i = 2 ; //don't use 100M!
 switch ( i ) {
 case 0: //200
  key = 12;
  mpll_val = (92<<12)|(4<<4)|(1);
  break;
 case 1: //300
  key = 13;
  mpll_val = (67<<12)|(1<<4)|(1);
  break;
 case 2: //400
  key = 14;
  mpll_val = (92<<12)|(1<<4)|(1);
  break;
 case 3: //440!!!
  key = 14;
  mpll_val = (102<<12)|(1<<4)|(1);
  break;
 default:
  key = 14;
  mpll_val = (92<<12)|(1<<4)|(1);
  break;
 }
 
 //init FCLK=400M, so change MPLL first
 ChangeMPllValue((mpll_val>>12)&0xff, (mpll_val>>4)&0x3f, mpll_val&3);
 

逐行计算：

1、mpll_val = (92<<12)|(1<<4)|(1);

算得mpll_val的二进制值为：0101 1100 0000 0001 0001

注意上式中92为十进制92，其十六进制为：0x5C;

2、代入ChangeMPllValue((mpll_val>>12)&0xff, (mpll_val>>4)&0x3f, mpll_val&3);
得ChangeMPllValue(0x5C,1,1);

3、ChangeMPllValue()函数原型为：

void ChangeMPllValue(int mdiv,int pdiv,int sdiv)
{
        rMPLLCON = (mdiv<<12) | (pdiv<<4) | sdiv;
}

将值代入得rMPLLCON二进制值为：0101 1100 0000 0001 0001.

4、S3C2440的Datasheet:

PLLCON        Bit              Description                Initial State
MDIV          [19:12]       Main divider control       0x96 / 0x4d
PDIV            [9:4]         Pre-divider control         0x03 / 0x03
SDIV            [1:0]         Post divider control        0x0 / 0x0

根据上表得到MDIV=92   PDIV=1   SDIV=1

 Datasheet:

MPLL Control Register
Mpll = (2 * m * Fin) / (p * 2S)
m = (MDIV + 8), p = (PDIV + 2), s = SDIV

所以：m = 92+8=100, p = 1+2 = 3 , s = 1 , Fin为默认输入的时钟频率12MHz.

Mpll = (2*100*12M)/(3*2) = 400M.