Where is the Utable Code Stored in a Microcontroller: A Comprehensive Guide

Understand the storage locations of the utable code in a microcontroller and the different types of non-volatile memory used. This article provides a detailed overview of where the utable code is stored, the common types of memory used, and the exceptions to this rule.

Introduction to Microcontroller Utable Code Storage

In a microcontroller, the utable code is typically stored in non-volatile memory, which retains data even when the power is turned off. This information is crucial for ensuring the microcontroller can operate correctly, whether for a short burst or continuously. The major types of non-volatile memory used include Flash Memory, ROM, EEPROM, and Boot ROM. Below, we will explore each type in depth, highlighting their advantages and limitations.

Types of Non-Volatile Memory for Microcontroller Storage

Flash Memory

Flash Memory is the most common type of non-volatile memory used in modern microcontrollers. It is favored for its ability to retain data without power and to be electrically erased and reprogrammed. This characteristic makes it ideal for storing firmware and utable code. Modern microcontrollers often use Flash Memory because it supports high volume writes and has relatively lower write cycle limitations compared to other non-volatile memory types.

ROM Read-Only Memory

older microcontrollers may rely on ROM (Read-Only Memory) to store the utable code. Rom is not reprogrammable, meaning the code must be written during the manufacturing process. While less flexible, ROM offers high stability and reliability, making it a reliable choice for critical applications where the code does not need to be updated post-deployment.

EEPROM Electrically Erasable Programmable Read-Only Memory

Some microcontrollers use EEPROM (Electrically Erasable Programmable Read-Only Memory) for storing smaller amounts of code or configuration data. EEPROM can be erased and reprogrammed, but it typically has a limited number of write cycles compared to Flash Memory. This makes it less suitable for large amounts of data but ideal for small, frequently updated segments of code.

Boot ROM

Boot ROM, a small piece of dedicated ROM, contains a bootloader program. This program assists in loading the primary firmware from other types of memory. Boot ROM is particularly useful for systems where the primary firmware might need to be updated periodically, providing a fail-safe method to load the latest version even if the main memory is compromised.

Special Cases: RP2040 and Other Exceptions

While most microcontrollers typically store their utable code internally, the RP2040 is an exception. It stores the utable code externally, meaning the program is not found on the chip itself. Instead, the RP2040 relies on an external memory source for its utable code. In contrast, a typical microcontroller (MCU) stores the utable code and related data within the chip itself, which is the nature of MCUs.

Median Implementation of Memory in a Microcontroller

From a median implementation perspective, non-volatile memory is often flash memory. Additionally, RAM can be flash memory but is more commonly SRAM (Static Random-Access Memory) or SDRAM (Synchronous Dynamic Random-Access Memory). FRAM (Ferroelectric Random-Access Memory) is rarely used and is usually external.

Most microcontrollers have an embedded bootloader provided by the manufacturer. This ensures that the code can be updated and modified as needed, maintaining the functionality and flexibility of the system.

Utable Code Execution in a Microcontroller

Typically, the utable code is stored contiguously starting at the address 00 of the flash memory. The process of turning C/C or assembly language code into CPU instructions and hex codes occurs through a combination of the compiler, assembler, linker, and the build system. The build system assembles the code, creates the object files, and ultimately produces the hex code that the CPU can understand.

It is important to note that while main() is a function, it may not be the entry point of the microcontroller. The build system usually includes a startup code at address 00 which initializes the stack and other necessary components before transferring control to the main() function, located elsewhere in the flash memory.

Conclusion

The utable code in a microcontroller is primarily stored in flash memory, but other forms of non-volatile memory like ROM and EEPROM may be used depending on the specific architecture and application requirements. The RP2040 is an exception, storing its utable code externally. Understanding the differences and choices between these memory types can greatly enhance the design and functionality of microcontroller-based systems.