Troubleshooting “Invalid ROM Table” error when debugging Cortex-M1

The “Invalid ROM Table” error is a common problem faced by developers when debugging applications on Cortex-M1 chips. This error occurs when the debugger is unable to locate the ROM table entries that contain vital debugging information about the Cortex-M1 core. The ROM table is located in the Cortex-M1 memory map and provides the debugger with details on hardware breakpoints, watchpoints, vector catch settings, etc. An invalid or corrupted ROM table prevents the debugger from initializing correctly and accessing necessary debug features of the chip.

Possible Causes

There are several potential reasons why the ROM table could be invalid on a Cortex-M1 chip:

• Corrupted ROM table – The ROM table could have been overwritten with invalid data, erasing the original contents.
• Invalid chip configuration – The Cortex-M1 configuration chosen during chip initialization may not match the actual hardware configuration.
• Faulty memory hardware – Issues with RAM or flash memory could lead to ROM table corruption.
• Invalid clock settings – Incorrect core or system clock configurations could prevent the ROM table from being read properly.
• Buggy firmware – Bugs in the firmware running on the Cortex-M1 could accidentally modify the ROM table.
• Access restrictions – The debugger may not have sufficient access permissions to read the ROM table entries.

Troubleshooting Steps

Debugging the “Invalid ROM Table” error requires methodically checking different aspects of the Cortex-M1 hardware and software configuration. Follow these troubleshooting steps to resolve the issue:

1. Verify chip configuration: Double check that the Cortex-M1 configuration (memory map, peripherals, etc.) matches the actual chip. CPU registers and reference manual can help identify the configuration.
2. Check ROM table location: Examine the memory map in the reference manual to ensure the debugger is accessing the correct ROM table memory addresses.
3. Reload firmware: Erase and re-flash the Cortex-M1 firmware in case the existing firmware is buggy. Use manufacturer-provided firmware or sample code if possible.
4. Initialize memory: Perform an explicit initialization of the Cortex-M1 memory using chip-specific code to detect any RAM/flash issues.
5. Examine vector table: The first entry in the vector table points to the initial stack pointer, verify this value is within expected range.
6. Check clock configurations: Ensure core, peripheral, and system clocks are at supported frequencies. Consult datasheet for chip-specific details.
7. Reset access permissions: Fully reset the Cortex-M1 core to default settings to eliminate any access restriction issues.
8. Use JTAG/SWD instead: Interface the debugger via JTAG or SWD instead of the debug port to bypass any debug port errors.
9. Update debugger firmware: Debugger firmware may require an update to support all ROM table entries for the specific Cortex-M1 chip.

Verifying ROM Table Contents

After addressing any potential issues, the next step is verifying the actual ROM table contents to determine if the issue has been resolved. Here are some methods to check the integrity of the ROM table data:

• Read debug subsystem identification register (DSIDR) – This contains a signature that indicates a valid ROM table.
• Check ROM table version number register (ROMTABLE_VERSION) – Should match known value for chip.
• Verify debug component ID registers (CID0-CID3) – First components should be ARM debug architecture IDs.
• Read debug peripheral identification registers (PID0-PID7) – Indicates presence of debugging components.
• Check ROM table end marker – Valid end of table marker value is 0x6FF04FF0.
• Compare checksums – Sum values in ROM table against reference manual checksums.

The ARMv6-M Architecture Reference Manual contains the full details on expected ROM table content. Individual chip reference manuals also provide specifics on the exact ROM table structure.

Analyzing Debug Behavior

A final method to validate the ROM table is testing actual debug functionality of the Cortex-M1 chip:

• Set hardware breakpoints and watchpoints
• Examine core registers and memory
• Step through code at breakpoint
• Clear and set vector catch for exceptions
• Check single-stepping behavior

If basic debug operations fail or behave unexpectedly, it indicates lingering ROM table issues. More advanced debug tests can also be constructed to thoroughly validate the ROM table contents.

Risks of Invalid ROM Table

An invalid ROM table prevents utilization of key Cortex-M1 debug capabilities. This has several consequences during application development and testing:

• No hardware breakpoints or watchpoints
• Limited visibility into core registers and memory state
• Reduced control over code execution flow
• Blind to exceptions and system state changes
• Difficulty detecting firmware crashes and hangs
• No post-mortem crash analysis

Together these limitations drastically slow down code development, testing, and optimization processes. Bugs and crashes become difficult to detect requiring extra logging output from the firmware. Complex software issues can no longer be efficiently analyzed and resolved.

Mitigation Strategies

If the “Invalid ROM Table” error persists after exhaustive troubleshooting, here are some strategies to mitigate the situation:

• Use JTAG/SWD interface with boundary scan chain access
• Leverage run-control debugging via RTOS plugins
• Insert debug logging code into the firmware
• Add assertions and state checks within code
• Serialize state over debug interface periodically
• Switch to using emulator for debugging
• Consider firmware refactor or port to different Cortex-M device

JTAG and SWD provide pin-level debugging that work independently of the ARM CoreSight debug architecture. Logging and self-checks within the firmware itself also compensate for lack of hardware breakpoints. And emulators fully simulate Cortex-M1 chips allowing unmodified firmware debugging.

Summary

The “Invalid ROM Table” error prevents utilization of hardware debugging capabilities for Cortex-M1 chips. This error can arise due to corrupted ROM tables, incorrect chip configuration, faulty memory hardware, invalid clock settings, buggy firmware, or access restrictions. A systematic troubleshooting methodology examining the ROM table location, contents, and debugger behavior can help resolve the underlying issue. For recalcitrant situations, JTAG/SWD access, debug logging, assertions, emulators and firmware refactors are mitigation strategies to consider. With sufficient analysis and the right tools, the “Invalid ROM Table” error can be overcome to enable debugging ARM Cortex-M1 applications.