Startup sequence and reset vector in Embedded C - Mini Project: Build & Apply

Startup Sequence and Reset Vector in Embedded C

📖 Scenario: You are working on a simple embedded system that needs to start correctly when powered on or reset. The system uses a reset vector to jump to the startup code, which initializes the system before running the main program.

🎯 Goal: Build a basic embedded C program that defines a reset vector and a startup sequence function. The startup sequence will initialize a variable and then call the main function. This simulates how embedded systems begin running after a reset.

📋 What You'll Learn

Define a reset vector function named Reset_Handler

Create a startup function named Startup_Code that initializes a variable system_ready to 1

Call the main function from the startup code

Define a main function that returns 0

Print the value of system_ready in main to confirm initialization

💡 Why This Matters

🌍 Real World

Embedded systems like microcontrollers use reset vectors and startup sequences to prepare hardware and software before running the main application.

💼 Career

Understanding startup sequences and reset vectors is essential for embedded software engineers working on firmware for devices like IoT gadgets, automotive controllers, and consumer electronics.

1

Create the system_ready variable

Create a global integer variable called system_ready and set it to 0.

Embedded C

# Create a global integer variable called system_ready and set it to 0
# Your code here

Hint

Use int system_ready = 0; to create the variable.

2

Define the Startup_Code function

Define a function called Startup_Code that sets system_ready to 1 and then calls the main function.

Embedded C

int system_ready = 0;

// Define Startup_Code function to initialize system_ready and call main
# Your code here

Hint

Write void Startup_Code() { system_ready = 1; main(); }.

3

Create the Reset_Handler function

Create a function called Reset_Handler that calls the Startup_Code function.

Embedded C

int system_ready = 0;

void Startup_Code() {
    system_ready = 1;
    main();
}

// Define Reset_Handler function to call Startup_Code
# Your code here

Hint

Write void Reset_Handler() { Startup_Code(); }.

4

Define the main function and print system_ready

Define the main function that prints the value of system_ready and returns 0.

Embedded C

int system_ready = 0;

void Startup_Code() {
    system_ready = 1;
    main();
}

void Reset_Handler() {
    Startup_Code();
}

// Define main function to print system_ready and return 0
# Your code here

Hint

Use printf("system_ready = %d\n", system_ready); inside main.

Practice

1. What is the primary role of the reset vector in an embedded C program?

easy

A. It manages power consumption modes.

B. It initializes all global variables to zero.

C. It handles interrupts during program execution.

D. It tells the microcontroller where to start executing code after a reset.

Solution

1. Step 1: Understand the reset vector concept

   The reset vector is a special address in memory that points to the first instruction executed after a reset.

2. Step 2: Identify its role in program start

   When the microcontroller resets, it jumps to the reset vector to begin running the startup code.

3. Final Answer:

   It tells the microcontroller where to start executing code after a reset. -> Option D

4. Quick Check:

   Reset vector = start address after reset [OK]

Hint: Reset vector points to start code after reset [OK]

Common Mistakes:

• Confusing reset vector with interrupt vector
• Thinking reset vector initializes variables
• Assuming reset vector manages power modes

2. Which of the following is the correct way to declare the reset vector in embedded C?

easy

A. int ResetVector = 0x0000;

B. void ResetVector() { main(); }

C. void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler;

D. char *ResetVector = "Reset_Handler";

Solution

1. Step 1: Understand reset vector declaration

   The reset vector is usually a pointer to a function placed in a special memory section.

2. Step 2: Check syntax correctness

   void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler; correctly declares a constant function pointer to Reset_Handler placed in the ".reset_vector" section using GCC attribute syntax.

3. Final Answer:

   void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler; -> Option C

4. Quick Check:

   Function pointer in reset vector section = correct declaration [OK]

Hint: Reset vector is a function pointer in special section [OK]

Common Mistakes:

• Declaring reset vector as int or char pointer
• Defining reset vector as a function instead of pointer
• Missing section attribute for placement

3. Given the following code snippet, what will be the output after reset?

void Reset_Handler(void) {
    int x = 5;
    x += 10;
    while(1) {}
}

void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler;

medium

A. The program will enter an infinite loop after setting x to 15.

B. The program will run main() and print 15.

C. The program will cause a runtime error due to missing main.

D. The program will reset repeatedly.

Solution

1. Step 1: Analyze Reset_Handler behavior

   Reset_Handler sets x to 5, adds 10 making x = 15, then enters an infinite loop.

2. Step 2: Understand program flow after reset

   After reset, the microcontroller jumps to Reset_Handler and stays in the infinite loop; no main() is called.

3. Final Answer:

   The program will enter an infinite loop after setting x to 15. -> Option A

4. Quick Check:

   Reset_Handler runs then loops forever = infinite loop [OK]

Hint: Reset_Handler runs then loops forever, no main called [OK]

Common Mistakes:

• Assuming main() is called automatically
• Expecting output or print statements
• Confusing variable x scope and effect

4. Identify the error in this startup code snippet:

void Reset_Handler(void) {
    // Initialize data
    extern int _sidata, _sdata, _edata;
    int *src = &_sidata;
    int *dst = &_sdata;
    while(dst < &_edata) {
        *dst++ = *src++;
    }
    main();
}

void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler;

medium

A. Reset_Handler should not call main() directly.

B. The pointers _sidata, _sdata, _edata should be declared as extern char, not int.

C. The while loop condition should use <= instead of <.

D. ResetVector should not be a function pointer.

Solution

1. Step 1: Check pointer types for memory copy

   Memory sections like _sidata, _sdata, _edata represent byte addresses and should be pointers to char or unsigned char, not int.

2. Step 2: Understand pointer arithmetic impact

   Using int pointers causes incorrect copying because pointer arithmetic advances by sizeof(int), skipping bytes.

3. Final Answer:

   The pointers _sidata, _sdata, _edata should be declared as extern char, not int. -> Option B

4. Quick Check:

   Memory pointers for data init = char* not int* [OK]

Hint: Use char pointers for memory copy in startup [OK]

Common Mistakes:

• Using int pointers for byte-wise memory copy
• Changing loop condition incorrectly
• Thinking main() call is wrong here

5. You want to create a startup sequence that initializes .data and .bss sections and then jumps to main(). Which of the following code snippets correctly implements this in embedded C?

hard

A. void Reset_Handler(void) {
extern char _sidata, _sdata, _edata, _sbss, _ebss;
char *src = &_sidata;
char *dst = &_sdata;
while(dst < &_edata) *dst++ = *src++;
dst = &_sbss;
while(dst < &_ebss) *dst++ = 0;
main();
}
void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler;

B. void Reset_Handler(void) {
int *src = &_sidata;
int *dst = &_sdata;
while(dst <= &_edata) *dst++ = *src++;
main();
}
void ResetVector = Reset_Handler;

C. void Reset_Handler(void) {
main();
}
void (* ResetVector)(void) = Reset_Handler;

D. void Reset_Handler(void) {
extern char _sidata, _sdata, _edata;
char *src = &_sdata;
char *dst = &_sidata;
while(src < &_edata) *dst++ = *src++;
main();
}
void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler;

Solution

1. Step 1: Verify .data section initialization

   void Reset_Handler(void) { extern char _sidata, _sdata, _edata, _sbss, _ebss; char *src = &_sidata; char *dst = &_sdata; while(dst < &_edata) *dst++ = *src++; dst = &_sbss; while(dst < &_ebss) *dst++ = 0; main(); } void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler; correctly copies initialized data from flash (_sidata) to RAM (_sdata to _edata) using char pointers and proper loop.

2. Step 2: Verify .bss section zero initialization

   void Reset_Handler(void) { extern char _sidata, _sdata, _edata, _sbss, _ebss; char *src = &_sidata; char *dst = &_sdata; while(dst < &_edata) *dst++ = *src++; dst = &_sbss; while(dst < &_ebss) *dst++ = 0; main(); } void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler; zeroes the .bss section (_sbss to _ebss) by setting bytes to 0 before calling main().

3. Step 3: Check reset vector declaration

   void Reset_Handler(void) { extern char _sidata, _sdata, _edata, _sbss, _ebss; char *src = &_sidata; char *dst = &_sdata; while(dst < &_edata) *dst++ = *src++; dst = &_sbss; while(dst < &_ebss) *dst++ = 0; main(); } void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler; declares ResetVector as a constant function pointer placed in the reset vector section, which is correct.

4. Final Answer:

   Option A correctly implements startup sequence with .data and .bss initialization and reset vector. -> Option A

5. Quick Check:

   Proper .data/.bss init + correct reset vector = void Reset_Handler(void) { extern char _sidata, _sdata, _edata, _sbss, _ebss; char *src = &_sidata; char *dst = &_sdata; while(dst < &_edata) *dst++ = *src++; dst = &_sbss; while(dst < &_ebss) *dst++ = 0; main(); } void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler; [OK]

Hint: Initialize .data from flash, zero .bss, then call main() [OK]

Common Mistakes:

• Using int pointers instead of char pointers
• Swapping source and destination pointers
• Missing .bss zero initialization
• Incorrect reset vector declaration