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                                样例使用说明
                                Sample Description
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功能描述：
此样例演示了TIM1的外部时钟模式2功能，选择ETR(PA12)引脚作为外部时钟输入源，并使能
更新中断，在中断中翻转LED灯。

Function descriptions:
This sample demonstrates the external clock mode 2 function of TIM1, selects the 
ETR(PA12) pin as the external clock input source, and enables the update 
interrupt and toggle the LED light in the interrupt
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测试环境：
测试用板：PY32F072_STK
MDK版本： 5.28
IAR版本： 9.20
GCC 版本：GNU Arm Embedded Toolchain 10.3-2021.10

Test environment:
Test board: PY32F072_STK
MDK Version: 5.28
IAR Version: 9.20
GCC Version: GNU Arm Embedded Toolchain 10.3-2021.10
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使用步骤：
1. 编译下载程序到MCU，并运行；
2. PA12引脚，输入外部时钟；
3. 可观察到MCU从PA12引脚上每检测到800个脉冲，会让LED灯翻转一次；

Example execution steps:
1. compile and download the program to MCU and run it;
2. Input external clock into PA12 pin;
3. It can be observed that the MCU will toggle the LED light once every 800 
pulses detected from the PA12 pin;
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注意事项：

Notes:

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