MC9S12单片机上电初始化过程及BOOTLOADER分析

上电过程

上电后，CPU从固定的位置读取复位向量。

这个固定的位置就是0xFFFE。是芯片厂商出厂时预设好的。

这个地址是NON_BANKED的。这个位置保存的是复位向量，实际上就是一个函数指针。该函数指针指向_Startup()函数。

map文件:

注意：程序的入口点是_Startup，不是main()。

_Startup函数是存放在0xC000位置。而0xFFFE地址的值就是0xC000.

S19文件：

_Startup（)函数分析

打开 Start12.c文件，可以看到_Startup（）函数。

/* The function _Startup must be called in order to initialize global variables and to call main */
/* You can adapt this function or call it from your startup code to implement a different startup */
/* functionality. */

/* You should also setup the needed IO registers as WINDEF (HC12A4 only) or the COP registers to run */
/* on hardware */

/* to set the reset vector several ways are possible : */
/* 1. define the function with "interrupt 0" as done below in the first case */
/* 2. add the following line to your prm file : VECTOR ADDRESS 0xfffe _Startup */
/* of course, even more posibilities exists */
/* the reset vector must be set so that the application has a defined entry point */

#if defined(__SET_RESET_VECTOR__)
__EXTERN_C void __interrupt 0 _Startup(void) {
#else
__EXTERN_C void _Startup(void) {
#endif
/*  purpose:    1)  initialize the stack
                2)  initialize the RAM, copy down init data etc (Init)
                3)  call main;
    parameters: NONE
    called from: _PRESTART-code generated by the Linker
                 or directly referenced by the reset vector */

   /* initialize the stack pointer */
   INIT_SP_FROM_STARTUP_DESC(); /*lint !e522 asm code */ /* HLI macro definition in hidef.h */

#if defined(_HCS12_SERIALMON)
   /* for Monitor based software remap the RAM & EEPROM to adhere
      to EB386. Edit RAM and EEPROM sections in PRM file to match these. */
   ___INITRG = 0x00;  /* lock registers block to 0x0000 */
   ___INITRM = 0x39;  /* lock Ram to end at 0x3FFF */
   ___INITEE = 0x09;  /* lock EEPROM block to end at 0x0fff */
#endif

   /* Here user defined code could be inserted, the stack could be used */
#if defined(_DO_DISABLE_COP_)
   _DISABLE_COP();
#endif

   /* Example : Set up WinDef Register to allow Paging */
#ifdef HC812A4 /* HC12 A4 derivative needs WINDEF to configure which pages are available */
#if  (__ENABLE_EPAGE__ != 0 ||  __ENABLE_DPAGE__ != 0 || __ENABLE_PPAGE__ != 0)
   WINDEF= __ENABLE_EPAGE__ | __ENABLE_DPAGE__  | __ENABLE_PPAGE__;
#endif
#endif

#if (defined(__MAP_RAM__) || defined(__MAP_FLASH__) || defined(__MAP_EXTERNAL__)) && !defined(__DO_SET_MMCTL1__)
#define __DO_SET_MMCTL1__
#endif


#if defined(__DO_SET_MMCTL1__)
  /* Set the MMCTL1 byte. Please use for HCS12XE and change the bits according   */
  /* to your configuration.                                                      */
  /* Note: MMCTL1 is write once therefore please adapt this initialization here. */
  /* This has to be done prior to the call to Init.                              */
#define _MMCTL1_ADR (0x00000013)
#define _MMCTL1_BIT_TGMRAMON (1<<7)  /* EEE Tag RAM and FTM SCRATCH RAM visible in the memory map */
#define _MMCTL1_BIT_EEEIFRON (1<<5)  /* EEE IFR visible in the memory map                         */
#define _MMCTL1_BIT_PGMIFRON (1<<4)  /* Program IFR visible in the memory map                     */
#define _MMCTL1_BIT_RAMHM    (1<<3)  /* RAM only in the higher half of the memory map             */
#define _MMCTL1_BIT_EROMON   (1<<2)  /* Enables emulated Flash or ROM memory in the memory map    */
#define _MMCTL1_BIT_ROMHM    (1<<1)  /* FLASH or ROM only in higher Half of Memory Map            */
#define _MMCTL1_BIT_ROMON    (1<<0)  /* Enable FLASH or ROM in the memory map                     */

#define _MMCTL1_SET(value)   ((*(volatile unsigned char*)_MMCTL1_ADR)= (value))

#if defined(__MAP_FLASH__)
  _MMCTL1_SET(_MMCTL1_BIT_ROMON | _MMCTL1_BIT_EROMON);
#elif defined(__MAP_EXTERNAL__)
  _MMCTL1_SET(_MMCTL1_BIT_ROMON | _MMCTL1_BIT_EROMON | _MMCTL1_BIT_ROMHM);
#else /* RAM */
  _MMCTL1_SET(_MMCTL1_BIT_ROMON | _MMCTL1_BIT_EROMON | _MMCTL1_BIT_RAMHM | _MMCTL1_BIT_ROMHM);
#endif    
#endif

#ifndef __ONLY_INIT_SP
   Init(); /* zero out, copy down, call constructors */
#endif

   /* Here user defined code could be inserted, all global variables are initilized */
#if defined(_DO_ENABLE_COP_)
   _ENABLE_COP(1);
#endif

   /* call main() */
   main();
}

其实，删除掉各种define，去繁存精，只干了三件事：设置栈指针SP，调用init()，然后调用main()。

从map文件中也可以验证：