What is the final step in the Cortex-M4 processor reset sequence?

The Cortex-M4 processor from ARM is a widely used 32-bit RISC CPU core designed for embedded applications. It features the ARMv7E-M architecture and includes features like digital signal processing (DSP) instructions, single cycle multiply, low power consumption, and memory protection unit (MPU). Understanding the reset sequence of the Cortex-M4 is important for developers working with this processor.

The final step in the Cortex-M4 reset sequence is exiting the reset handler and jumping to the application code entry point. Here is a breakdown of the full reset sequence:

Power Up

When power is first applied to the Cortex-M4 processor, the system logic automatically holds the processor in a reset state. This gives time for the power supply and clocks to stabilize before execution begins.

Boot Code

After the power supplies have stabilized, the reset logic releases the Cortex-M4 reset signal, allowing boot code execution to begin. This boot code is stored in on-chip ROM or flash memory and initializes important system components like clocks, memory controllers, and IO peripherals.

Vector Table

The boot code sets up the vector table, which contains the memory addresses for key exception and interrupt handlers. The first entry in the vector table is the initial stack pointer value, and the second entry is the reset handler address.

Reset Handler

After the vector table setup, the processor loads the initial stack pointer and executes the reset handler code. The reset handler has these key tasks:

• Initialize core registers and status flags
• Set up memory regions for code and data
• Clear or initialize RAM contents
• Initialize the system tick or real-time clock
• Initialize debug system
• Copy initialized variables from ROM to RAM

Application Entry Point

The final step in the reset sequence is when the reset handler executes a branch instruction to jump to the application code entry point. This entry point is often the main() function in embedded C programs. Execution of the user application now begins.

Reset Sequence Summary

In summary, the complete Cortex-M4 reset sequence consists of:

1. Processor held in reset state during power on
2. Boot code execution after reset release
3. Vector table setup
4. Reset handler execution
5. Jump to application code entry point

The final step of jumping to the application entry point hands over control from the system initialization code to the user’s main program.

Reset Configuration Options

There are some configuration options that affect the Cortex-M4 reset behavior:

• Stack pointer initialization – The reset value of the stack pointer can be set to different memory regions as needed.
• Reset handler location – The reset handler can be configured to execute from flash or internal RAM.
• Boot code flexibility – Some Cortex-M4 microcontrollers allow custom boot code to be used instead of built-in boot ROM.

Reset Handling Tips

Here are some tips for working with Cortex-M4 reset handling:

• Initialize RAM contents to a known state to avoid random data on startup.
• Handle clock and peripheral initialization early in the reset sequence.
• Use the reset handler to configure memory protections and privilege levels.
• Keep reset handler execution time short for quick wakeup from sleep modes.
• Verify reset behavior with debugger breakpoints at key points.

Reset Troubleshooting

Some common reset handling issues include:

• Processor does not come out of reset – Check power supply voltages and reset signal assertion.
• Crashes or lockup after reset – Memory/clock initializations may be incorrect in boot code.
• Stack randomly corrupted – Initialize stack pointer to valid memory region.
• Wrong code executed – Incorrect vector table entries or reset handler branch.

Confirming the proper reset sequence on prototype hardware is an important step during Cortex-M4 system development.

Advanced Reset Options

In addition to the standard power-on reset sequence, the Cortex-M4 supports other advanced reset capabilities:

• Wakeup from sleep modes – Cortex-M4 sleep modes allow fast wakeup and restart.
• Watchdog reset – The watchdog timer can reset the system if not periodically refreshed.
• Software system reset – The Application Interrupt and Reset Control Register (AIRCR) can initiate a soft reset.
• Debug reset – Debuggers can reset the core and execute new code without full system reset.

These reset options provide flexibility for reset handling in complex, real-world Cortex-M4 applications.

Related Resources

For more information on working with Cortex-M4 reset handling, refer to these ARM resources:

• Cortex-M4 Devices Generic User Guide – Covers reset modes, boot sequence, and debug reset.
• Cortex-M4 Technical Reference Manual – Provides detailed reset handler description.
• AN311 – Cortex-M4 Boot Sequence – Example code for reset handler and bootloader.

Properly configuring reset handling lays the foundation for a stable and robust Cortex-M4 system. Following ARM’s application notes and reference guides will give developers the key details needed for the Cortex-M4 processor reset sequence.