如果使用内部RC振荡器而不使用外部晶振，请按照下面方法处理：
1）对于100脚或144脚的产品，OSC_IN应接地，OSC_OUT应悬空。
2）对于少于100脚的产品，有2种接法：
2.1）OSC_IN和OSC_OUT分别通过10K电阻接地。此方法可提高EMC性能。
2.2）分别重映射OSC_IN和OSC_OUT至PD0和PD1，再配置PD0和PD1为推挽输出并输出'0'。此方法可以减小功耗并(相对上面2.1)节省2个外部电阻。
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以下是在ST英文网站FAQ上抄下的详细解释：
1) In 100 and 144 packages, the OSC_IN /OSC_Out pins are a dedicated pins for HSE and mapped respectively on pins 12, 13 and pins 23, 24 for LQFP packages. Although, for BGA 100 and BGA 144 packages, they are mapped respectively on pins C1,D1 and pins D1,E1. In this case the recommended configuration is:
OSC_In pin must be connected to ground as it is always an input pin
OSC_Out must be left not connected as it is always an output pin.
2) In LQFP48 and LQFP64 packages, the OSC_IN/OSC_Out pins are mapped on pins 5,6 whereas in the VFQFPN36 package they are mapped on pins 2,3. However the PD0 and PD1 functionality can be remapped by software on these pins. There are two possible configurations:
Hardware configuration: Connect the OSC_IN/OSC_Out pins to ground through a 10K pull down resistors.
Software configuration: First, the OSC_IN/OSC_Out pins are left not connected on PCB layout. Then, remap the PD0 and PD1 functionality by software on these pins and configure the PD0/PD1 to Output push-pull forced to 0 logic level(Data Register = 0). This will maintain those I/Os to a fixed level minimizing Noise and external stress on these pins.
The software configuration is recommended to reduce the power consumption and saves cost by avoiding the use of external pull down resistors as the I/O is already connected to the ground internally. Whereas the hardware configuration is recommended to increase the EMC performance.
STM32中如何使用PC14和PC15
在STM32的数据手册的管脚分配图中可以看到：PC14与OSC32_IN公用一个引脚，PC15与OSC32_OUT公用一个引脚，它们的使用方法如下：
当LSE（低速外部时钟信号）开启时，这两个公用管脚的功能是OSC32_IN和OSC32_OUT。
当LSE（低速外部时钟信号）关闭时这两个公用管脚的功能是PC14和PC15。
备用区域控制寄存器(RCC_BDCR)的LSEON用于控制LSE的开启或关闭。关于这个寄存器的用法请参看《STM3210x技术参考手册》。
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作为GPIO输出的配置过程：
(1).使能GPIOC时钟
RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOC, ENABLE);
(2).配置GPIOC
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_14 | GPIO_Pin_15;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_2MHz;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_Out_PP;
GPIO_Init(GPIOC, &GPIO_InitStructure);
(3).然后写高写低
GPIO_WriteBit(GPIOC, GPIO_Pin_14, Bit_SET);
GPIO_WriteBit(GPIOC, GPIO_Pin_15, Bit_SET);
GPIO_WriteBit(GPIOC, GPIO_Pin_14, Bit_RESET);
GPIO_WriteBit(GPIOC, GPIO_Pin_15, Bit_RESET);
以上代码跑在两个板子上，一个是backup区域里的RCC_DBCR的LSEON为0，即LSE关闭，以上(3)的操作从示波器上看到了电平相应变换；另外一个板子，backup区域里的RCC_DBCR的LSEON为1，即LSE打开，则以上(3)的操作从示波器上看到无效。
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作为GPIO输入的配置过程：
只是把以上的(2)稍微改一下，(3)就不用了
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_14 | GPIO_Pin_15;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN_FLOATING;
GPIO_Init(GPIOC, &GPIO_InitStructure);
外部连接为：在PC14引脚焊了一个电阻，电阻另一端通过跳线在上拉到3.3V和下拉到地的两种情况下，读出GPIOC_IDR.14分别为"1"和"0"。
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作为外部中断输入的配置过程：
1.时钟使能
// + osc32_in/out --> pc14/15
RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOC |RCC_APB2Periph_AFIO, ENABLE);
注意要使能AFIO的时钟哦
2.中断配置
// + for EXTI on PC.14 at falling edge
NVIC_PriorityGroupConfig(NVIC_PriorityGroup_1);
NVIC_InitStructure.NVIC_IRQChannel = EXTI15_10_IRQChannel;
NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;
NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
NVIC_Init(&NVIC_InitStructure);
3. GPIO配置同上
4. EXTI配置
// + for PC14 EXTI @ falling edge
GPIO_EXTILineConfig(GPIO_PortSourceGPIOC, GPIO_PinSource14);
EXTI_InitStructure.EXTI_Line = EXTI_Line14;
EXTI_InitStructure.EXTI_Mode = EXTI_Mode_Interrupt;
EXTI_InitStructure.EXTI_Trigger = EXTI_Trigger_Falling;
EXTI_InitStructure.EXTI_LineCmd = ENABLE;
EXTI_Init(&EXTI_InitStructure);
5. 中断处理ISR
void EXTI15_10_IRQHandler(void)
{
if(EXTI_GetITStatus(EXTI_Line14) != RESET)
{
EXTI_ClearITPendingBit(EXTI_Line14);
}
}
在中断处设置断点，只要按下key，就进入中断，跑出中断，再按key，再次进入，屡试不爽，试验完成~~~
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