Debugging any microcontroller can be challenging, but debugging ARM Cortex processors like the Cortex-M1 and Cortex-M3 adds another layer of complexity. While both processors are based on the ARM architecture, there are key differences between the two that impact the debugging process.
Core Architecture
The Cortex-M1 uses the ARMv6-M architecture which is a simpler, smaller microcontroller-focused design. The Cortex-M3 uses the more advanced ARMv7-M architecture which includes a 3-stage pipeline, faster clock speeds, and additional instructions. These architectural differences mean the Cortex-M3 has more advanced debugging capabilities built-in.
Debug Access Port
Both processors contain an ARM Debug Access Port (DAP) to facilitate debugging. The Cortex-M1 DAP only supports JTAG for debug access. The Cortex-M3 DAP supports both JTAG and Serial Wire Debug (SWD). SWD provides higher performance debugging over a two pin interface making it easier to debug the Cortex-M3.
Breakpoints and Watchpoints
The Cortex-M1 supports up to 4 hardware breakpoints. The Cortex-M3 supports up to 6 hardware breakpoints. Additionally, the Cortex-M3 includes 2 watchpoints for monitoring memory access. Watchpoints allow monitoring reads, writes or both on a particular memory address. This helps track down difficult memory related bugs.
Debugging Modes
Both processors support halting and stepping through code one instruction at a time. The Cortex-M3 adds support for Monitor mode debugging. Monitor mode uses a dedicated debug exception handler to avoid disrupting the processor state during debugging. This results in a less invasive, more seamless debugging experience.
Trace Support
The Cortex-M3 contains an Embedded Trace Macrocell (ETM) which supports instruction trace capabilities. Tracing allows recording a log of program execution that can be used to reconstruct code execution flow. This helps identify bugs that may not be reproducible with breakpoints alone.
Memory Access
Debugging often requires directly accessing memory to view or modify values. The Cortex-M1 only allows memory access while code is halted. The Cortex-M3 includes additional coprocessor registers to allow memory access while code is running. This enables more advanced debugging techniques.
Toolchain Support
There are more robust debug tool options available for the Cortex-M3. Tools like SEGGER J-Link, ULINKpro, and others provide developers advanced debugging features leveraging the Cortex-M3’s capabilities. Less toolchain support is available for the aging Cortex-M1.
Processor Availability
Since the Cortex-M1 is an older design, fewer silicon partners are still manufacturing chips based on it. The Cortex-M3 is newer and continues to be widely supported by the ARM ecosystem. The availability of Cortex-M3 test boards and development kits also aids debugging.
Debugging Workflow
The increased debugging capabilities of the Cortex-M3 enable a more advanced debugging workflow. The additional breakpoints, watchpoints, trace support, and less invasive modes allow tracking down bugs faster. With the Cortex-M1, more time is spent working around the limited debugging functionality.
Summary
In summary, while both processors can be debugged, the Cortex-M3 provides a clear advantage with its robust debugging feature set. The architectural improvements like the 3-stage pipeline, addition of coprocessors, and inclusion of an ETM trace module greatly assist debugging complex issues. The Cortex-M1 relies more on basic breakpoints and memory access while halted making for a slower, more difficult debugging process. When choosing a new ARM Cortex processor, developers should consider the debugging experience in addition to other factors.
