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DESIGN.md

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GOAL

We want an ultra-minimal GDB Remote Serial Protocol (RSP) “probe” firmware that talks SWD to a Cortex‑M target (initially: generic Cortex‑M0+/Cortex‑M, not MSPM0-specific). The probe runs on a cheap MSPM0 MCU and is controlled over UART by GDB.

STARTING POINT

This repo now uses a split, module-based layout:

  • main.c: calls board_init() and runs probe_poll().
  • hal.h: minimal HAL (UART, delay, SWD GPIO).
  • src/board_mspm0c1104.c, src/board_mspm0c1105.c: MSPM0 DriverLib bring-up + HAL implementation.
  • src/swd_bitbang.c: SWD wire protocol bit-bang (turnaround, ACK, parity).
  • src/adiv5.c: ADIv5 Debug Port / Access Port transactions (this is ARM “Debug Interface v5”, not “division”).
  • src/target_mem.c: memory reads/writes via AHB‑AP.
  • src/cortex.c: Cortex‑M debug (DHCSR/DCRSR), halt/step/continue, FPB breakpoints.
  • src/rsp.c: UART RSP packet parser + command handling.
  • src/probe.c: glue between RSP and SWD/ADIv5/Cortex.

Hardware Notes

Probe clocks:

  • MSPM0C1104 (“tiny”): use SYSOSC base frequency (24 MHz per MSPM0C110x datasheet/DFP metadata).
  • MSPM0C1105 (“bigger”): use SYSOSC base frequency (32 MHz).

Timing uses a free-running SysTick (24-bit) in delay_us().

GPIO's : just pick any for now. The schematic is not set. But it should only be a few pins, and we can just edit this later.

When we have this all building, we check the consumed resources (linker prints memory usage, and we also run arm-none-eabi-size).

TARGET HARDWARE

Current focus is the MSPM0C1 series:

  • MSPM0C1104 (tiny target; 16KB flash / 1KB SRAM / 24 MHz)
  • Optional: MSPM0C1105 (more headroom; 32KB flash / 8KB SRAM / 32 MHz)

The original L-series notes are considered deprecated for this repo’s current direction.

BUILD SYSTEM

Toolchain: arm-none-eabi-gcc via cmake/toolchains/arm-gcc.cmake.

SDK path is a CMake cache variable:

  • MSPM0_SDK_PATH defaults to /Applications/ti/mspm0_sdk_2_08_00_03/

Targets:

  • C1104 tiny (recommended for “does it fit in 1KB SRAM?”):
    • cmake -S . -B build_c1104 -DCMAKE_TOOLCHAIN_FILE=cmake/toolchains/arm-gcc.cmake -DPROBE_DEVICE=MSPM0C1104 -DPROBE_TINY_RAM=ON
  • C1105 bigger (“full feature”):
    • cmake -S . -B build_c1105 -DCMAKE_TOOLCHAIN_FILE=cmake/toolchains/arm-gcc.cmake -DPROBE_DEVICE=MSPM0C1105 -DPROBE_TINY_RAM=OFF
      • Defaults to enabling qXfer:features:read target XML and DWT watchpoints; override with:
        • -DPROBE_ENABLE_QXFER_TARGET_XML=OFF
        • -DPROBE_ENABLE_DWT_WATCHPOINTS=OFF

Then build:

  • cmake --build build_c1104 -j
  • cmake --build build_c1105 -j

Notes:

  • Link uses -Wl,--print-memory-usage (and produces mspm0_debugger.map).
  • If you change PROBE_DEVICE inside an existing build directory, clear the cache or use a fresh build_* dir to avoid stale per-device settings.