单片机DCO时钟频率校准

在前段时间的一个项目中，因为项目需求我们修安排使用单片机中非标准频率即非1MHz、8MHz、12MHz、16MHz这几个已经在出厂的时候即校准好的频率，因此采用了通过外部时钟校准单片机DCO时钟频率的方法。

校准流程如下图所示：

对于单片机通过Timer定时器的捕获模式来测量外界方波频率的例子相信大家都很熟悉，本次时钟校准方法与之类似，只不过这次是外部时钟已经确定，通过每次外界时钟输入上升沿的捕获时Timer计数器的数值大小来修改于DCO频率相关的DCOCTL以及BCSCTL1这两个寄存器的值。

所用代码如下所示：

//******************************************************************************
//  MSP430F22x4 Demo - DCO Calibration Constants Programmer
//
//  NOTE: THIS CODE REPLACES THE TI FACTORY-PROGRAMMED DCO CALIBRATION
//  CONSTANTS LOCATED IN INFOA WITH NEW VALUES. USE ONLY IF THE ORIGINAL
//  CONSTANTS ACCIDENTALLY GOT CORRUPTED OR ERASED.
//
//  Description: This code re-programs the F2xx DCO calibration constants.
//  A software FLL mechanism is used to set the DCO based on an external
//  32kHz reference clock. After each calibration, the values from the
//  clock system are read out and stored in a temporary variable. The final
//  frequency the DCO is set to is 1MHz, and this frequency is also used
//  during Flash programming of the constants. The program end is indicated
//  by the blinking LED.
//  ACLK = LFXT1/8 = 32768/8, MCLK = SMCLK = target DCO
//  //* External watch crystal installed on XIN XOUT is required for ACLK *//
//
//           MSP430F22x4
//         ---------------
//     /|\|            XIN|-
//      | |               | 32kHz
//      --|RST        XOUT|-
//        |               |
//        |           P1.0|--> LED
//        |           P2.1|--> SMLCK = target DCO
//
//  A. Dannenberg
//  Texas Instruments Inc.
//  April 2006
//  Built with IAR Embedded Workbench Version: 3.41A
//******************************************************************************
#include "msp430x21x1.h"#define DELTA_1MHZ    244                   // 244 x 4096Hz = 999.4Hz
#define DELTA_8MHZ    2205                  // 2205 x 4096Hz = 9.03MHz
#define DELTA_12MHZ   2894                  // 2894 x 4096Hz = 11.85MHz
#define DELTA_16MHZ   3906                  // 3906 x 4096Hz = 15.99MHzunsigned char CAL_DATA[8];                  // Temp. storage for constants
volatile unsigned int i;
int j;
char *Flash_ptrA;                           // Segment A pointer
void Set_DCO(unsigned int Delta);void main(void)
{WDTCTL = WDTPW + WDTHOLD;                 // Stop WDTfor (i = 0; i < 0xfffe; i++);             // Delay for XTAL stabilizationP1OUT = 0x10;                             // Clear P1 output latchesP1DIR = 0x11;                             // P1.0 outputP1SEL = 0X10;                             //P1.4 SMCLK//P2SEL |= 0x02;                            // P2.1 SMCLK output//P2DIR |= 0x02;                            // P2.1 outputj = 0;                                    // Reset pointerSet_DCO(DELTA_16MHZ);                     // Set DCO and obtain constantsCAL_DATA[j++] = DCOCTL;CAL_DATA[j++] = BCSCTL1;Set_DCO(DELTA_12MHZ);                     // Set DCO and obtain constantsCAL_DATA[j++] = DCOCTL;CAL_DATA[j++] = BCSCTL1;Set_DCO(DELTA_8MHZ);                      // Set DCO and obtain constantsCAL_DATA[j++] = DCOCTL;CAL_DATA[j++] = BCSCTL1;Set_DCO(DELTA_1MHZ);                      // Set DCO and obtain constantsCAL_DATA[j++] = DCOCTL;CAL_DATA[j++] = BCSCTL1;Flash_ptrA = (char *)0x10C0;              // Point to beginning of seg AFCTL2 = FWKEY + FSSEL0 + FN1;             // MCLK/3 for Flash Timing GeneratorFCTL1 = FWKEY + ERASE;                    // Set Erase bitFCTL3 = FWKEY + LOCKA;                    // Clear LOCK & LOCKA bits*Flash_ptrA = 0x00;                       // Dummy write to erase Flash seg AFCTL1 = FWKEY + WRT;                      // Set WRT bit for write operationFlash_ptrA = (char *)0x10F8;              // Point to beginning of cal constsfor (j = 0; j < 8; j++)*Flash_ptrA++ = CAL_DATA[j];            // re-flash DCO calibration dataFCTL1 = FWKEY;                            // Clear WRT bitFCTL3 = FWKEY + LOCKA + LOCK;             // Set LOCK & LOCKA bitDCOCTL = CALDCO_8MHZ;BCSCTL1 = CALBC1_8MHZ;BCSCTL2 = 0;//P1DIR |= 0X10;//P1SEL |= 0X10;while (1){P1OUT ^= 0x01;                          // Toggle LEDfor (i = 0; i < 0x4000; i++);           // SW Delay}
}void Set_DCO(unsigned int Delta)            // Set DCO to selected frequency
{unsigned int Compare, Oldcapture = 0,Totalcapture = 0;unsigned int CaptureTime = 0;BCSCTL1 |= DIVA_3;                        // ACLK = LFXT1CLK/8TACCTL2 = CM_1 + CCIS_1 + CAP;            // CAP, ACLKTACTL = TASSEL_2 + MC_2 + TACLR;          // SMCLK, cont-mode, clearwhile (1){Totalcapture = 0;for(CaptureTime=0;CaptureTime<8;CaptureTime++){while (!(CCIFG & TACCTL2));             // Wait until capture occuredTACCTL2 &= ~CCIFG;                      // Capture occured, clear flagCompare = TACCR2;                       // Get current captured SMCLKCompare = Compare - Oldcapture;         // SMCLK differenceOldcapture = TACCR2;                    // Save current captured SMCLKTotalcapture +=Compare;                 // Save the total capture SMCLK}Totalcapture = Totalcapture>>3;         // Get average captured SMCLKif (Delta == Compare)break;                                // If equal, leave "while(1)"else if (Delta < Compare){DCOCTL--;                             // DCO is too fast, slow it downif (DCOCTL == 0xFF)                   // Did DCO roll under?if (BCSCTL1 & 0x0f)BCSCTL1--;                        // Select lower RSEL}else{DCOCTL++;                             // DCO is too slow, speed it upif (DCOCTL == 0x00)                   // Did DCO roll over?if ((BCSCTL1 & 0x0f) != 0x0f)BCSCTL1++;                        // Sel higher RSEL}}TACCTL2 = 0;                              // Stop TACCR2TACTL = 0;                                // Stop Timer_ABCSCTL1 &= ~DIVA_3;                       // ACLK = LFXT1CLK
}

在本程序中我们通过Timer定时器去捕获外部32768/8=4096Hz时钟，并计数，通过该数值与正确的数值进行比较，然后修改DCO参数继续捕获，直到Timer计数与正确数值一致。一共需要校准出9.032MHz以及11.85MHz的时钟频率并将数据存储在FLASH中。（注：每片MSP430在FLASH中均会存储DCO频率设置为1MHz,8MHz,12MHz,16MHz等四个频率时所用的参数。）

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