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：

BOOTLOADER设计

在有BOOT的系统中，程序分为两部分：BOOT和APP。

这里面就有两套中断。需要在BOOT中做特殊处理。

在升级过程中，接收到上位机发过来的APP的bin文件时，需要调整中断向量保存的位置。（因为APP工程中生成的中断向量也是保存在0xFF10开始的位置）

上电后，CPU还是先从BOOT开始执行。在BOOT中作一些判断，确认APP正常后，跳转到APP中去执行。但是在跳转前，需要先配置中断向量基地址寄存器（IVBR），确保中断向量指向正确的位置。

注意：IVBR是个8位的寄存器，而中断向量起始地址是16位的，其中低8位默认为0.