/**
  ******************************************************************************
  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
  * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
  * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
  * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
  * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  *
  * Copyright (C) 2023 Shanghai Eastsoft Microelectronics Co., Ltd. ALL rights reserved.
  *******************************************************************************
  */

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Version history:
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2023-04-24	V1.0.0	AE TEAM

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Demo introduction:
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本示例功能：外部中断嵌套内核计时中断。
按键触发外部中断后，在中断服务中闪烁LED灯，闪烁频率由计时中断控制。

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测试环境:
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测试用板；
ES-PDS-ES32VF2264

辅助工具：
无

辅助软件：
无

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测试步骤：
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1) 编译工程，编译通过后将程序下载到目标芯片；
2) 在线调试，或复位，运行程序；
3) 按下按键后松开，LED闪烁两次，频率1Hz。

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注意：
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1) 若未正确进行中断嵌套所需的配置，则无法嵌套到计时中断，按下按键松开后，LED不会闪烁，而保持长亮。
2) Compiler标签页的Define栏需包含NEST_INT_ENABLE。

中断嵌套前提条件：
1、使能全局中断；
2、确保需要响应的中断使能；
3、确保需要响应的中断产生；
4、确保需要响应的中断优先级高于现有中断。

中断嵌套流程是 ：
1、mstatus/mepc/mcause压栈；
2、打开全局中断；
3、中断服务回调；
4、关闭全局中断；
5、mstatus/mepc/mcause弹栈。
注意：只有第3个阶段可以被嵌套。
参考本例程本例程的EXTI0_3_Handler。

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System clock:
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PLL 72MHz

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Pin connection: 
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KEY  ---- PD2[GPIO_FUNC_1]
LED  ---- PA15[GPIO_FUNC_1]
