TMS320F28335时钟     TMS320F28335上有一个基于PLL电路的片上时钟模块，如图1所示，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。
三种时钟输入的接法：



下面是InitSysCtrl函数中调用的InitPll函数进行时钟设置： //--------------------------------------------------------------------------- // This function initializes the PLLCR register. 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。
说白了就相当于乘10，除2 (30*10/2 = 150MHZ) 更详细内容参考： http://wenku.baidu.com/link?url=vL0bh97ETShHDNcARicm11dkcvSac4VbUE-mm9uvkZSxq4wDpYi3x9Xxevtq7moLFlBX3KSYSQ6icRGo21ePg7MWfu8hI4cnfrn_aAFRUUi