ARM Cortex-M7 Boot Sequence

The ARM Cortex-M7 is a high-performance processor core designed for advanced embedded applications. It features a 3-stage pipeline, floating point unit, and optional MMU for efficient execution of complex tasks. The boot process of the Cortex-M7 initializes critical hardware components and transfers execution to the application code.

Reset Sequence

When power is first applied to a Cortex-M7 system, the processor undergoes a reset sequence to initialize internal registers and prepare for boot. This involves the following steps:

1. The reset signal is asserted, halting execution and resetting pipeline registers.
2. The voltage regulator and oscillator are enabled to power up the system and generate a clock signal.
3. The processor initializes internal registers like the stack pointer and exception vectors.
4. The reset signal is deasserted, allowing the boot process to proceed.

Boot ROM

After reset, the Cortex-M7 fetches the initial instructions from a built-in boot ROM. The key functions of the boot ROM include:

• Initializing main system components like the clock, memory controllers, and I/O peripherals.
• Copying code and data sections from non-volatile flash memory to volatile RAM.
• Setting up the stack and heap memory areas.
• Enabling the floating point unit and MMU if present.
• Transitioning to execution of code from the application flash area.

System Initialization

The system initialization code executed out of the boot ROM sets up critical hardware for operation. Key steps include:

• Clock Initialization: The system clock source is configured and PLLs are programmed to generate required core, memory, and peripheral clocks.
• Memory Initialization: The flash memory controller is configured and any on-chip SRAM banks are set up.
• Peripheral Initialization: Common peripherals like timers, watchdog, interrupts are initialized based on system requirements.
• Stack Setup: The MSP stack pointer register is initialized to point to the end of the RAM area.
• FPU Initialization: If an FPU is present, it is enabled and stack memory allocated.
• MMU Initialization: The optional MMU is enabled and page tables set up if present.

Data Relocation

Since internal RAM is limited, the boot process must copy initialized data from flash to RAM:

1. The .data section is copied from flash to the RAM data area.
2. Values in the .bss section are cleared in RAM to zero.
3. The data relocation process uses memory mapping and cache management units to maximize efficiency.

Platform Initialization

The boot ROM will call a platform initialization function provided by the application code. This enables boards and SoCs to configure specific peripherals and options like:

• Board-level multiplexing and GPIO assignments.
• Drivers for external memories and devices.
• Security configurations including firewalls and cryptography.
• Advanced performance enhancements and power optimizations.

Startup Code Execution

After hardware initialization, the boot process jumps to executable application code:

1. The vector table provides the reset handler address.
2. The reset handler initializes the C runtime environment.
3. Register and stack values are set up as defined in ABI conventions.
4. The main() function of the application is called.

Execution now transitions from boot firmware to the runtime OS and main application.

C Runtime Initialization

The C runtime contains startup code that executes prior to main() to complete the boot process. It performs tasks like:

• Initialize heap memory for dynamic allocation needs.
• Initialize C standard libraries used by the application.
• Copy initialized C variables from flash to RAM.
• Clear uninitialized C variables.
• Set up argc and argv constructs.
• Call constructors for global C++ objects.

Operating System Initialization

Embedded Cortex-M7 systems often run a real-time operating system (RTOS) to manage concurrent tasks. The OS initialization at boot may involve:

• Creating and scheduling key system tasks/threads.
• Setting up inter-process communication mechanisms like queues.
• Initializing peripherals, device drivers and file systems.
• Allocating resources between applications.

This completes the multi-stage boot process for Cortex-M7 systems, loading the application program and runtime environment needed for functional operation.

Boot Customization

The standard Cortex-M7 boot sequence can be customized by embedded developers to suit specific needs:

• Override default configurations via engineering boot modes.
• Add board-specific hardware tests and diagnostics.
• Modify clock frequencies, memory maps and stack/heap sizes.
• Integrate security measures like encrypted bootloaders.
• Load different OS or runtime environments like FreeRTOS.
• Support remote firmware updates.

Boot Time Optimization

Various techniques can optimize the Cortex-M7 boot time to minimize delays:

• Streamline clock initialization code to quickly enable key clocks.
• Use memory mapping units to hide flash latency.
• Utilize caches to speed up data transfer.
• Initialize only required hardware during boot.
• Create efficient platform initialization code.
• Use optimized C libraries and minimal runtime.
• Choose a lightweight OS and efficient drivers.

With careful optimization, the Cortex-M7 boot time can be reduced to just hundreds of microseconds, enabling rapid startup times for embedded systems.

Boot Errors

Common boot errors on Cortex-M7 systems include:

• Invalid reset vector – Processor reset fails.
• Faulty system clock configuration – Crashes or hangs.
• Corrupted boot ROM – Processor faults early.
• Invalid memory initialization – Data corruption.
• Failed data relocation – Code crashes during execute.
• Platform init bugs – System fails to initialize.
• Stack overflow – Hard faults or hangs.
• Heap corruption – Memory overwrites.

These errors can be debugged via JTAG/SWD tools, counters, logs and debug breakpoints. Memory protection units and other hardware features can also detect invalid accesses and faults during boot.

Summary

The ARM Cortex-M7 undergoes a complex multi-stage boot process to transition from reset to application execution. The hardware initialization, data relocation, C runtime setup, and OS bootstrap are key phases in this sequence. Customization for specific platforms is enabled while keeping critical low-level tasks consistent. With optimizations, the boot time can be reduced substantially to support real-time embedded use cases.