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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USART的中断方式发送和接收不定长数据，USART配置为115200，数据位8，
停止位1，校验位None,下载并运行程序后，然后通过上位机下发任意长度个数据
（不超过128bytes），例如0x1~0xC,则MCU会把接收到的数据再次发送到上位机。

Function descriptions:
This example demonstrates the interrupt method of USART to send and receive
variable length data. USART is configured as 115200, with data bit 8, stop bit
1, and check bit None. After downloading and running the program, the MCU will
send any length of data (not exceeding 128bytes) through the upper computer,
such as 0x1~0xC. The MCU will send the received data to the upper computer again.
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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. 编译并下载程序到MCU，并运行；
2. 通过USB转TTL模块连接PC与STK板,STK板与USB转TTL模块的连线方式如下；
STK板        USB转TTL模块
PA02(TX) --> RX
PA03(RX) --> TX
GND      --> GND
3. PC端打开串口调试助手，正确连接上通讯COM口，波特率设置为115200；
4. 上位机发送任意长度个数据（不超过128bytes），MCU会将接收到的数据回发给上位机

Example execution steps:
1. compile and download the program to MCU and run it;
2. Connect the PC to the STK board through the USB-to-TTL module. The connection 
method between the STK board and the USB-to-TTL module is as follows.
STK board    USB-to-TTL module
PA02(TX) --> RX
PA03(RX) --> TX
GND      --> GND
3. Start the serial assistant on the PC, correctly connect the COM port, and set
the baud rate to 115200.
4. The upper computer sends any length of data (not exceeding 128 bytes), and
the MCU will send the received data back to the upper computer
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注意事项：
用户可根据需要修改main.h中宏RX_MAX_LEN的值，RX_MAX_LEN定义了MCU单次接收数据长度
（当前样例为128bytes）。

Notes:
Users can modify the values of macros RX_MAX_LEN in main.h as  needed. RX_MAX_LEN
defines the length of data received by MCU for a single time(currently 128bytes).
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