+浅谈TMS320F28335的时钟

TMS320F28335上有一个基于PLL电路的片上时钟模块，为CPU及外设提供时钟有两种方式：一种是用外部的时钟源，将其连接到X1引脚上或者 XCLKIN引脚上，X2接地；另一种是使用振荡器产生时钟，用30MHz的晶体和两个20PF的电容组成的电路分别连接到X1和X2引脚 上，XCLKIN引脚接地。我们常用第二种来产生时钟。此时钟将通过一个内部PLL锁相环电路，进行倍频。由于F28335的最大工作频率是150M，所 以倍频值最大是5。其中倍频值由PLLCR的低四位和PLLSTS的第7、8位来决定。其详细的倍频值可以参照TMS320F28335的 Datasheet。下面是F28335的时钟设置：

void InitPll(Uint16 val, Uint16 divsel)
{

// Make sure the PLL is not running in limp mode
   if (SysCtrlRegs.PLLSTS.bit.MCLKSTS != 0)
   {
      // Missing external clock has been detected
      // Replace this line with a call to an appropriate
      // SystemShutdown(); function.
      asm("        ESTOP0");
   }

   // divSEL MUST be 0 before PLLCR can be changed from
   // 0x0000. It is set to 0 by an external reset XRSn
   // This puts us in 1/4
   if (SysCtrlRegs.PLLSTS.bit.divSEL != 0)
   {
       EALLOW;
       SysCtrlRegs.PLLSTS.bit.divSEL = 0;
       EDIS;
   }

   // Change the PLLCR
   if (SysCtrlRegs.PLLCR.bit.div != val)
   {

      EALLOW;
      // Before setting PLLCR turn off missing clock detect logic
      SysCtrlRegs.PLLSTS.bit.MCLKOFF = 1;
      SysCtrlRegs.PLLCR.bit.div = val;
      EDIS;

      // Optional: Wait for PLL to lock.
      // During this time the CPU will switch to OSCCLK/2 until
      // the PLL is stable.  Once the PLL is stable the CPU will
      // switch to the new PLL value.
      //
      // This time-to-lock is monitored by a PLL lock counter.
      //
      // Code is not required to sit and wait for the PLL to lock.
      // However, if the code does anything that is timing critical,
      // and requires the correct clock be locked, then it is best to
      // wait until this switching has completed.

      // Wait for the PLL lock bit to be set.

      // The watchdog should be disabled before this loop, or fed within
      // the loop via ServiceDog().

   // Uncomment to disable the watchdog
      DisableDog();

      while(SysCtrlRegs.PLLSTS.bit.PLLLOCKS != 1)
      {
       // Uncomment to service the watchdog
          // ServiceDog();
      }

      EALLOW;
      SysCtrlRegs.PLLSTS.bit.MCLKOFF = 0;
      EDIS;
    }

    // If switching to 1/2
if((divsel == 1)||(divsel == 2))
{
  EALLOW;
     SysCtrlRegs.PLLSTS.bit.divSEL = divsel;
     EDIS;
}

// If switching to 1/1
// * First go to 1/2 and let the power settle
//   The time required will depend on the system, this is only an example
// * Then switch to 1/1
if(divsel == 3)
{
  EALLOW;
     SysCtrlRegs.PLLSTS.bit.divSEL = 2;
     DELAY_US(50L);
     SysCtrlRegs.PLLSTS.bit.divSEL = 3;
     EDIS;
    }
}

如果我们希望DSP工作在某一个频率下，我们就可以对Uint16 val, Uint16 divsel两个参数进行设定。

方法一X2必须接地吗？不接地会是什么情况

应该验证一下 

C2000的时钟一般是先倍频再分频。所以最大5是不是有点不合理