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AT91SAM7S64C-MU 开发手册 - ATMEL（爱特美尔）
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ATMEL（爱特美尔）
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微控制器
封装:
QFN-64
描述:
AT91 系列 55 MHz 64 kB 闪存 16 kB SRAM 微控制器 - QFN-64
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AT91SAM7S64C-MU数据手册
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2
6131A–ATARM–04-Mar-05
AT91 ARM Thumb
2. C-Startup Sequence
Following reset, the processor starts to fetch instructions from 0x0, therefore there must be
some executable code accessible from that address. In an AT91SAM7S embedded system, this
requires embedded Flash to be present, at least initially, at address 0x0.
The simplest layout is accomplished by locating the application in embedded Flash at address
0x0 in the memory map. The application can then branch to the real entry point when it executes
its first instruction at the reset vector at address 0x0.
All applications written for AT91SAM7S ARM-based systems are embedded applications that
are contained in embedded Flash and execute on reset.
There are a number of factors that must be considered when writing embedded operating sys-
tems, or embedded applications that execute from reset without an operating system, including:
• Reset entry point in embedded Flash.
• Initializing the execution environment, such as exception vectors, stacks, I/Os.
• Initializing the application.
• For example, copying initialization values for initialized variables from Flash to RAM and
resetting all other variables to zero.
• Linking an embedded executable image to place code and data in specific locations in RAM
memory.
For an embedded application without an operating system, the code in Flash must provide a
way for the application to initialize itself and start executing. No automatic initialization takes
place on reset, therefore the application entry point must perform some initialization before it can
call any C code.
The initialization code, located at address zero after reset, must:
• Mark the default entry point for the initialization code.
• Set up exception vectors.
• Initialize the memory system.
• Initialize any critical I/O devices.
• Initialize the stack pointer registers.
• Initialize any Register required by the interrupt system.
• Enable interrupts (if handled by the initialization code).
• Change processor mode if necessary.
• Change processor state if necessary.
After the environment has been initialized, the sequence continues with the application initializa-
tion and should enter the C code.
The C-startup file is the first file executed at power on and performs initialization of the microcon-
troller from the reset vector up to the calling of the application’s main routine.
The main program should be a closed loop and should not return. The ARM core begins execut-
ing instructions from address 0x0 at reset. For an AT91SAM7S embedded system this means in
embedded Flash at address 0x0 when the system is reset.
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