Memory layout on a microcontroller (Flash, SRAM, EEPROM) in Embedded C - Mini Project: Build & Apply

Memory layout on a microcontroller (Flash, SRAM, EEPROM)

📖 Scenario: You are working with a simple microcontroller that has three types of memory: Flash, SRAM, and EEPROM. Each memory type stores different kinds of data. Flash stores the program code, SRAM stores variables that change while the program runs, and EEPROM stores data that must be saved even when the power is off.

🎯 Goal: You will create variables in the correct memory sections and then print their values to understand how memory layout works on a microcontroller.

📋 What You'll Learn

Create a constant string stored in Flash memory

Create a variable stored in SRAM memory

Create a variable stored in EEPROM memory

Print the values of all three variables

💡 Why This Matters

🌍 Real World

Microcontrollers use different memory types to store code, temporary data, and persistent data. Understanding this helps write efficient embedded programs.

💼 Career

Embedded software engineers must manage memory layout carefully to optimize performance and data retention in devices like sensors, appliances, and IoT gadgets.

1

Create a constant string in Flash memory

Create a constant character array called flash_message with the value "Hello Flash" stored in Flash memory using the const keyword.

Embedded C

# Create a constant string called flash_message stored in Flash memory
// Your code here

Hint

Use const char to store data in Flash memory.

2

Create a variable in SRAM memory

Create a variable called sram_counter of type int and initialize it to 0. This variable will be stored in SRAM memory.

Embedded C

const char flash_message[] = "Hello Flash";
// Create an int variable called sram_counter initialized to 0
// Your code here

Hint

Variables that change during program execution are stored in SRAM.

3

Create a variable in EEPROM memory

Create a variable called eeprom_value of type unsigned char and initialize it to 100. Use the __attribute__((section(".eeprom"))) to place it in EEPROM memory.

Embedded C

const char flash_message[] = "Hello Flash";
int sram_counter = 0;
// Create an unsigned char variable called eeprom_value initialized to 100 in EEPROM memory
// Your code here

Hint

Use __attribute__((section(".eeprom"))) to store variables in EEPROM.

4

Print all variable values

Write printf statements to print the values of flash_message, sram_counter, and eeprom_value. Use the format "Flash: %s\nSRAM: %d\nEEPROM: %d\n".

Embedded C

const char flash_message[] = "Hello Flash";
int sram_counter = 0;
unsigned char eeprom_value __attribute__((section(".eeprom"))) = 100;

// Print flash_message, sram_counter, and eeprom_value
// Your code here

Hint

Use printf with the correct format specifiers: %s for strings and %d for integers.

Practice

1. Which type of memory in a microcontroller is used to store the program code permanently?

easy

A. SRAM

B. Flash memory

C. EEPROM

D. Cache memory

Solution

1. Step 1: Understand memory types

   Flash memory is non-volatile and stores the program code permanently.

2. Step 2: Compare with other memories

   SRAM is temporary for variables, EEPROM stores small data but not the main program.

3. Final Answer:

   Flash memory -> Option B

4. Quick Check:

   Program code = Flash memory [OK]

Hint: Program code is always in Flash memory [OK]

Common Mistakes:

• Confusing SRAM with Flash memory
• Thinking EEPROM stores program code
• Assuming cache memory is used in microcontrollers

2. Which of the following declarations correctly places a variable in EEPROM memory in embedded C?

easy

A. int var __attribute__((section(".eeprom")));

B. int var __attribute__((section(".sram")));

C. int var __attribute__((section(".flash")));

D. int var __attribute__((section(".stack")));

Solution

1. Step 1: Identify EEPROM section syntax

   Variables placed in EEPROM use the section attribute ".eeprom" in embedded C.

2. Step 2: Check other options

   ".sram" is for SRAM, ".flash" for Flash, ".stack" is not a valid section for variables.

3. Final Answer:

   int var __attribute__((section(".eeprom"))); -> Option A

4. Quick Check:

   EEPROM section attribute = .eeprom [OK]

Hint: EEPROM variables use section ".eeprom" attribute [OK]

Common Mistakes:

• Using .sram or .flash instead of .eeprom
• Forgetting __attribute__ syntax
• Confusing stack with memory sections

3. Consider this embedded C code snippet:

const int x = 10;
int y = 20;
int main() {
  static int z = 30;
  return 0;
}

Where are the variables x, y, and z stored respectively?

medium

A. x in SRAM, y in EEPROM, z in Flash

B. x in EEPROM, y in Flash, z in EEPROM

C. x in Flash, y in EEPROM, z in SRAM

D. x in Flash, y in SRAM, z in SRAM

Solution

1. Step 1: Analyze variable storage rules

   Constants like x are stored in Flash memory. Global variables like y go to SRAM. Static variables like z also reside in SRAM during runtime.

2. Step 2: Match variables to memory types

   x = Flash, y = SRAM, z = SRAM.

3. Final Answer:

   x in Flash, y in SRAM, z in SRAM -> Option D

4. Quick Check:

   const = Flash, global/static = SRAM [OK]

Hint: const vars in Flash, globals and statics in SRAM [OK]

Common Mistakes:

• Assuming static variables go to EEPROM
• Thinking const variables are in SRAM
• Mixing EEPROM with Flash for constants

4. This embedded C code tries to store a variable in EEPROM:

int var __attribute__((section(".eeprom"))) = 5;
void main() {
  var = 10;
}

What is the main problem with this code?

medium

A. You cannot assign a new value to EEPROM variable at runtime like this

B. The section attribute syntax is incorrect

C. Variables in EEPROM must be declared as const

D. The main function must return int, not void

Solution

1. Step 1: Understand EEPROM write limitations

   EEPROM variables cannot be assigned new values by simple assignment at runtime; special functions are needed to write EEPROM.

2. Step 2: Check other code aspects

   The section attribute syntax is correct, variables need not be const, and main can be void in embedded C.

3. Final Answer:

   You cannot assign a new value to EEPROM variable at runtime like this -> Option A

4. Quick Check:

   EEPROM write needs special functions [OK]

Hint: EEPROM writes need special functions, not simple assignment [OK]

Common Mistakes:

• Assuming normal assignment writes EEPROM
• Thinking section attribute is wrong
• Confusing main return type with memory issues

5. You want to store a configuration value that must persist after power off and be modifiable during runtime on a microcontroller. Which memory type should you use and why?

hard

A. Cache memory, because it stores frequently used data

B. Flash, because it is fast and can be rewritten easily during runtime

C. EEPROM, because it retains data without power and allows runtime updates with special functions

D. SRAM, because it is volatile and fast for temporary storage

Solution

1. Step 1: Identify memory needs

   The value must persist after power off and be modifiable during runtime.

2. Step 2: Match memory type to needs

   EEPROM is non-volatile and supports runtime updates via special functions. Flash is non-volatile but rewriting is slower and more complex. SRAM is volatile and loses data on power off. Cache is not used for persistent storage.

3. Final Answer:

   EEPROM, because it retains data without power and allows runtime updates with special functions -> Option C

4. Quick Check:

   Persistent modifiable data = EEPROM [OK]

Hint: Use EEPROM for persistent, modifiable data [OK]

Common Mistakes:

• Choosing Flash for frequent runtime updates
• Using SRAM for persistent storage
• Confusing cache with persistent memory