Detailed Overview of Memory in Embedded Systems

Memory in Embedded Systems

Memory is a critical component in embedded systems, as it stores both the programs that drive the system’s behavior and the data that the system processes. In embedded systems, memory can be classified into two primary types: volatile memory and non-volatile memory. Each plays a distinct role, and both are essential for the proper functioning of any embedded system.

In this article, I will dive deeply into volatile and non-volatile memory, discussing their roles, types, applications, and practical usage in embedded systems.

Volatile Memory (RAM)

Volatile memory refers to memory that loses its stored information once power is removed. The most common type of volatile memory in embedded systems is Random Access Memory (RAM). RAM is used for temporary data storage during program execution, acting as a workspace for the CPU to store variables, stacks, and intermediate computations.

Types of Volatile Memory:

Static RAM (SRAM): SRAM stores data in flip-flops and doesn’t require refreshing. It’s fast but more expensive and used in small memory caches.
Dynamic RAM (DRAM): DRAM stores data in capacitors and needs periodic refreshing. It’s slower but cheaper and more compact than SRAM, making it ideal for larger memory needs.

Practical Example: Using RAM in Embedded Systems

In an embedded system like an STM32 microcontroller, RAM is used to store temporary data such as sensor readings or variable calculations during program execution. For instance:

int temperature_buffer[100]; // Allocates space in RAM for 100 temperature readings

This memory is wiped when the system loses power, making it suitable for short-term data storage but not for permanent data retention.

Key Functions of RAM in Embedded Systems:

Temporary data storage: RAM is used to hold temporary data that is created and destroyed during program execution.
Variable storage: Any variable or buffer that changes frequently during program execution resides in RAM.
Stack and heap: RAM stores the stack, which holds function call information, and the heap, which is used for dynamic memory allocation.

Non-Volatile Memory (Flash/EEPROM)

Non-volatile memory retains its data even when power is removed, making it ideal for storing programs, configuration settings, and any data that must survive power cycles.

Types of Non-Volatile Memory:

Flash Memory: Flash memory is used for firmware storage and larger blocks of data. It’s slower to write to but offers higher storage capacity. Commonly used for firmware updates and code storage in embedded systems.
EEPROM: EEPROM allows byte-level writing and is often used to store small configuration settings or calibration data. It has a limited number of write cycles, making it suitable for data that’s updated infrequently.

Practical Example: Using Flash in Embedded Systems

In an STM32 microcontroller, flash memory stores the firmware code that is executed when the microcontroller is powered on. For example, the program that reads data from sensors and displays it on a screen would be stored in the flash memory. Whenever the system is turned on, the code is retrieved from flash and executed.

EEPROM is commonly used to store user settings, such as the preferred temperature in a thermostat. Even after a power cycle, the setting is retained as it is stored in non-volatile EEPROM:

EEPROM.write(0x00, preferred_temperature);  // Store preferred temperature in EEPROM

Flash vs. EEPROM:

Flash: Used for large storage tasks, such as storing entire programs or large blocks of data, but requires erasing large sectors before writing.
EEPROM: Allows for more granular byte-wise access, making it ideal for saving settings or configuration parameters, though it has a limited number of write cycles.

Comparison of Volatile and Non-Volatile Memory

Type	Function	Advantages	Disadvantages
SRAM	Stores temporary data, very fast	Fast access, no refreshing required	More expensive, less storage density
DRAM	Main memory storage for larger systems	High density, cheaper	Slower, requires refreshing
Flash Memory	Stores firmware, bootloaders, large blocks of data	Non-volatile, high capacity	Slower write speeds, needs block erasure
EEPROM	Stores user settings, small configurations	Non-volatile, byte-level access	Limited write cycles, slower than RAM

Applications in Embedded Systems

Both volatile and non-volatile memory play essential roles in embedded systems, with practical applications including:

Firmware Updates: Flash memory is often used for over-the-air (OTA) firmware updates in IoT devices. The new firmware is downloaded and written to flash memory, which is then executed on the next reboot.
Data Logging Systems: Non-volatile memory is vital in data logging applications, where sensor data must be stored persistently. For example, a temperature monitoring system may log readings in flash memory and periodically transmit the data to a server.
User Configuration: EEPROM is commonly used to store user preferences, such as volume levels, screen brightness, or custom settings. These values are retained even if the device is powered off.
Real-World Example: In a washing machine controller, flash memory stores the control program (for washing modes), while EEPROM stores user settings (e.g., preferred water temperature), both of which are retained across power cycles.