freemodbus stm32 hal库porttimer代码，中文注释

以下是FreeModbus STM32 HAL库中porttimer文件的示例代码，其中包含了中文注释：

/* 
 * FreeModbus Libary: STM32Fxxx Port Timer For Modbus 
 * 
 * Copyright (C) 2013 Real Time Engineers ltd.
 * 
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 * 
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 * 
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 * 
 * File: porttimer.c
 * 
 * Description: STM32Fxxx port timer implementation for FreeModbus.
 */

/* ----------------------- Modbus includes ----------------------------------*/
#include "mb.h"
#include "mbport.h"

/* ----------------------- STM32 includes ----------------------------------*/
#include "stm32f10x.h"

/* ----------------------- Defines ------------------------------------------*/
#define MB_TIMER_PRESCALER      ( ( uint16_t )( ( SystemCoreClock / 2 ) / 1000000 ) - 1 )   // 定时器预分频器，用于将系统时钟分频到1MHz
#define MB_TIMER_TICKS          ( 20000UL )     // 定时器重装值，用于定时20ms

/* ----------------------- Static variables ---------------------------------*/
static USHORT   usTimerOCRADelta;

/* ----------------------- Function prototypes ------------------------------*/
static void prvvTIMERExpiredISR( void );

/* ----------------------- Start implementation -----------------------------*/
BOOL
xMBPortTimersInit( USHORT usTim1Timerout50us )
{
    TIM_TimeBaseInitTypeDef TIM_TimeBaseStructure;
    NVIC_InitTypeDef NVIC_InitStructure;

    /* Enable the TIM2 global Interrupt */
    NVIC_InitStructure.NVIC_IRQChannel = TIM2_IRQn;
    NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 3;
    NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;
    NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
    NVIC_Init( &NVIC_InitStructure );

    /* Time base configuration */
    TIM_TimeBaseStructInit( &TIM_TimeBaseStructure );
    TIM_TimeBaseStructure.TIM_Period = ( uint16_t )( MB_TIMER_TICKS - 1 );
    TIM_TimeBaseStructure.TIM_Prescaler = MB_TIMER_PRESCALER;
    TIM_TimeBaseStructure.TIM_ClockDivision = 0;
    TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
    TIM_TimeBaseInit( TIM2, &TIM_TimeBaseStructure );

    /* Clear TIM2 update flag */
    TIM_ClearFlag( TIM2, TIM_FLAG_Update );

    /* Enable TIM2 Update interrupt */
    TIM_ITConfig( TIM2, TIM_IT_Update, ENABLE );

    /* Preload enable */
    TIM_OC1PreloadConfig( TIM2, TIM_OCPreload_Disable );
    TIM_ARRPreloadConfig( TIM2, ENABLE );

    /* TIM2 enable counter */
    TIM_Cmd( TIM2, ENABLE );

    /* Timer delta value for Modbus tick */
    usTimerOCRADelta = ( uint16_t )( ( ( uint32_t )usTim1Timerout50us * ( uint32_t )( MB_TIMER_TICKS ) ) / ( uint32_t )20000 );

    return TRUE;
}

void
vMBPortTimersEnable(  )
{
    /* Enable the timer with the timeout passed to xMBPortTimersInit( ) */
    TIM_SetAutoreload( TIM2, ( uint16_t )( MB_TIMER_TICKS - 1 ) );
    TIM_SetCompare1( TIM2, TIM_GetCounter( TIM2 ) + usTimerOCRADelta );
    TIM_Cmd( TIM2, ENABLE );
}

void
vMBPortTimersDisable(  )
{
    /* Disable any pending timers. */
    TIM_Cmd( TIM2, DISABLE );
    TIM_SetCounter( TIM2, 0 );
}

/* ----------------------- Timers functions ---------------------------------*/
void
prvvTIMERExpiredISR( void )
{
    ( void )pxMBPortCBTimerExpired(  );
}

void
TIM2_IRQHandler( void )
{
    if( TIM_GetITStatus( TIM2, TIM_IT_Update ) != RESET )
    {
        TIM_ClearITPendingBit( TIM2, TIM_IT_Update );
        prvvTIMERExpiredISR(  );
    }
}

这段代码实现了FreeModbus STM32 HAL库的定时器功能。其中，`MB_TIMER_PRESCALER`和`MB_TIMER_TICKS`分别是定时器的预分频器和重装值，用于将定时器时钟分频到1MHz并定时20ms。在`xMBPortTimersInit()`函数中，首先配置了定时器的各项参数，并设置了定时器中断的优先级和使能。在`vMBPortTimersEnable()`函数中，设置了定时器到期时间，并使能了定时器。而在`vMBPortTimersDisable()`函数中，则禁用了定时器。

此外，代码还实现了定时器到期时的中断处理函数`prvvTIMERExpiredISR()`和定时器中断服务函数`TIM2_IRQHandler()`，以便在定时器到期时触发Modbus超时处理。