Cortex-M0 Reset Sequence and Address Non-Sequential Behavior

Cortex-M0 Reset Sequence and Initial Memory Accesses

The Cortex-M0 reset sequence is a critical process that initializes the processor and sets up the environment for executing the first instructions. During this sequence, the processor fetches the initial Main Stack Pointer (MSP) value and the Program Counter (PC) value from the vector table located at the beginning of the memory map. The vector table is a predefined structure that contains the addresses of exception handlers, with the first two entries being the initial MSP and the reset handler address (PC).

In the observed scenario, the Cortex-M0 performs the following memory accesses during the reset sequence:

• The first read operation fetches the initial MSP value from address 0x00000000.
• The second read operation fetches the initial PC value from address 0x00000004.
• Subsequent read operations are performed at addresses 0x0000020C, 0x000002C8, 0x000002C4, 0x000002C8, and 0x000002CC.

The initial MSP value is 0x20000208, and the initial PC value is 0x010002C5. The PC value indicates that the processor will jump to the reset handler located at address 0x010002C5. However, the non-sequential nature of the subsequent memory accesses raises questions about the behavior of the address bus and the underlying memory system.

Non-Sequential Address Access and Memory System Behavior

The non-sequential address access pattern observed during the reset sequence can be attributed to several factors related to the memory system and the Cortex-M0’s bus interface. The Cortex-M0 uses the AHB-Lite bus protocol, which allows for non-sequential transfers. Non-sequential transfers occur when the address of the current transfer is not related to the address of the previous transfer, which is common in control flow operations such as branches and jumps.

In this case, the non-sequential address access pattern is likely due to the following reasons:

• The reset handler code may contain branches or jumps that cause the processor to fetch instructions from non-sequential addresses.
• The memory system may have specific address decoding logic that maps certain address ranges to different memory regions or peripherals, leading to non-sequential address accesses.
• The Cortex-M0’s prefetch unit may fetch instructions from non-sequential addresses to optimize performance, especially if the code contains loops or frequently executed branches.

The observed address sequence (0x0000020C, 0x000002C8, 0x000002C4, 0x000002C8, 0x000002CC) suggests that the reset handler code is executing a loop or a series of branches that cause the processor to repeatedly access specific memory locations. This behavior is consistent with the execution of a bootloader or initialization code that sets up the system environment before jumping to the main application code.

Debugging and Resolving Address and Data Mismatches

The mismatch between the observed HRDATA values and the expected values from the bootloader.hex file can be attributed to several potential issues in the memory system or the bootloader implementation. To debug and resolve these mismatches, the following steps should be taken:

1. Verify the Vector Table Configuration: Ensure that the vector table is correctly configured in the bootloader.hex file and that the initial MSP and PC values match the expected values. The vector table should be located at the beginning of the memory map, and the first two entries should contain the initial MSP and PC values.

2. Check Memory Mapping and Address Decoding: Verify that the memory system correctly maps the addresses accessed during the reset sequence to the appropriate memory regions or peripherals. Ensure that the address decoding logic is correctly implemented and that there are no conflicts or overlaps in the address space.

3. Inspect the Reset Handler Code: Disassemble the reset handler code to understand the control flow and identify any branches or jumps that may cause non-sequential address accesses. Ensure that the reset handler code correctly initializes the system and sets up the environment for executing the main application code.

4. Compare HRDATA with Memory Contents: Compare the observed HRDATA values with the contents of the memory at the corresponding addresses. Ensure that the memory contents match the expected values from the bootloader.hex file. If there are discrepancies, investigate potential issues in the memory initialization or bootloader implementation.

5. Analyze Bus Transactions: Use a bus analyzer or simulation tool to capture and analyze the bus transactions during the reset sequence. Verify that the address and data values on the bus match the expected values and that there are no errors or unexpected behavior in the bus transactions.

6. Check for Memory Initialization Issues: Ensure that the memory is correctly initialized before the reset sequence begins. If the memory is not