In microcontroller (MCU) or microprocessor security features. This typically involves using a security fuse, sometimes called a "security bit" or "lock bit," to protect the code stored in the MCU's flash memory or to control access to certain features of the MCU. Here's how it generally works: Security Fuse or Bit: Many MCUs have a security fuse or a specific bit in a configuration register that can be set to a locked state. This fuse can be programmed only once, and once set, it cannot be reset. Setting this fuse signifies that the MCU should enter a secure mode. Secure Code Storage: Once the security fuse is set, the MCU's flash memory is often divided into secure and non-secure regions. The secure region is where critical or sensitive code is stored, and the non-secure region is for regular application code. Access Control: The security fuse may also control access to certain MCU features or peripherals. In some MCUs, setting the fuse might disable external programming interfaces (like JTAG or SWD) or limit access to specific peripherals. Authentication: To run code in the secure region or access restricted features, the MCU often requires authentication. This could be through cryptographic keys, passwords, or some other mechanism. Secure Boot: In some cases, the MCU will have a secure boot process, ensuring that only authenticated code can run on the device. The secure boot process verifies the integrity and authenticity of the code before allowing it to execute. Tamper Detection: MCUs with security features may also include tamper detection mechanisms. These can trigger actions (like erasing memory or disabling the device) if tampering is detected. Secure Updates: Secure MCUs often have a mechanism for securely updating the firmware or software, ensuring that only authorized updates can be installed. The specific implementation of these security features can vary widely between different MCU manufacturers and models. Common manufacturers like Atmel/Microchip, STMicroelectronics, NXP, and others provide documentation and tools for configuring and using security features in their MCUs. The use of security fuses or bits is a critical component of securing embedded systems, as it helps protect sensitive intellectual property, prevent unauthorized access, and maintain the integrity of firmware and code. It's important to carefully read the documentation provided by the MCU manufacturer and follow best practices for implementing security in your embedded applications.