STM32F0 单片机使用内部RC振荡器作为系统，软件修改
系统上电复位后，
运行如下
$ v" n' x* _/ [# x8 G% @& f

1. ; Reset handler routine
   ' t4 r6 B- t* |6 Z" o% G
2. Reset_Handler PROC
   
3. EXPORT Reset_Handler [WEAK]
   
4. IMPORT __main
   3 X% N' D" S: R% U1 q6 M9 f
5. IMPORT SystemInit
   
6. LDR R0, =SystemInit
   
7. BLX R0
   ; o( c; n! A3 [
8. LDR R0, =__main
   3 N3 W# u5 D% ~1 U* V
9. BX R0
   0 S3 `6 H  E# L7 Q2 K
10. ENDP

复制代码

然后进入 system_stm32f0xx.c 文件
1 E' y4 _! P( P! G' k4 z在SystemInit函数里有 SetSysClock(); ，我们修改这个函数即可
8 M: n2 m+ [6 j1 T
9 e$ v" \6 k8 Z7 ~

1. void SystemInit (void)
   
2. {
   
3. /* Set HSION bit */
   
4. RCC->CR |= (uint32_t)0x00000001;
   
5. 
   
6. #if defined (STM32F031) || defined (STM32F072) || defined (STM32F042)
   
7. /* Reset SW[1:0], HPRE[3:0], PPRE[2:0], ADCPRE and MCOSEL[2:0] bits /
   
8. RCC->CFGR &= (uint32_t)0xF8FFB80C;
   * D+ l6 J# d. P1 y' o3 Y
9. #else
   
10. / Reset SW[1:0], HPRE[3:0], PPRE[2:0], ADCPRE, MCOSEL[2:0], MCOPRE[2:0] and PLLNODIV bits /
    : H  v' E+ X5 T! i, G
11. RCC->CFGR &= (uint32_t)0x08FFB80C;
    
12. #endif / STM32F031*/
    ! T6 S7 h+ Q7 P; t
13. 
    
14. /* Reset HSEON, CSSON and PLLON bits */
    - g6 {9 S$ a2 |0 n: }$ C: ?
15. RCC->CR &= (uint32_t)0xFEF6FFFF; //1111 1110 1111 0110 1111 1111 1111 1111
    
16. 
    
17. /* Reset HSEBYP bit */
    ) L4 u) S3 j% ]* p& g% w" z; m9 ~
18. RCC->CR &= (uint32_t)0xFFFBFFFF;
    2 M( y' R; Z2 h  s; a" _
19. 
    8 W9 W) H. i$ R3 q  e
20. /* Reset PLLSRC, PLLXTPRE and PLLMUL[3:0] bits */
    
21. RCC->CFGR &= (uint32_t)0xFFC0FFFF;
    
22. 
    
23. /* Reset PREDIV1[3:0] bits */
    
24. RCC->CFGR2 &= (uint32_t)0xFFFFFFF0;
    % w' P- U9 ~4 u9 }9 l
25. 
    ) a9 {. n5 R/ \7 v% \8 r
26. /* Reset USARTSW[1:0], I2CSW, CECSW and ADCSW bits */
    % ?5 _% L0 P" x% T- t
27. RCC->CFGR3 &= (uint32_t)0xFFFFFEAC;
    
28. 
    
29. /* Reset HSI14 bit */
    & j+ O: J4 B, i, W
30. RCC->CR2 &= (uint32_t)0xFFFFFFFE;
    
31. 
    8 @; N9 v2 l- d5 g& e
32. /* Disable all interrupts */
    1 B; p3 p2 l" l7 Y7 m; ~4 ^
33. RCC->CIR = 0x00000000;
    $ ?1 }  r5 G0 ?2 `3 ]9 j
34. 
    1 ?8 G) A2 ^: C7 l( O% G! a: p: i( v
35. /* Configure the System clock frequency, AHB/APBx prescalers and Flash settings */
    . b5 G- k0 u( u* L! r* e' [7 v( u
36. SetSysClock();
    / `( O$ y+ v: E8 B; B$ V' a8 k
37. }

复制代码

* e' I' G- O/ @% G9 G6 C( W2 D) r在文件system_stm32f0xx.c下修改static void SetSysClock(void)
如下：

1. static void SetSysClock(void)
   
2. {
   
3. __IO uint32_t StartUpCounter = 0, HSIStatus = 0;
   
4. 
   
5. /* SYSCLK, HCLK, PCLK configuration ----------------------------------------/
   
6. / Enable HSI*/
   1 C- V- p) m4 [6 h
7. RCC->CR |= ((uint32_t)RCC_CR_HSION); //RCC_CR_HSION,RCC_CR_HSEON
   
8. 
   
9. /* Wait till HSE is ready and if Time out is reached exit */
   
10. do
    / O7 ~. Y; C& f- M6 a
11. {
    
12. HSIStatus = RCC->CR & RCC_CR_HSIRDY;
    
13. StartUpCounter++;
    ( |; Y: ^9 b  e2 ]1 q
14. } while((HSIStatus == 0) && (StartUpCounter != HSI_STARTUP_TIMEOUT));
    
15. 
    - i9 {/ m/ ^. X
16. if ((RCC->CR & RCC_CR_HSIRDY) != RESET)
    1 W7 E, P/ I2 i1 y" H" c; L
17. {
    
18. HSIStatus = (uint32_t)0x01;
    
19. }
    ! x3 G1 M/ U8 J* [2 o2 l
20. else
    
21. {
    * ^& K' d2 E3 W- f4 Y; J+ ~  v5 h) p
22. HSIStatus = (uint32_t)0x00;
    9 z0 B" Y0 j4 j* s( P7 @" w
23. }
    
24. 
    
25. if (HSIStatus == (uint32_t)0x01)
    
26. {
    
27. /* Enable Prefetch Buffer and set Flash Latency */
    # j( B* C( b8 R/ N( [; }& u4 |
28. FLASH->ACR = FLASH_ACR_PRFTBE | FLASH_ACR_LATENCY;
    ( v$ x, g( z  V6 m
29. 
    : U4 F7 i3 [8 X4 m' X! V5 z
30. /* HCLK = SYSCLK */
    
31. RCC->CFGR |= (uint32_t)RCC_CFGR_HPRE_DIV1;
    
32. 
    : Z0 F3 @  I. c  _2 P0 ~5 S4 L
33. /* PCLK = HCLK */
    % d! G9 g8 t4 E# J
34. RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE_DIV1;
    
35. 
    4 o# U: m! ^( m& M
36. /* PLL configuration = HSE * 6 = 48 MHz */
    
37. RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_PLLSRC | RCC_CFGR_PLLXTPRE | RCC_CFGR_PLLMULL));
    
38. RCC->CFGR |= (uint32_t)(RCC_CFGR_PLLSRC_HSI_Div2  | RCC_CFGR_PLLMULL12);
    + Q* G2 d, L$ v5 G
39. 
    
40. /* Enable PLL */
    ' j9 c* s- I/ u1 Z! K
41. RCC->CR |= RCC_CR_PLLON;
    
42. 
    
43. /* Wait till PLL is ready */
    
44. while((RCC->CR & RCC_CR_PLLRDY) == 0)
    4 V9 L3 r; T9 w& c0 p& p3 [
45. {
    
46. }
    / |/ T2 s( k8 E! B
47. 
    9 [: C, v7 N7 `4 R2 y8 |
48. /* Select PLL as system clock source */
    0 [, v- w/ U3 J* P
49. RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_SW));
    
50. RCC->CFGR |= (uint32_t)RCC_CFGR_SW_PLL;   
    % h' B' b) o% Z& _
51. 
    
52. /* Wait till PLL is used as system clock source */
    
53. while ((RCC->CFGR & (uint32_t)RCC_CFGR_SWS) != (uint32_t)RCC_CFGR_SWS_PLL)
    
54. {
    % b5 |& n# B) _: t; X0 t0 [
55. }
    
56. 
    
57. }
    
58. else
    ; Y5 M+ f! \) v" t* P/ x. v
59. { /* If HSE fails to start-up, the application will have wrong clock
    1 ]1 A" O/ g: M" H7 u9 z5 _
60. configuration. User can add here some code to deal with this error */
    
61. }
    ! T" u' r* t+ V8 H2 W
62. }

复制代码

% V0 y& l, S6 g) r& A. W
) l; }& m0 Z  x0 {! I
& i: `4 u& z9 K, r5 O+ U