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Reset Sequence

Reset Sequence

1. When you reset the processor (e.g., press the reset button on the boad), the PC is loaded with the value 0x0000_0000. (Memory location 0x0000_0000 belongs to a Flash memory, or ROM memory, or Program memory.)

2. Then the processor reads the value at the memory location 0x0000_0000 into the MSP.

   • MSP = value at 0x0000_0000

   • MSP is the Main Stack Pointer register. This means that the processor, upon reset, first initializes the Stack Pointer.

3. Then the processor reads the value at the memory location 0x000_0004 into PC. That value is actually the address of the reset handler. (In other words, the address of the reset handler gets stored into PC.)

4. PC jumps to the reset handler.

   A reset handler is just a C or assembly function written by you to carry out any initialization required.

5. From the reset handler, main() function of the application gets called.

This is how the control reaches your program's main() function after reset.

Reset Handler

• Reset handler does the early initialization after which the call to the user program (main()) is made:

  Processor reset

  $\to$$\to$ data section initialization

  $\to$$\to$ BSS section initialization

  $\to$$\to$ initialization of a standard C library if it is used (e.g., __libc_init_array();)

  $\to$$\to$ calls the user program (main())

• Remember! The main() function cannot be called without these important early initializations.

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  43
  1
  /* a section of startup file (startup_stm32f407vgtx.s) */
  2
  
  3
  Reset_Handler:
  4
    ldr   r0, =_estack
  5
    mov   sp, r0          /* set stack pointer */
  6
  /* Call the clock system initialization function.*/
  7
    bl  SystemInit
  8
  
  9
  /* Copy the data segment initializers from flash to SRAM */
  10
    ldr r0, =_sdata
  11
    ldr r1, =_edata
  12
    ldr r2, =_sidata
  13
    movs r3, #0
  14
    b LoopCopyDataInit
  15
  
  16
  CopyDataInit:
  17
    ldr r4, [r2, r3]
  18
    str r4, [r0, r3]
  19
    adds r3, r3, #4
  20
  
  21
  LoopCopyDataInit:
  22
    adds r4, r0, r3
  23
    cmp r4, r1
  24
    bcc CopyDataInit
  25
  
  26
  /* Zero fill the bss segment. */
  27
    ldr r2, =_sbss
  28
    ldr r4, =_ebss
  29
    movs r3, #0
  30
    b LoopFillZerobss
  31
  
  32
  FillZerobss:
  33
    str  r3, [r2]
  34
    adds r2, r2, #4
  35
  
  36
  LoopFillZerobss:
  37
    cmp r2, r4
  38
    bcc FillZerobss
  39
  
  40
  /* Call static constructors */
  41
    bl __libc_init_array
  42
  /* Call the application's entry point.*/
  43
    bl main
  

   

References

Nayak, K. (2022). Embedded Systems Programming on ARM Cortex-M3/M4 Processor [Video file]. Retrieved from https://www.udemy.com/course/embedded-system-programming-on-arm-cortex-m3m4/