System memory in a microcontroller, based on the provided reference, is a specific type of read-only memory (ROM). It serves a critical function by storing essential, non-volatile data that the microcontroller needs to operate correctly from the moment it powers on.
Key Characteristics of System Memory
Here's a breakdown of what system memory is and does:
- Read-Only (ROM): As the name implies and the reference states, system memory is read-only (ROM). This means the data stored within it cannot be easily altered or erased by the user program during normal operation.
- Essential Content: It contains crucial information like the default bootloader and calibration values.
- The bootloader is a small program that runs when the microcontroller starts up. Its primary role is often to initialize the system and load the main user program from another memory area (like FLASH) into RAM for execution, or to facilitate programming the device via communication interfaces (like UART, USB, etc.).
- Calibration values are data points used by internal peripherals (like analog-to-digital converters, oscillators, etc.) to ensure they function accurately. These values are typically factory-programmed.
System Memory vs. FLASH Memory
It's important to distinguish system memory from other types of memory in a microcontroller, particularly FLASH memory:
- FLASH Memory: This is where you typically store code (your program) and often some user data. Unlike system memory (ROM), FLASH is electrically erasable and programmable, allowing you to update your microcontroller's firmware.
- Purpose Distinction:
- System Memory: Stores system-level, fixed code (bootloader) and data (calibration values) required for basic operation and programming.
- FLASH Memory: Stores the user application code and potentially user data.
- Execution Considerations (as per reference): While you can store user data in FLASH, the reference notes a practical limitation: unless you have one of the dual-bank STM32 models, program execution from FLASH waits while any other part of FLASH is being erased/programmed. This highlights the distinct roles and access characteristics compared to the always-available system memory.
Here's a simple comparison:
| Feature | System Memory (ROM) | FLASH Memory |
|---|---|---|
| Type | Read-Only (ROM) | Electrically Erasable/Programmable |
| Primary Role | Bootloader, Calibration Values | User Code (Firmware), User Data |
| Modifiability | Not easily user-modifiable | Erasable & Programmable |
| Typical Size | Relatively small | Larger (kilobytes to megabytes) |
In essence, system memory provides the foundational layer for the microcontroller to start up and function, containing the built-in mechanisms for initialization and programming, while FLASH memory holds the application logic you develop.
