
POSTDIV=0x00; //pllclock=2*osc*(1+SYNR)/(1+REFDV)=208MHz;
_asm(nop); //BUS CLOCK=104M
_asm(nop);
while(!(CRGFLG_LOCK==1)); //when pll is steady ,then use it;
CLKSEL_PLLSEL =1; //engage PLL to system;
}
void SetBusCLK_120M(void)
{
CLKSEL=0X00; //disengage PLL to system
PLLCTL_PLLON=1; //turn on PLL
SYNR =0xc0 | 0x0d;
REFDV=0x80 | 0x01;
POSTDIV=0x00; //pllclock=2*osc*(1+SYNR)/(1+REFDV)=240MHz;
_asm(nop); //BUS CLOCK=120M
_asm(nop);
while(!(CRGFLG_LOCK==1)); //when pll is steady ,then use it;
CLKSEL_PLLSEL =1; //engage PLL to system;
}
void main(void)
{
EnableInterrupts;
for(;;)
{
_asm(nop);
}
}
后面会尽快把PWM模块、AD模块、PIT模块等等都整理出来的……还请大家挑错或是共同讨论，共同进步嘛！
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