{"id":2489,"date":"2019-06-26T11:30:08","date_gmt":"2019-06-26T11:30:08","guid":{"rendered":"http:\/\/fastbitlab.com\/?p=2489"},"modified":"2023-08-29T17:20:45","modified_gmt":"2023-08-29T11:50:45","slug":"arm-cortex-m-processor","status":"publish","type":"post","link":"https:\/\/fastbitlab.com\/blog\/arm-cortex-m-processor\/","title":{"rendered":"ARM Cortex M processor reset sequence"},"content":{"rendered":"
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ARM Cortex M processor reset sequence<\/span><\/strong><\/h2>\n
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\"ARM<\/p>\n

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In this post let\u2019s understand the RESET sequence of the Cortex M3\/M4 processor. It will answer the question, what happens when you reset the processor. Remember, the addressable memory space of the processor always starts with zero i.e 0x00000000. <\/strong><\/span><\/p>\n

The beginning of the memory space starting from zero, actually contains the \u201cvector table\u201d<\/strong>. <\/span><\/p>\n

Vector table is nothing but a table of information about the initial stack pointer value and various exception handler addresses. We will talk a lot about vector tables and its contents in the later videos of the course<\/a><\/span>. So don\u2019t worry about vector table as of now.<\/span><\/p>\n

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What I wanted, you to understand here is that, the sequences after you reset your processor or say your board.<\/p>\n

1) After reset, PC is loaded with the address 0x00000000<\/strong><\/p>\n

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2) Then the processor fetches the value at 0x00000000 in to the MSP. i.e., Main Stack Pointer.<\/strong><\/p>\n

that\u2019s the initial value of the MSP.<\/em><\/p>\n

So , processor basically first initializes the main stack pointer.<\/em><\/p>\n

Now the question is,  who puts the valid value into address location 0x00000000.<\/em><\/p>\n

Yes , it\u2019s the programmer\u2019s responsibility to put the valid value at the location 0x00000000 ( Usually taken care by the startup code )<\/em><\/p>\n

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3) Next the processor reads the address of the reset handler from the location 0x00000004 in to the Program Counter.<\/strong><\/p>\n

So , It is very important to store the address of your reset handler in the location 0x00000004<\/em><\/p>\n

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4) Then the processor jumps to your reset handler and start executing the first instruction which you wrote there .<\/strong><\/p>\n

If you are wondering what’s the RESET handler. Let me explain to you .<\/em><\/p>\n

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Reset handler is nothing but a normal function written in assembly or C language, which you want to get called whenever processor resets. Remember that RESET is also a processor system exception, so when you hit the reset button , it raises RESET exception, as a result your reset handler will run.<\/p>\n<\/blockquote>\n

In the reset handler usually we do  initial device specific initialization such as configuring clocks, configuring hardware block, re-initializing the stack space, before calling main function of your application source code.<\/em><\/p>\n

Application writer guy may not be knowing how to Configure the clock, so \u201cResent handler\u201d may take care of that before calling the main function.<\/em><\/p>\n

Here is a small code segment which you can see in the startup code of the STM32 related MCUs.<\/p>\n

;Reset handler<\/span>
\nReset_Handler         PROC<\/span>
\n                                     EXPORT Reset_Handler [WEAK]<\/span>
\nIMPORT            SystemInit<\/span>
\nIMPORT            __main<\/span>
\nLDR R0,           =SystemInit  \/* First calls for the SystemInit , which does MCU Clock init *\/<\/span><\/span>
\nBLX R0<\/span>
\nLDR R0,           =__main  \/* then call main() *\/<\/span><\/span>
\nBX R0<\/span>
\nEND<\/span><\/p>\n

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5) After the required initialization, you can then call your main() from Reset Handler ,that\u2019s how the control comes to your main() function in your Application<\/strong> .<\/p>\n

Watch the below video for detailed explanation .<\/strong><\/span><\/p>\n