Debugging ARM Cortex-M4 DAP Access Issues to PPB Trace Components

ARM Cortex-M4 DAP Access Failures to PPB Trace Registers

When working with ARM Cortex-M4 processors, particularly in debugging scenarios, accessing the Private Peripheral Bus (PPB) region through the Debug Access Port (DAP) can present significant challenges. The PPB region houses critical debug and trace components such as the Data Watchpoint and Trace (DWT), Instrumentation Trace Macrocell (ITM), and Trace Port Interface Unit (TPIU). These components are essential for real-time debugging and performance analysis. However, attempts to configure these registers via the DAP, especially while the processor is running, can result in access faults, as highlighted in the provided discussion. This post delves into the root causes of such issues and provides detailed troubleshooting steps to resolve them.

Memory Access Permissions and Debug State Configuration

The primary issue revolves around the inability to write to PPB-registered trace components like DWT->COMP0 at address 0xE0001020 through the DAP while the processor is operational. The fault acknowledgment during the SWD transaction suggests a violation of access permissions or misconfiguration in the debug state.

The ARM Cortex-M4 architecture enforces strict memory access permissions, particularly for the PPB region. This region is accessible only through privileged accesses, and unprivileged attempts result in fault conditions. However, the debug state, entered via a halt request from the debugger or debug events, should theoretically allow the debugger to access or modify processor registers and system memory, including peripherals inside and outside the processor, regardless of privilege level.

The confusion arises from the interpretation of the debug state’s capabilities. While the debug state does permit access to the PPB region, this access is contingent upon the proper configuration of the Debug Exception and Monitor Control Register (DEMCR) and the Debug Halting Control and Status Register (DHCSR). Misconfigurations in these registers can lead to access faults, even in the debug state.

Additionally, the DAP’s ability to access the PPB region is influenced by the Internal Bus Interconnect’s configuration. The DAP and the core must share a consistent view of the memory map, particularly for debug registers. Discrepancies in the address mapping between the DAP and the core can lead to access failures, as the DAP might attempt to access an incorrect or non-existent memory location.

SWD Protocol Misconfigurations and DAP Initialization

The SWD protocol, used for communication between the debugger and the target microcontroller, is another potential source of access faults. The protocol requires precise initialization and configuration to ensure reliable communication. The DAP’s Target Address Register (TAR) and Data Register Write (DRW) must be correctly set to facilitate memory accesses.

In the provided code snippet, the TAR is set to 0x20000000 for SRAM access, which succeeds, but the subsequent attempt to set the TAR to 0xE0001020 for PPB access fails. This discrepancy suggests that the DAP’s initialization or the SWD protocol’s configuration might be incomplete or incorrect. The DAP’s Access Port (AP) must be properly configured to enable access to the PPB region. This configuration includes setting the AP’s Control and Status Word (CSW) to enable privileged access and configuring the AP’s Transfer Address Register (TAR) to point to the correct memory location.

Furthermore, the SWD protocol’s acknowledgment mechanism must be carefully handled. A fault acknowledgment (ACK) indicates that the target device has rejected the access request. This rejection can stem from various factors, including incorrect address mapping, insufficient access permissions, or protocol violations. The debugger must interpret these acknowledgments correctly and adjust its configuration accordingly.

Implementing Proper DAP Configuration and Debug State Management

To resolve the access faults and enable successful configuration of the PPB trace components, a comprehensive approach to DAP configuration and debug state management is required. The following steps outline the necessary actions to achieve this:

1. Verify DAP and Core Memory Map Consistency: Ensure that the DAP and the core share a consistent view of the memory map, particularly for the PPB region. This verification involves cross-referencing the ARM Cortex-M4 Technical Reference Manual (TRM) and the microcontroller’s datasheet to confirm the address mappings for the DWT, ITM, and TPIU registers.

2. Configure the Debug Exception and Monitor Control Register (DEMCR): Set the DEMCR’s TRCENA bit to enable access to the trace components. This bit must be set before attempting to access the DWT, ITM, or TPIU registers. The DEMCR is located at address 0xE000EDFC, and setting the TRCENA bit (bit 24) ensures that the trace components are powered and accessible.

3. Set the Debug Halting Control and Status Register (DHCSR): Ensure that the DHCSR’s C_DEBUGEN bit is set to enable debug mode. This bit allows the debugger to halt the processor and access its registers. The DHCSR is located at address 0xE000EDF0, and setting the C_DEBUGEN bit (bit 0) is crucial for entering the debug state.

4. Initialize the DAP’s Access Port (AP): Configure the AP’s Control and Status Word (CSW) to enable privileged access and set the appropriate transfer size. The CSW is located at offset 0x00 in the AP’s memory map, and setting the PROT bit (bit 30) enables privileged access. Additionally, configure the AP’s Transfer Address Register (TAR) to point to the correct memory location for the PPB region.

5. Handle SWD Protocol Acknowledgment: Carefully interpret the SWD protocol’s acknowledgment mechanism to identify and resolve access faults. If a fault acknowledgment is received, verify the address mapping, access permissions, and protocol configuration. Adjust the DAP’s initialization and configuration as necessary to ensure successful access to the PPB region.

6. Implement Error Handling and Retry Mechanisms: Incorporate error handling and retry mechanisms in the debugger’s firmware to manage transient access faults. This implementation involves monitoring the SWD protocol’s acknowledgment signals and retrying the access request if a fault acknowledgment is received. Additionally, log the fault conditions for further analysis and debugging.

By following these steps, the access faults during DAP configuration of the PPB trace components can be resolved, enabling successful debugging and performance analysis on the ARM Cortex-M4 processor. Proper configuration of the DAP, SWD protocol, and debug state management is essential for reliable access to the PPB region and its associated trace components.