The ARM Cortex M4 is a 32-bit core integrated in several microcontrollers from various manufacturers. It is based on the ARMv7-M architecture and includes features like the Nested Vectored Interrupt Controller, optional Memory Protection Unit, and DSP extensions. Understanding the boot process for Cortex M4 devices is important for developers working on these microcontrollers.
At a high level, the Cortex M4 boot process consists of the following key stages:
- Power on Reset
- Configuration of Clock and Memory
- Initialization of RAM
- Copying code from ROM to RAM
- Branching to application code in RAM
Let’s look at each of these stages in more detail:
1. Power on Reset
When power is first applied to the microcontroller, the Power on Reset circuitry detects the power ramp up and holds the device in reset mode. This ensures proper operation as the voltage reaches operational levels. The reset is held active for a minimum period (specified in datasheet) to allow voltages and clocks to stabilize.
During this reset period, the processor core is held in inactive state and all peripherals are reset. The reset also initializes some key processor registers like the stack pointer. The values loaded during reset are device specific but generally configure a basic state for further initialization.
2. Configuration of Clocks and Memory
After the Power on Reset completes, the boot sequence jumps to a defined reset vector. This is usually location 0x00000000 in code memory (flash). The code at the reset vector then configures the system clocks, PLLs, bus frequencies and sets up external memory interface.
Key tasks during clock configuration include:
- Enabling the high speed external oscillator
- Configuring the PLL module
- Setting up flash wait states based on clocks
- Enabling systick timer
For memory configuration, the code typically:
- Configures external SRAM or SDRAM controllers
- Sets up memory remapping or MPU regions if needed
- Enables caching or buffers if supported
The details of clock and memory setup are device specific based on the microcontroller. But in general, the goal is to enable fast external memory and clock speeds for maximum performance.
3. Initialization of RAM
With the clocks and memory configured, the processor stacks and heap areas in RAM need to be initialized. The ARM Cortex M4 contains Main Stack Pointer (MSP) and Process Stack Pointer (PSP) which point to memory regions for the stack.
A simple RAM initialization will involve:
- Clear .bss section to zero initialize variables
- Setup MSP and PSP stack pointers
- Initialize heap region for dynamic memory allocation
An MPU or MMU may also need to be configured at this stage to set up address regions and access permissions in RAM. The details are device specific.
4. Copying Code from ROM to RAM
Most microcontrollers execute code from relatively slower flash memory. For better performance, a common technique is to copy some or all code from flash to faster RAM. The Cortex M4 reset sequence will include code to perform this copy.
A simple approach is to copy the .text and .data sections from flash to RAM. Some microcontrollers may support more advanced options like executing code directly from RAM. After copying to RAM, the application code branches to the entry point in RAM.
Important considerations for code copy include:
- Handling interrupts during copy
- Data and code sizes to avoid overwriting
- Code relocation if addresses change
Optimized copy routines utilize DMA transfers for fastest speed. The code copy is usually done before jumping to main application.
5. Branching to Application Code
After the initialization steps, the boot code finally passes control to the user application. This is done by branching to the pre-defined entry point or main() function. The application code then kicks off and completes the Cortex M4 boot sequence.
Some key points about application execution:
- Code now runs from initialized RAM region
- Stacks, heap have been setup
- Interrupts are still disabled at this stage
- Application enables global interrupts when ready
The application completes the boot process by calling various peripheral drivers, middleware, RTOS hooks. With that, the Cortex M4 MCU is operational and boot sequence is complete.
In summary, the ARM Cortex M4 boot involves critical steps to correctly initialize the processor, clocks, memory regions and stacks before branching to firmware. Understanding this flow is useful for developers working on the configuration and applications.
The boot process ensures a reliable and optimized start up sequence. Configuring it properly sets the stage for building robust and high performance applications on the Cortex M4 platform.
Additional points to note:
- Boot process is mostly implemented in C and Assembly
- Can leverage SVC or PendSV for flexible boot steps
- ROM libraries provide common initialization functions
- Boot time is usually a few milliseconds
- Peripherals are enabled after boot by firmware
There are many chip level optimizations possible in the boot firmware, memory layouts and clock configurations. The overall flow remains similar. Understanding the key stages helps developers customize, debug and enhance their Cortex M4 boot sequence.
References:
- ARMv7-M Architecture Reference Manual
- Cortex-M4 Technical Reference Manual
- AN4879 – Code Optimization Techniques for ARM Cortex M4
- ARM Cortex M-Series Boot Sequences
The ARM Cortex M4 boot process sets up the optimal environment for executing high performance embedded applications. Tracing through the boot stages provides valuable insights for developers working with M4 microcontrollers.
