The ARM Cortex-M0 is a 32-bit RISC processor core designed for microcontroller applications. It is one of the simplest and most energy-efficient processors in the Cortex-M family. Understanding the boot sequence of Cortex-M0 microcontrollers is important for developers working on embedded and IoT projects using these devices.
On reset or power-on, the Cortex-M0 microcontroller begins executing boot code located in internal ROM or flash memory. This boot code is responsible for initializing the processor and peripherals to get the system into an operational state. The key steps in the Cortex-M0 boot sequence are:
1. Processor Mode and Stack Initialization
After reset, the processor starts executing code in Privileged Thread mode with main stack pointer initialized. The standard boot code provided by ARM initializes the stack pointer for the current execution mode.
2. Copy Initialization Code to RAM
The initial boot code is executed from non-volatile flash memory. For better performance, this code is copied over to faster RAM. The size of initialization code to be copied depends on the specific microcontroller and compiler used.
3. Setup System Clock
The system clock source (internal RC oscillator or external crystal) is configured along with PLL and dividers to generate the desired system clock frequency. Clock setup involves configuring the Cortex-M0 SysTick timer for basic timing and RTOS ticks.
4. Initialize Memory Subsystem
This step sets up various memory/cache related controller registers as required for correct functioning of the memory subsystem. Instruction and data caches are enabled if present. MPU regions, bus interfaces like AHB/APB bridges are also initialized.
5. Configure Microcontroller Peripherals
Device specific peripherals like GPIO pins, timers, communication interfaces, analog modules, etc are initialized to put them in known disabled or low power state. Default configurations are setup for various power and clock domains.
6. Zero Out BSS Segment
Uninitialized global and static variables located in BSS segment are cleared to zero. This step ensures these variables start with expected initial values before main() is called.
7. Call System Initialization Functions
Low-level processor specific system initialization functions are called to enable FPU if present, perform stack limit checking and other system checks. This also triggers calling constructors for global C++ objects.
8. Call main()
With all initialization complete, control is passed to main() application entry point. The application firmware takes over after this point to run as per its logic.
9. Handle Exceptions and Interrupts
Throughout the boot process and during application execution, the processor continuously monitors and responds to exceptions and interrupts. The NVIC handles enabling, prioritization and exception/interrupt dispatching.
That covers the typical steps in the start-up sequence for an ARM Cortex-M0 based microcontroller. There are additional considerations like bootloader support, image validation, fault handling, debug access and wake-up from low power modes that extend this basic flow. Vendor specific customizations and optimizations are also common in real-world implementations.
Understanding the purpose behind each phase of the boot flow allows developers to properly initialize the target device, peripherals and application firmware. This is crucial for creating robust and efficient embedded and IoT products with Cortex-M0 microcontrollers. Modifying the default boot sequence or injecting custom boot code requires intimate knowledge of the startup behavior.
Key Components of Cortex-M0 Boot Process
There are several key hardware and software components that come together to facilitate booting up a Cortex-M0 system successfully:
Boot ROM
An on-chip boot ROM stores the initial boot code executed after reset. This boot ROM is mapped at the start of the memory map.
Vector Table
This data structure located at the memory address 0x00000000 contains the reset value for stack pointer and exceptions/interrupt handler vectors.
Startup Code
Startup code or bootloader is responsible for initializing the device and preparing it to execute the user application. It is stored in flash memory.
Linker Script
The linker script defines the memory map and address spaces. It places code and data sections like text, rodata, bss, stack and heap at appropriate locations.
Reset Handler
The reset handler function defined in startup code is the first code executed on microcontroller reset. It sets up stacks, initializes hardware, copies data sections to RAM before calling main().
Device Configuration
The microcontroller device header files contain processor specific configurations for clocks, memory, peripherals, interrupts and other resources.
Typical Steps in Cortex-M0 Reset Handler
The reset handler is the first piece of code that runs on a Cortex-M0 device after coming out of reset. It is responsible for the initial hardware setup:
- Configure processor stack pointer
- Initialize .data and .bss sections
- Setup system clock source and PLL
- Configure NVIC and exceptions
- Initialize static peripherals
- Call SystemInit() and software initialization
- Branch to main() application entry point
Simplifying Cortex-M0 Boot Process
The Cortex-M0 boot sequence can be simplified for applications with minimal hardware and software initialization needs. Steps like memory copy and BSS zeroing can be avoided to reduce startup time and code size overhead. The key aspects to retain are:
- Configure clocks and power for necessary peripherals only
- Initialize critical peripherals like GPIO, watchdog, etc
- Setup stack pointer and vector table
- Branch to main() or bootloader code quickly
Startup code should be optimized for the target application requirements. Unused hardware need not be configured saving boot time and memory.
Role of Toolchain in Cortex-M0 Boot Sequence
The toolchain plays an important role in shaping the Cortex-M0 boot sequence. Key functions provided by the toolchain include:
- Linker script defines memory map addresses
- Startup code handles early initializations
- Boot code libraries provided by compiler
- Vector table and interrupt handling
- Hardware abstraction libraries
- Configuration for heap/stack setup
Compiler optimizations impact code size and performance during boot. The toolchain choice affects boot time, flexibility and bootloader integration options.
Boot Time Optimization Techniques
To optimize the boot time on Cortex-M0 microcontrollers following techniques can be applied:
- Efficient exception handling and interrupt vector table
- Optimized toolchain startup code
- tuned linker scripts aligning critical code and data sections
- Boot from higher speed memory like SRAM instead of flash
- Use optimized library versions
- Inline critical functions
- Initialize only required hardware
- Configure components like debug, RTOS, etc only when needed
An optimized boot sequence ensures quick start-up, reduced power consumption and improved real-time response for embedded applications.
Conclusion
In summary, understanding the ARM Cortex-M0 boot flow is important for leveraging these microcontrollers fully in designing embedded and IoT products. The start-up sequence initializes the core, clocks, memory, peripherals and application firmware. Developers can tune the boot process for different trade-offs between performance, size and simplicity. With careful configuration and optimization, Cortex-M0 microcontrollers can deliver the desired boot behavior for time-critical embedded applications.
