请问 stm32f103 外部晶体 何时工作？
没有下载程序的stm32f103最小系统上电，晶体上没测到波形，二脚都是低电平，是否正常？谢谢！
中文手册第57页：
HSE晶体可以通过设置时钟控制寄存器里RCC_CR中的HSEON位被启动和关闭。
你要 在主程序里设置呀，不然程序不知道你要用外置还是内置的 刚启动的时候是使用内部的HSI,内部HSI稳定后如果设置使用HSE则待HSE稳定后切换至HSE时钟，你的代码有可能未使用外部时钟，所以没有波 本帖最后由 stm1024 于 2018-5-28 17:25 编辑
首先，你要选择HSE……
建议你先看看版本为3.5的标准库，system_stm32f10x.c这个文件，最开始的描述是这样的：/*
* 1.This file provides two functions and one global variable to be called from
* user application:
* - SystemInit(): Setups the system clock (System clock source, PLL Multiplier
* factors, AHB/APBx prescalers and Flash settings).
* This function is called at startup just after reset and
* before branch to main program. This call is made inside
* the "startup_stm32f10x_xx.s" file.
*
* - SystemCoreClock variable: Contains the core clock (HCLK), it can be used
* by the user application to setup the SysTick
* timer or configure other parameters.
*
* - SystemCoreClockUpdate(): Updates the variable SystemCoreClock and must
* be called whenever the core clock is changed
* during program execution.
*
* 2. After each device reset the HSI (8 MHz) is used as system clock source.
* Then SystemInit() function is called, in "startup_stm32f10x_xx.s" file, to
* configure the system clock before to branch to main program.
*
* 3. If the system clock source selected by user fails to startup, the SystemInit()
* function will do nothing and HSI still used as system clock source. User can
* add some code to deal with this issue inside the SetSysClock() function.
*
* 4. The default value of HSE crystal is set to 8 MHz (or 25 MHz, depedning on
* the product used), refer to "HSE_VALUE" define in "stm32f10x.h" file.
* When HSE is used as system clock source, directly or through PLL, and you
* are using different crystal you have to adapt the HSE value to your own
* configuration.
*/ 以前我们在3.5之前的标准库里面，总要在main中调用一下SystemInit()，但是在3.5中，此项工作在启动代码中执行。
文件“startup_stm32f10x_xx.s“中的SystemInit()函数在所有设备会在进入main之前配置系统时钟，如果配置失败，HSI还是作为系统时钟源。
检查一下你的代码，是否有这个过程，如果有的话，最好调试一下看看代码挂在哪里了。
不管寄存器，标准库还是HAL库，我们要使用外部高速时钟都是需要在程序里设置代码的。没有程序的stm32f103应该是默认就没有使用外部晶振的，所以晶体本来就没工作。
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