在MRS的默认配置中，在main函数执行之前，就已经执行了时钟的初始化配置程序，这部分程序被放在了一个名为system_ch32v00x.c的文件中，这个文件默认被加载到MRS的User目录下（可双击打开它）。在该文件中，最重要的一个函数就是SystemInit，它负责系统的初始化工作，其代码如下所示。
void SystemInit (void)
{
RCC->CTLR |= (uint32_t)0x00000001;
RCC->CFGR0 &= (uint32_t)0xFCFF0000;
RCC->CTLR &= (uint32_t)0xFEF6FFFF;
RCC->CTLR &= (uint32_t)0xFFFBFFFF;
RCC->CFGR0 &= (uint32_t)0xFFFEFFFF;
RCC->INTR = 0x009F0000;
SetSysClock();
}
在末尾调用了一个设置系统时钟的函数SetSysClock，其代码如下。
static void SetSysClock(void)
{
#ifdef SYSCLK_FREQ_8MHz_HSI
SetSysClockTo_8MHz_HSI();
#elif defined SYSCLK_FREQ_24MHZ_HSI
SetSysClockTo_24MHZ_HSI();
#elif defined SYSCLK_FREQ_48MHZ_HSI
SetSysClockTo_48MHZ_HSI();
#elif defined SYSCLK_FREQ_8MHz_HSE
SetSysClockTo_8MHz_HSE();
#elif defined SYSCLK_FREQ_24MHz_HSE
SetSysClockTo_24MHz_HSE();
#elif defined SYSCLK_FREQ_48MHz_HSE
SetSysClockTo_48MHz_HSE();
#endif
}
从上可看出，该函数主要依据宏定义SYSCLK_FREQ的值来决定使用哪一个配置函数，并没有具体内容。这里不妨选取最后一个函数SetSysClockTo_48MHz_HSE（即使用外部晶振的48MHz主频模式）来展开讨论，其代码如下所示。
static void SetSysClockTo_48MHz_HSE(void)
{
__IO uint32_t StartUpCounter = 0, HSEStatus = 0;
/* Close PA0-PA1 GPIO function */
RCC->APB2PCENR |= RCC_AFIOEN;
AFIO->PCFR1 |= (1<<15);
RCC->CTLR |= ((uint32_t)RCC_HSEON);
/* Wait till HSE is ready and if Time out is reached exit */
do
{
HSEStatus = RCC->CTLR & RCC_HSERDY;
StartUpCounter++;
} while((HSEStatus == 0) && (StartUpCounter != HSE_STARTUP_TIMEOUT));
if ((RCC->CTLR & RCC_HSERDY) != RESET)
{
HSEStatus = (uint32_t)0x01;
}
else
{
HSEStatus = (uint32_t)0x00;
}
if (HSEStatus == (uint32_t)0x01)
{
/* Flash 0 wait state */
FLASH->ACTLR &= (uint32_t)((uint32_t)~FLASH_ACTLR_LATENCY);
FLASH->ACTLR |= (uint32_t)FLASH_ACTLR_LATENCY_1;
/* HCLK = SYSCLK = APB1 */
RCC->CFGR0 |= (uint32_t)RCC_HPRE_DIV1;
/* PLL configuration: PLLCLK = HSE * 2 = 48 MHz */
RCC->CFGR0 &= (uint32_t)((uint32_t)~(RCC_PLLSRC));
RCC->CFGR0 |= (uint32_t)(RCC_PLLSRC_HSE_Mul2);
/* Enable PLL */
RCC->CTLR |= RCC_PLLON;
/* Wait till PLL is ready */
while((RCC->CTLR & RCC_PLLRDY) == 0)
{
}
/* Select PLL as system clock source */
RCC->CFGR0 &= (uint32_t)((uint32_t)~(RCC_SW));
RCC->CFGR0 |= (uint32_t)RCC_SW_PLL;
/* Wait till PLL is used as system clock source */
while ((RCC->CFGR0 & (uint32_t)RCC_SWS) != (uint32_t)0x08)
{
}
}
else
{
/*
* If HSE fails to start-up, the application will have wrong clock
* configuration. User can add here some code to deal with this error
*/
}
}
以上就是MRS中系统初始化的具体流程，看起来是不是有些绕（仅为了方便开发者），能不能自己实现一个精简的初始化过程呢？答案是肯定的。以下就一个是非常精简的初始化代码实例。
void SetSysClockTo_48MHz_HSE(void)
{
RCC->CFGR0 = 0; //AHB时钟不分频
RCC->APB2PCENR |= 0x00000001; //使能I/O辅助功能模块时钟
AFIO->PCFR1 |= (1<<15); //引脚PA1&PA2重映射位，改为接外部晶振引脚
RCC->CTLR |= 0x00010000; //使能外部振荡器
while(!(RCC->CTLR & 0x00020000)); //等待外部晶振稳定
FLASH->ACTLR &= 0x00000003;
FLASH->ACTLR |= 0x00000001; //设置FLASH等待
RCC->CFGR0 |= 0x00010000; //选择外部晶振
RCC->CTLR |= 0x01000000; //开启PLL
while(!(RCC->CTLR & 0x02000000)); //等待PLL稳定
RCC->CFGR0 &= ~0x00000003;
RCC->CFGR0 |= 0x00000002; //选择PLL作为系统时钟
}
上面的函数可直接放在main.c文件中，提供给main函数调用。为此，还可以移出默认在User目录下的系统配置文件system_ch32v00x.c及其头文件system_ch32v00x.h，只留下一个main.c文件，同时把头文件ch32v00x.h中的“#include <system_ch32v00x.h>”一句注释掉，让开发环境做到极简。但是，文件移出后编译会报错，原因是缺失了系统初始化函数SystemInit。如何解决？有两种方法。其一，把上面的函数名称改为“void SystemInit (void)”，让系统在复位后直接执行初始化，这就不需要在main函数中来调用它了。其二，更改系统在复位后的程序入口，直接执行main函数，这需要修改系统启动文件startup_ch32v00x.S。启动文件是用汇编写成的，下面是它的具体内容。
.section .init, "ax", @progbits
.globl _start
.align 2
_start:
.option norvc;
j handle_reset
.word 0
.word NMI_Handler /* NMI Handler */
.word HardFault_Handler /* Hard Fault Handler */
.word 0
.word 0
.word 0
.word 0
.word 0
.word 0
.word 0
.word 0
.word SysTick_Handler /* SysTick Handler */
.word 0
.word SW_Handler /* SW Handler */
.word 0
/* External Interrupts */
.word WWDG_IRQHandler /* Window Watchdog */
.word PVD_IRQHandler /* PVD through EXTI Line detect */
.word FLASH_IRQHandler /* Flash */
.word RCC_IRQHandler /* RCC */
.word EXTI7_0_IRQHandler /* EXTI Line 7..0 */
.word AWU_IRQHandler /* AWU */
.word DMA1_Channel1_IRQHandler /* DMA1 Channel 1 */
.word DMA1_Channel2_IRQHandler /* DMA1 Channel 2 */
.word DMA1_Channel3_IRQHandler /* DMA1 Channel 3 */
.word DMA1_Channel4_IRQHandler /* DMA1 Channel 4 */
.word DMA1_Channel5_IRQHandler /* DMA1 Channel 5 */
.word DMA1_Channel6_IRQHandler /* DMA1 Channel 6 */
.word DMA1_Channel7_IRQHandler /* DMA1 Channel 7 */
.word ADC1_IRQHandler /* ADC1 */
.word I2C1_EV_IRQHandler /* I2C1 Event */
.word I2C1_ER_IRQHandler /* I2C1 Error */
.word USART1_IRQHandler /* USART1 */
.word SPI1_IRQHandler /* SPI1 */
.word TIM1_BRK_IRQHandler /* TIM1 Break */
.word TIM1_UP_IRQHandler /* TIM1 Update */
.word TIM1_TRG_COM_IRQHandler /* TIM1 Trigger and Commutation */
.word TIM1_CC_IRQHandler /* TIM1 Capture Compare */
.word TIM2_IRQHandler /* TIM2 */
.option rvc;
.section .text.vector_handler, "ax", @progbits
.weak NMI_Handler
.weak HardFault_Handler
.weak SysTick_Handler
.weak SW_Handler
.weak WWDG_IRQHandler
.weak PVD_IRQHandler
.weak FLASH_IRQHandler
.weak RCC_IRQHandler
.weak EXTI7_0_IRQHandler
.weak AWU_IRQHandler
.weak DMA1_Channel1_IRQHandler
.weak DMA1_Channel2_IRQHandler
.weak DMA1_Channel3_IRQHandler
.weak DMA1_Channel4_IRQHandler
.weak DMA1_Channel5_IRQHandler
.weak DMA1_Channel6_IRQHandler
.weak DMA1_Channel7_IRQHandler
.weak ADC1_IRQHandler
.weak I2C1_EV_IRQHandler
.weak I2C1_ER_IRQHandler
.weak USART1_IRQHandler
.weak SPI1_IRQHandler
.weak TIM1_BRK_IRQHandler
.weak TIM1_UP_IRQHandler
.weak TIM1_TRG_COM_IRQHandler
.weak TIM1_CC_IRQHandler
.weak TIM2_IRQHandler
NMI_Handler: 1: j 1b
HardFault_Handler: 1: j 1b
SysTick_Handler: 1: j 1b
SW_Handler: 1: j 1b
WWDG_IRQHandler: 1: j 1b
PVD_IRQHandler: 1: j 1b
FLASH_IRQHandler: 1: j 1b
RCC_IRQHandler: 1: j 1b
EXTI7_0_IRQHandler: 1: j 1b
AWU_IRQHandler: 1: j 1b
DMA1_Channel1_IRQHandler: 1: j 1b
DMA1_Channel2_IRQHandler: 1: j 1b
DMA1_Channel3_IRQHandler: 1: j 1b
DMA1_Channel4_IRQHandler: 1: j 1b
DMA1_Channel5_IRQHandler: 1: j 1b
DMA1_Channel6_IRQHandler: 1: j 1b
DMA1_Channel7_IRQHandler: 1: j 1b
ADC1_IRQHandler: 1: j 1b
I2C1_EV_IRQHandler: 1: j 1b
I2C1_ER_IRQHandler: 1: j 1b
USART1_IRQHandler: 1: j 1b
SPI1_IRQHandler: 1: j 1b
TIM1_BRK_IRQHandler: 1: j 1b
TIM1_UP_IRQHandler: 1: j 1b
TIM1_TRG_COM_IRQHandler: 1: j 1b
TIM1_CC_IRQHandler: 1: j 1b
TIM2_IRQHandler: 1: j 1b
.section .text.handle_reset, "ax", @progbits
.weak handle_reset
.align 1
handle_reset:
.option push
.option norelax
la gp, __global_pointer$
.option pop
1:
la sp, _eusrstack
2:
/* Load data section from flash to RAM */
la a0, _data_lma
la a1, _data_vma
la a2, _edata
bgeu a1, a2, 2f
1:
lw t0, (a0)
sw t0, (a1)
addi a0, a0, 4
addi a1, a1, 4
bltu a1, a2, 1b
2:
/* clear bss section */
la a0, _sbss
la a1, _ebss
bgeu a0, a1, 2f
1:
sw zero, (a0)
addi a0, a0, 4
bltu a0, a1, 1b
2:
li t0, 0x80
csrw mstatus, t0
li t0, 0x3
csrw 0x804, t0
la t0, _start
ori t0, t0, 3
csrw mtvec, t0
jal SystemInit
la t0, main
csrw mepc, t0
mret
在系统复位后，程序指针从handle_reset的地方开始执行。这里只需要把下面的“jal SystemInit”一句（倒数第4行处）注释掉就可以了，不让程序指针跳转到SystemInit函数去，而是直接跳转到main函数去执行。