ARM Cortex-M1 Initial SP and Reset Vector Configuration Issues

ARM Cortex-M1 Stack Pointer and Reset Vector Misconfiguration

The ARM Cortex-M1 microcontroller is designed to execute code from a specific memory region, with the stack pointer (SP) and reset vector configured to point to valid memory addresses during startup. In this scenario, the program code is intended to execute from an external memory region starting at address 0xF000 with a size of 0x8000, while the data region is allocated at 0x20100000 with a size of 0x8000. However, the program fails to execute correctly during simulation, indicating a potential misconfiguration in the initial stack pointer (SP) value or the reset vector.

The Cortex-M1 architecture requires the initial SP value and the reset vector to be correctly set in the vector table, which is typically located at the beginning of the memory map. The vector table’s first entry is the initial SP value, and the second entry is the reset vector, which points to the reset handler. If these values are not correctly configured, the processor will fail to initialize properly, leading to a system hang or undefined behavior.

The scatter file provided in the discussion maps the load region (LR_IROM1) to 0x000F0000 with a size of 0x8000, and the execution region (ER_IROM1) to the same address. The RW data region (RW_IRAM1) is mapped to 0x20100000 with a size of 0x8000. The startup file defines the stack and heap sizes and initializes the vector table. However, the issue lies in the alignment and configuration of the initial SP value and the reset vector, which are critical for the Cortex-M1’s boot process.

Memory Alignment and Vector Table Configuration Errors

One of the primary causes of the issue is the misalignment or incorrect configuration of the initial SP value and the reset vector in the vector table. The Cortex-M1 expects the initial SP value to be set to the top of the stack memory region, and the reset vector to point to the reset handler function. If these values are not correctly set, the processor will not be able to initialize the stack or jump to the reset handler, resulting in a system hang.

The scatter file defines the load and execution regions, but it does not explicitly ensure that the initial SP value and the reset vector are correctly aligned and configured. The startup file defines the stack and heap sizes and initializes the vector table, but it does not guarantee that the initial SP value is set to the correct address. The stack memory region is defined in the startup file, but the initial SP value must be explicitly set to the top of this region.

Another potential cause is the use of external memory for code execution. The Cortex-M1 may have specific requirements for accessing external memory, such as wait states or specific initialization sequences. If these requirements are not met, the processor may fail to fetch the initial SP value or the reset vector from external memory, leading to a system hang.

Additionally, the scatter file and startup file must be consistent in their memory region definitions. If the scatter file defines a different memory region for the stack or heap than the startup file, the initial SP value or the reset vector may point to an invalid memory address, causing the system to hang.

Correcting Initial SP and Reset Vector Configuration

To resolve the issue, the initial SP value and the reset vector must be correctly configured in the vector table, and the memory regions must be properly aligned and consistent between the scatter file and the startup file. The following steps outline the necessary corrections:

1. Ensure Correct Initial SP Value in Vector Table: The initial SP value must be set to the top of the stack memory region. In the startup file, the stack memory region is defined as follows:

   Stack_Size EQU 0x00000400
   AREA STACK, NOINIT, READWRITE, ALIGN=3
   __stack_limit
   Stack_Mem SPACE Stack_Size
   __initial_sp
   

   The __initial_sp symbol must be set to the top of the stack memory region. This can be achieved by ensuring that the __initial_sp symbol is correctly defined and aligned in the vector table:

   __Vectors DCD __initial_sp ; Top of Stack
   

2. Verify Reset Vector Points to Reset Handler: The reset vector must point to the reset handler function. In the startup file, the reset handler is defined as follows:

   Reset_Handler PROC
   EXPORT Reset_Handler [WEAK]
   IMPORT SystemInit
   IMPORT __main
   LDR R0, =SystemInit
   BLX R0
   LDR R0, =__main
   BX R0
   ENDP
   

   The reset vector in the vector table must point to the Reset_Handler function:

   __Vectors DCD __initial_sp ; Top of Stack
   DCD Reset_Handler ; Reset Handler
   

3. Align Memory Regions in Scatter File and Startup File: The scatter file and startup file must be consistent in their memory region definitions. The scatter file defines the load and execution regions as follows:

   LR_IROM1 0x000F0000 0x00008000 {
       ER_IROM1 0x000F0000 0x00008000 {
           *.o (RESET, +First)
           *(InRoot$$Sections)
           .ANY (+RO)
           .ANY (+XO)
       }
       RW_IRAM1 0x20100000 0x00008000 {
           .ANY (+RW +ZI)
       }
   }
   

   The startup file must define the stack and heap memory regions to match the scatter file:

   Stack_Size EQU 0x00000400
   AREA STACK, NOINIT, READWRITE, ALIGN=3
   __stack_limit
   Stack_Mem SPACE Stack_Size
   __initial_sp
   
   Heap_Size EQU 0x00000400
   IF Heap_Size != 0
   AREA HEAP, NOINIT, READWRITE, ALIGN=3
   __heap_base
   Heap_Mem SPACE Heap_Size
   __heap_limit
   ENDIF
   

4. Configure External Memory Access: If external memory is used for code execution, ensure that the Cortex-M1’s memory interface is correctly configured to access the external memory. This may involve setting up wait states, configuring the memory controller, or performing specific initialization sequences. Refer to the Cortex-M1 technical reference manual for detailed instructions on configuring external memory access.

5. Verify Vector Table Placement: The vector table must be placed at the correct memory address, typically at the beginning of the memory map. The scatter file ensures that the vector table is placed at the correct address by using the RESET section:

   LR_IROM1 0x000F0000 0x00008000 {
       ER_IROM1 0x000F0000 0x00008000 {
           *.o (RESET, +First)
           *(InRoot$$Sections)
           .ANY (+RO)
           .ANY (+XO)
       }
   }
   

   The startup file must ensure that the vector table is correctly placed and aligned:

   AREA RESET, DATA, READONLY
   EXPORT __Vectors
   EXPORT __Vectors_End
   EXPORT __Vectors_Size
   __Vectors DCD __initial_sp ; Top of Stack
   DCD Reset_Handler ; Reset Handler
   ; Additional vector table entries
   __Vectors_End
   __Vectors_Size EQU __Vectors_End - __Vectors
   

By following these steps, the initial SP value and the reset vector will be correctly configured, ensuring that the Cortex-M1 microcontroller can properly initialize and execute code from the specified memory regions. This will resolve the system hang issue and allow the program to execute as intended.