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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1的功能，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 functionality of TIM1 in external clock mode 1. 
It selects ETR (PA12) pin as the external clock input source and enables the 
update interrupt. In the interrupt, the LED light is toggled.
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测试环境：
测试用板：PY32T020_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: PY32T020_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 every 800 pulses detected by the MCU from the PA12 
pin, the LED light will be turned over once;
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注意事项：

Notes:
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