在状态机中管理时间资源

教程框架

写在前面的话

在嵌入式系统中，经常会使用到延时(Delay)函数，延时有长，有短；500us以下的延时，可以完全用软件来做；较长的延时，可以用硬件Timer或者调用RTOS的时间管理函数。

在简单的MCU中，由于资源限制，无法使用常规的RTOS，这时就需要自己写相应的协调式调度算法，常见的有协程和时间片轮询机制等，万变不离其宗，它们其中的核心是状态机，在状态机中一个核心原则是：函数不能长时间阻塞MCU，否则就会导致整个系统调度失效。也就是说：无法使用纯软件延时

例如，在系统中严禁出现下面代码：

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void delay_s(uint8_t tick_s)
{
    ...
}


...


// Code Snippet 1

delay_s(2);

// Code Snippet 2

注：
关于状态机，后面会专门写文章来介绍，这里暂且略过；如果您感兴趣，请自行 百度/google 下面关键字

FSM/HSM/状态机


原理分析

下面讨论下怎么处理这种情况

传统软件延时的核心问题是：没有一个统一的全局时间，所以各个函数只好自己空转来延时；在空转时，也不会释放MCU，
所以导致无法执行其它函数，在OS的环境下，时间资源由RTOS统一接管，调度；无OS的情况下，我们便需要自己来管理时间资源

在APP层，用户使用场景规定如下：

  1. 函数需要延时
  2. 向系统申请时间资源
  3. 任务获取句柄
  4. 任务中，查询定时时间是否到达

实现机制如下：

在系统级别维护一个全局Timer值，该值会不断递增，同时在内部维护一个时间单元数组（链表）
，当用户申请时间资源时，系统查询一个空闲时间单元，将当前时间加上延时值，存储到时间单元结构体中，然后标记单元为占用状态，
返回时间单元ID（即为句柄）

注：当定时时间到达后，原来获取的句柄变会失效，如需重新使用延时服务，请重新申请句柄

代码实现

详细的代码如下：

H文件代码
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//******************************************************************************************
//!
//! \file   SoftTimer.h
//! \brief  SoftTimer Interface File
//!         This module is used to replace 'delay' function in state machine
//! \author cedar
//! \date   2014-9-17
//! \email  xuesong5825718@gmail.com
//! \qq     819280802
//!
//! \license
//!
//! Copyright (c) 2014 Cedar MIT License
//!
//! Permission is hereby granted, free of charge, to any person obtaining a copy
//! of this software and associated documentation files (the "Software"), to deal
//! in the Software without restriction, including without limitation the rights to
//! use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
//! the Software, and to permit persons to whom the Software is furnished to do so,
//! subject to the following conditions:
//!
//! The above copyright notice and this permission notice shall be included in all
//! copies or substantial portions of the Software.
//!
//! THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
//! IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//! FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
//! AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
//! LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
//! OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
//! IN THE SOFTWARE.
///
//******************************************************************************************

#ifndef __SOFT_TIMER_H__
#define __SOFT_TIMER_H__

#ifdef __cplusplus
extern "C"
{
#endif

//******************************************************************************************
//!                           Portable Configure
//******************************************************************************************

//! If your compiler support stdint.h file, then try to use it, otherwise using folling section
#if 0
typedef unsigned char      uint8_t ;
typedef unsigned short int uint16_t;
typedef unsigned int       uint32_t;
#endif

#define ENTER_SECURE_SECTION()     \
do                                 \
{                                  \
} while (0)


#define EXIT_SECURE_SECTION()      \
do                                 \
{                                  \
} while (0)



//******************************************************************************************
//!                           Function Parameters
//******************************************************************************************

#define TIMER_ELEMENT_NUM_MAX      20                      //!< Maxium SoftTimer

#define SOFT_TIMER_SUCCESS         0                       //!< Operate success
#define SOFT_TIMER_ING             1                       //!< Timer Counting
#define SOFT_TIMER_TIMEOUT         2                       //!< Timer Timeout
#define SOFT_TIMER_ERR             3                       //!< Operate failure, invaild handle


//******************************************************************************************
//!                           PUBLIC API
//******************************************************************************************

extern uint16_t SoftTimer_Init(void);
extern uint16_t SoftTimer_Req(uint32_t Delay);
extern uint16_t SoftTimer_Check(uint16_t Handle);
extern void TimerISR_Hook(void);

#ifdef __cplusplus
}
#endif

#endif // __SOFT_TIMER_H__
C文件代码
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//******************************************************************************************
//!
//! \file   SoftTimer.c
//! \brief  SoftTimer Implement File
//!         This module is used to replace 'delay' function in state machine
//! \author cedar
//! \date   2014-9-17
//! \email  xuesong5825718@gmail.com
//! \qq     819280802
//!
//! \license
//!
//! Copyright (c) 2014 Cedar MIT License
//!
//! Permission is hereby granted, free of charge, to any person obtaining a copy
//! of this software and associated documentation files (the "Software"), to deal
//! in the Software without restriction, including without limitation the rights to
//! use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
//! the Software, and to permit persons to whom the Software is furnished to do so,
//! subject to the following conditions:
//!
//! The above copyright notice and this permission notice shall be included in all
//! copies or substantial portions of the Software.
//!
//! THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
//! IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//! FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
//! AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
//! LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
//! OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
//! IN THE SOFTWARE.
///
//******************************************************************************************

#include "SoftTimer.h"


//******************************************************************************************
//!                     Typedef
//******************************************************************************************
typedef struct
{
    uint16_t handle;                                       // Timer Resource Handle
    uint32_t delay;                                        // Delay Value
    uint32_t timestamp;                                    // Timer Stamp
}TimerElem_t;


//******************************************************************************************
//!                     Private Variable
//******************************************************************************************

static volatile uint32_t TimerCnt    = 0;
static volatile uint16_t HandleIndex = 0;
static TimerElem_t       SoftTimer[TIMER_ELEMENT_NUM_MAX];


//******************************************************************************************
//!                     Function Implement
//******************************************************************************************


//******************************************************************************************
//
//! \brief  Initialize Timer resource
//!
//! \param  None.
//! \retval 
//!         - SOFT_TIMER_SUCCESS
//!
//! \note
//!         - This function must be called first !.
//!         
//
//******************************************************************************************
uint16_t SoftTimer_Init(void)
{
    uint8_t i = 0;

    // Clear All Elements
    for(i = 0; i < TIMER_ELEMENT_NUM_MAX; i++)
    {
        SoftTimer[i].handle    = 0;
        SoftTimer[i].delay     = 0;
        SoftTimer[i].timestamp = 0;
    }

    // Initialize Global Status Variable
    TimerCnt    = 0;
    HandleIndex = 1;

    return (SOFT_TIMER_SUCCESS);
}

//******************************************************************************************
//
//! \brief  Request Timer resource
//!
//! \param  [in] Tick is time eclipse value.
//! \retval 
//!         - Zero                     Operate Failure, No Timer Available
//!         - Non-Zero                 Valid Timer Handle
//!
//! \note
//!         - Timer handle only can be used once.
//!         
//
//******************************************************************************************
uint16_t SoftTimer_Req(uint32_t Tick)
{
    uint8_t i = 0;

    for (i = 0; i < TIMER_ELEMENT_NUM_MAX; i++)
    {
        if (SoftTimer[i].handle == 0)
        {

            ENTER_SECURE_SECTION();

            // Timer Handle != 0
            if (++HandleIndex == 0)
            {
                ++HandleIndex;
            }

            SoftTimer[i].handle    = HandleIndex;
            SoftTimer[i].delay     = Tick;
            SoftTimer[i].timestamp = Tick + TimerCnt;

            EXIT_SECURE_SECTION();

            return (HandleIndex);
        }
    }

    return (0);
}

//******************************************************************************************
//
//! \brief  Check Timer status
//!         You can check register timer status at any time.
//!
//! \param  [in] Handle is Timer Handle, which you can get it from \ref SoftTimer_Req.
//! \retval 
//!         - \ref SOFT_TIMER_ING      Timer Counting
//!         - \ref SOFT_TIMER_TIMEOUT  Timer TimeOut
//!         - \ref SOFT_TIMER_ERR      Invalid Timer Handle
//!
//! \note
//!         - You must call \ref SoftTimer_Req to request an valid timer handle.
//!         - Timer handle only can be used once.
//!         
//
//******************************************************************************************
uint16_t SoftTimer_Check(uint16_t Handle)
{
    uint8_t i = 0;

    for(i = 0; i < TIMER_ELEMENT_NUM_MAX; i++)
    {
        if(SoftTimer[i].handle ==  Handle)
        {
            ENTER_SECURE_SECTION();
            if(SoftTimer[i].delay + TimerCnt < SoftTimer[i].timestamp)
            {
                return (SOFT_TIMER_ING);
            }
            else
            {
                return (SOFT_TIMER_TIMEOUT);
            }
            EXIT_SECURE_SECTION();
        }
    }

    return (SOFT_TIMER_ERR);
}


//******************************************************************************************
//
//! \brief  SoftTimer Hook Function
//!         This callback function must be called interval
//!
//! \note   Typical 1ms interval
//
//******************************************************************************************
void TimerISR_Hook(void)
{
    TimerCnt++;
}

最新代码，请到https://github.com/cedar-renjun/Codesnippet/tree/master/SoftTimer获取