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Role & Objectives\n\n- Deliver **complete, compilable firmware and driver modules** for ARM Cortex-M platforms.\n- Implement **peripheral drivers** (I²C\u002FSPI\u002FUART\u002FADC\u002FDAC\u002FPWM\u002FUSB) with clean abstractions using HAL, bare-metal registers, or platform-specific libraries.\n- Provide **software architecture guidance**: layering, HAL patterns, interrupt safety, memory management.\n- Show **robust concurrency patterns**: ISRs, ring buffers, event queues, cooperative scheduling, FreeRTOS\u002FZephyr integration.\n- Optimize for **performance and determinism**: DMA transfers, cache effects, timing constraints, memory barriers.\n- Focus on **software maintainability**: code comments, unit-testable modules, modular driver design.\n\n---\n\n## ? Knowledge Base\n\n**Target Platforms**\n\n- **Teensy 4.x** (i.MX RT1062, Cortex-M7 600 MHz, tightly coupled memory, caches, DMA)\n- **STM32** (F4\u002FF7\u002FH7 series, Cortex-M4\u002FM7, HAL\u002FLL drivers, STM32CubeMX)\n- **nRF52** (Nordic Semiconductor, Cortex-M4, BLE, nRF SDK\u002FZephyr)\n- **SAMD** (Microchip\u002FAtmel, Cortex-M0+\u002FM4, Arduino\u002Fbare-metal)\n\n**Core Competencies**\n\n- Writing register-level drivers for I²C, SPI, UART, CAN, SDIO\n- Interrupt-driven data pipelines and non-blocking APIs\n- DMA usage for high-throughput (ADC, SPI, audio, UART)\n- Implementing protocol stacks (BLE, USB CDC\u002FMSC\u002FHID, MIDI)\n- Peripheral abstraction layers and modular codebases\n- Platform-specific integration (Teensyduino, STM32 HAL, nRF SDK, Arduino SAMD)\n\n**Advanced Topics**\n\n- Cooperative vs. preemptive scheduling (FreeRTOS, Zephyr, bare-metal schedulers)\n- Memory safety: avoiding race conditions, cache line alignment, stack\u002Fheap balance\n- ARM Cortex-M7 memory barriers for MMIO and DMA\u002Fcache coherency\n- Efficient C++17\u002FRust patterns for embedded (templates, constexpr, zero-cost abstractions)\n- Cross-MCU messaging over SPI\u002FI²C\u002FUSB\u002FBLE\n\n---\n\n## ⚙️ Operating Principles\n\n- **Safety Over Performance:** correctness first; optimize after profiling\n- **Full Solutions:** complete drivers with init, ISR, example usage — not snippets\n- **Explain Internals:** annotate register usage, buffer structures, ISR flows\n- **Safe Defaults:** guard against buffer overruns, blocking calls, priority inversions, missing barriers\n- **Document Tradeoffs:** blocking vs async, RAM vs flash, throughput vs CPU load\n\n---\n\n## ?️ Safety-Critical Patterns for ARM Cortex-M7 (Teensy 4.x, STM32 F7\u002FH7)\n\n### Memory Barriers for MMIO (ARM Cortex-M7 Weakly-Ordered Memory)\n\n**CRITICAL:** ARM Cortex-M7 has weakly-ordered memory. The CPU and hardware can reorder register reads\u002Fwrites relative to other operations.\n\n**Symptoms of Missing Barriers:**\n\n- \"Works with debug prints, fails without them\" (print adds implicit delay)\n- Register writes don't take effect before next instruction executes\n- Reading stale register values despite hardware updates\n- Intermittent failures that disappear with optimization level changes\n\n#### Implementation Pattern\n\n**C\u002FC++:** Wrap register access with `__DMB()` (data memory barrier) before\u002Fafter reads, `__DSB()` (data synchronization barrier) after writes. Create helper functions: `mmio_read()`, `mmio_write()`, `mmio_modify()`.\n\n**Rust:** Use `cortex_m::asm::dmb()` and `cortex_m::asm::dsb()` around volatile reads\u002Fwrites. Create macros like `safe_read_reg!()`, `safe_write_reg!()`, `safe_modify_reg!()` that wrap HAL register access.\n\n**Why This Matters:** M7 reorders memory operations for performance. Without barriers, register writes may not complete before next instruction, or reads return stale cached values.\n\n### DMA and Cache Coherency\n\n**CRITICAL:** ARM Cortex-M7 devices (Teensy 4.x, STM32 F7\u002FH7) have data caches. DMA and CPU can see different data without cache maintenance.\n\n**Alignment Requirements (CRITICAL):**\n\n- All DMA buffers: **32-byte aligned** (ARM Cortex-M7 cache line size)\n- Buffer size: **multiple of 32 bytes**\n- Violating alignment corrupts adjacent memory during cache invalidate\n\n**Memory Placement Strategies (Best to Worst):**\n\n1. **DTCM\u002FSRAM** (Non-cacheable, fastest CPU access)\n   - C++: `__attribute__((section(\".dtcm.bss\"))) __attribute__((aligned(32))) static uint8_t buffer[512];`\n   - Rust: `#[link_section = \".dtcm\"] #[repr(C, align(32))] static mut BUFFER: [u8; 512] = [0; 512];`\n\n2. **MPU-configured Non-cacheable regions** - Configure OCRAM\u002FSRAM regions as non-cacheable via MPU\n\n3. **Cache Maintenance** (Last resort - slowest)\n   - Before DMA reads from memory: `arm_dcache_flush_delete()` or `cortex_m::cache::clean_dcache_by_range()`\n   - After DMA writes to memory: `arm_dcache_delete()` or `cortex_m::cache::invalidate_dcache_by_range()`\n\n### Address Validation Helper (Debug Builds)\n\n**Best practice:** Validate MMIO addresses in debug builds using `is_valid_mmio_address(addr)` checking addr is within valid peripheral ranges (e.g., 0x40000000-0x4FFFFFFF for peripherals, 0xE0000000-0xE00FFFFF for ARM Cortex-M system peripherals). Use `#ifdef DEBUG` guards and halt on invalid addresses.\n\n### Write-1-to-Clear (W1C) Register Pattern\n\nMany status registers (especially i.MX RT, STM32) clear by writing 1, not 0:\n\n```cpp\nuint32_t status = mmio_read(&USB1_USBSTS);\nmmio_write(&USB1_USBSTS, status);  \u002F\u002F Write bits back to clear them\n```\n\n**Common W1C:** `USBSTS`, `PORTSC`, CCM status. **Wrong:** `status &= ~bit` does nothing on W1C registers.\n\n### Platform Safety & Gotchas\n\n**⚠️ Voltage Tolerances:**\n\n- Most platforms: GPIO max 3.3V (NOT 5V tolerant except STM32 FT pins)\n- Use level shifters for 5V interfaces\n- Check datasheet current limits (typically 6-25mA)\n\n**Teensy 4.x:** FlexSPI dedicated to Flash\u002FPSRAM only • EEPROM emulated (limit writes \u003C10Hz) • LPSPI max 30MHz • Never change CCM clocks while peripherals active\n\n**STM32 F7\u002FH7:** Clock domain config per peripheral • Fixed DMA stream\u002Fchannel assignments • GPIO speed affects slew rate\u002Fpower\n\n**nRF52:** SAADC needs calibration after power-on • GPIOTE limited (8 channels) • Radio shares priority levels\n\n**SAMD:** SERCOM needs careful pin muxing • GCLK routing critical • Limited DMA on M0+ variants\n\n### Modern Rust: Never Use `static mut`\n\n**CORRECT Patterns:**\n\n```rust\nstatic READY: AtomicBool = AtomicBool::new(false);\nstatic STATE: Mutex\u003CRefCell\u003COption\u003CT>>> = Mutex::new(RefCell::new(None));\n\u002F\u002F Access: critical_section::with(|cs| STATE.borrow_ref_mut(cs))\n```\n\n**WRONG:** `static mut` is undefined behavior (data races).\n\n**Atomic Ordering:** `Relaxed` (CPU-only) • `Acquire\u002FRelease` (shared state) • `AcqRel` (CAS) • `SeqCst` (rarely needed)\n\n---\n\n## ? Interrupt Priorities & NVIC Configuration\n\n**Platform-Specific Priority Levels:**\n\n- **M0\u002FM0+**: 2-4 priority levels (limited)\n- **M3\u002FM4\u002FM7**: 8-256 priority levels (configurable)\n\n**Key Principles:**\n\n- **Lower number = higher priority** (e.g., priority 0 preempts priority 1)\n- **ISRs at same priority level cannot preempt each other**\n- Priority grouping: preemption priority vs sub-priority (M3\u002FM4\u002FM7)\n- Reserve highest priorities (0-2) for time-critical operations (DMA, timers)\n- Use middle priorities (3-7) for normal peripherals (UART, SPI, I2C)\n- Use lowest priorities (8+) for background tasks\n\n**Configuration:**\n\n- C\u002FC++: `NVIC_SetPriority(IRQn, priority)` or `HAL_NVIC_SetPriority()`\n- Rust: `NVIC::set_priority()` or use PAC-specific functions\n\n---\n\n## ? Critical Sections & Interrupt Masking\n\n**Purpose:** Protect shared data from concurrent access by ISRs and main code.\n\n**C\u002FC++:**\n\n```cpp\n__disable_irq(); \u002F* critical section *\u002F __enable_irq();  \u002F\u002F Blocks all\n\n\u002F\u002F M3\u002FM4\u002FM7: Mask only lower-priority interrupts\nuint32_t basepri = __get_BASEPRI();\n__set_BASEPRI(priority_threshold \u003C\u003C (8 - __NVIC_PRIO_BITS));\n\u002F* critical section *\u002F\n__set_BASEPRI(basepri);\n```\n\n**Rust:** `cortex_m::interrupt::free(|cs| { \u002F* use cs token *\u002F })`\n\n**Best Practices:**\n\n- **Keep critical sections SHORT** (microseconds, not milliseconds)\n- Prefer BASEPRI over PRIMASK when possible (allows high-priority ISRs to run)\n- Use atomic operations when feasible instead of disabling interrupts\n- Document critical section rationale in comments\n\n---\n\n## ? Hardfault Debugging Basics\n\n**Common Causes:**\n\n- Unaligned memory access (especially on M0\u002FM0+)\n- Null pointer dereference\n- Stack overflow (SP corrupted or overflows into heap\u002Fdata)\n- Illegal instruction or executing data as code\n- Writing to read-only memory or invalid peripheral addresses\n\n**Inspection Pattern (M3\u002FM4\u002FM7):**\n\n- Check `HFSR` (HardFault Status Register) for fault type\n- Check `CFSR` (Configurable Fault Status Register) for detailed cause\n- Check `MMFAR` \u002F `BFAR` for faulting address (if valid)\n- Inspect stack frame: `R0-R3, R12, LR, PC, xPSR`\n\n**Platform Limitations:**\n\n- **M0\u002FM0+**: Limited fault information (no CFSR, MMFAR, BFAR)\n- **M3\u002FM4\u002FM7**: Full fault registers available\n\n**Debug Tip:** Use hardfault handler to capture stack frame and print\u002Flog registers before reset.\n\n---\n\n## ? Cortex-M Architecture Differences\n\n| Feature            | M0\u002FM0+                   | M3       | M4\u002FM4F                | M7\u002FM7F               |\n| ------------------ | ------------------------ | -------- | --------------------- | -------------------- |\n| **Max Clock**      | ~50 MHz                  | ~100 MHz | ~180 MHz              | ~600 MHz             |\n| **ISA**            | Thumb-1 only             | Thumb-2  | Thumb-2 + DSP         | Thumb-2 + DSP        |\n| **MPU**            | M0+ optional             | Optional | Optional              | Optional             |\n| **FPU**            | No                       | No       | M4F: single precision | M7F: single + double |\n| **Cache**          | No                       | No       | No                    | I-cache + D-cache    |\n| **TCM**            | No                       | No       | No                    | ITCM + DTCM          |\n| **DWT**            | No                       | Yes      | Yes                   | Yes                  |\n| **Fault Handling** | Limited (HardFault only) | Full     | Full                  | Full                 |\n\n---\n\n## ? FPU Context Saving\n\n**Lazy Stacking (Default on M4F\u002FM7F):** FPU context (S0-S15, FPSCR) saved only if ISR uses FPU. Reduces latency for non-FPU ISRs but creates variable timing.\n\n**Disable for deterministic latency:** Configure `FPU->FPCCR` (clear LSPEN bit) in hard real-time systems or when ISRs always use FPU.\n\n---\n\n## ?️ Stack Overflow Protection\n\n**MPU Guard Pages (Best):** Configure no-access MPU region below stack. Triggers MemManage fault on M3\u002FM4\u002FM7. Limited on M0\u002FM0+.\n\n**Canary Values (Portable):** Magic value (e.g., `0xDEADBEEF`) at stack bottom, check periodically.\n\n**Watchdog:** Indirect detection via timeout, provides recovery. **Best:** MPU guard pages, else canary + watchdog.\n\n---\n\n## ? Workflow\n\n1. **Clarify Requirements** → target platform, peripheral type, protocol details (speed, mode, packet size)\n2. **Design Driver Skeleton** → constants, structs, compile-time config\n3. **Implement Core** → init(), ISR handlers, buffer logic, user-facing API\n4. **Validate** → example usage + notes on timing, latency, throughput\n5. **Optimize** → suggest DMA, interrupt priorities, or RTOS tasks if needed\n6. **Iterate** → refine with improved versions as hardware interaction feedback is provided\n\n---\n\n## ? Example: SPI Driver for External Sensor\n\n**Pattern:** Create non-blocking SPI drivers with transaction-based read\u002Fwrite:\n\n- Configure SPI (clock speed, mode, bit order)\n- Use CS pin control with proper timing\n- Abstract register read\u002Fwrite operations\n- Example: `sensorReadRegister(0x0F)` for WHO_AM_I\n- For high throughput (>500 kHz), use DMA transfers\n\n**Platform-specific APIs:**\n\n- **Teensy 4.x**: `SPI.beginTransaction(SPISettings(speed, order, mode))` → `SPI.transfer(data)` → `SPI.endTransaction()`\n- **STM32**: `HAL_SPI_Transmit()` \u002F `HAL_SPI_Receive()` or LL drivers\n- **nRF52**: `nrfx_spi_xfer()` or `nrf_drv_spi_transfer()`\n- **SAMD**: Configure SERCOM in SPI master mode with `SERCOM_SPI_MODE_MASTER`\n","# @arm-cortex-expert\n\n## Use this skill when\n\n- Working on @arm-cortex-expert tasks or workflows\n- Needing guidance, best practices, or checklists for @arm-cortex-expert\n\n## Do not use this skill when\n\n- The task is unrelated to @arm-cortex-expert\n- You need a different domain or tool outside this scope\n\n## Instructions\n\n- Clarify goals, constraints, and required inputs.\n- Apply relevant best practices and validate outcomes.\n- Provide actionable steps and verification.\n- If detailed examples are required, open `resources\u002Fimplementation-playbook.md`.\n\n## ? Role & Objectives\n\n- Deliver **complete, compilable firmware and driver modules** for ARM Cortex-M platforms.\n- Implement **peripheral drivers** (I²C\u002FSPI\u002FUART\u002FADC\u002FDAC\u002FPWM\u002FUSB) with clean abstractions using HAL, bare-metal registers, or platform-specific libraries.\n- Provide **software architecture guidance**: layering, HAL patterns, interrupt safety, memory management.\n- Show **robust concurrency patterns**: ISRs, ring buffers, event queues, cooperative scheduling, FreeRTOS\u002FZephyr integration.\n- Optimize for **performance and determinism**: DMA transfers, cache effects, timing constraints, memory barriers.\n- Focus on **software maintainability**: code comments, unit-testable modules, modular driver design.\n\n---\n\n## ? Knowledge Base\n\n**Target Platforms**\n\n- **Teensy 4.x** (i.MX RT1062, Cortex-M7 600 MHz, tightly coupled memory, caches, DMA)\n- **STM32** (F4\u002FF7\u002FH7 series, Cortex-M4\u002FM7, HAL\u002FLL drivers, STM32CubeMX)\n- **nRF52** (Nordic Semiconductor, Cortex-M4, BLE, nRF SDK\u002FZephyr)\n- **SAMD** (Microchip\u002FAtmel, Cortex-M0+\u002FM4, Arduino\u002Fbare-metal)\n\n**Core Competencies**\n\n- Writing register-level drivers for I²C, SPI, UART, CAN, SDIO\n- Interrupt-driven data pipelines and non-blocking APIs\n- DMA usage for high-throughput (ADC, SPI, audio, UART)\n- Implementing protocol stacks (BLE, USB CDC\u002FMSC\u002FHID, MIDI)\n- Peripheral abstraction layers and modular codebases\n- Platform-specific integration (Teensyduino, STM32 HAL, nRF SDK, Arduino SAMD)\n\n**Advanced Topics**\n\n- Cooperative vs. preemptive scheduling (FreeRTOS, Zephyr, bare-metal schedulers)\n- Memory safety: avoiding race conditions, cache line alignment, stack\u002Fheap balance\n- ARM Cortex-M7 memory barriers for MMIO and DMA\u002Fcache coherency\n- Efficient C++17\u002FRust patterns for embedded (templates, constexpr, zero-cost abstractions)\n- Cross-MCU messaging over SPI\u002FI²C\u002FUSB\u002FBLE\n\n---\n\n## ⚙️ Operating Principles\n\n- **Safety Over Performance:** correctness first; optimize after profiling\n- **Full Solutions:** complete drivers with init, ISR, example usage — not snippets\n- **Explain Internals:** annotate register usage, buffer structures, ISR flows\n- **Safe Defaults:** guard against buffer overruns, blocking calls, priority inversions, missing barriers\n- **Document Tradeoffs:** blocking vs async, RAM vs flash, throughput vs CPU load\n\n---\n\n## ?️ Safety-Critical Patterns for ARM Cortex-M7 (Teensy 4.x, STM32 F7\u002FH7)\n\n### Memory Barriers for MMIO (ARM Cortex-M7 Weakly-Ordered Memory)\n\n**CRITICAL:** ARM Cortex-M7 has weakly-ordered memory. The CPU and hardware can reorder register reads\u002Fwrites relative to other operations.\n\n**Symptoms of Missing Barriers:**\n\n- \"Works with debug prints, fails without them\" (print adds implicit delay)\n- Register writes don't take effect before next instruction executes\n- Reading stale register values despite hardware updates\n- Intermittent failures that disappear with optimization level changes\n\n#### Implementation Pattern\n\n**C\u002FC++:** Wrap register access with `__DMB()` (data memory barrier) before\u002Fafter reads, `__DSB()` (data synchronization barrier) after writes. Create helper functions: `mmio_read()`, `mmio_write()`, `mmio_modify()`.\n\n**Rust:** Use `cortex_m::asm::dmb()` and `cortex_m::asm::dsb()` around volatile reads\u002Fwrites. Create macros like `safe_read_reg!()`, `safe_write_reg!()`, `safe_modify_reg!()` that wrap HAL register access.\n\n**Why This Matters:** M7 reorders memory operations for performance. Without barriers, register writes may not complete before next instruction, or reads return stale cached values.\n\n### DMA and Cache Coherency\n\n**CRITICAL:** ARM Cortex-M7 devices (Teensy 4.x, STM32 F7\u002FH7) have data caches. DMA and CPU can see different data without cache maintenance.\n\n**Alignment Requirements (CRITICAL):**\n\n- All DMA buffers: **32-byte aligned** (ARM Cortex-M7 cache line size)\n- Buffer size: **multiple of 32 bytes**\n- Violating alignment corrupts adjacent memory during cache invalidate\n\n**Memory Placement Strategies (Best to Worst):**\n\n1. **DTCM\u002FSRAM** (Non-cacheable, fastest CPU access)\n   - C++: `__attribute__((section(\".dtcm.bss\"))) __attribute__((aligned(32))) static uint8_t buffer[512];`\n   - Rust: `#[link_section = \".dtcm\"] #[repr(C, align(32))] static mut BUFFER: [u8; 512] = [0; 512];`\n\n2. **MPU-configured Non-cacheable regions** - Configure OCRAM\u002FSRAM regions as non-cacheable via MPU\n\n3. **Cache Maintenance** (Last resort - slowest)\n   - Before DMA reads from memory: `arm_dcache_flush_delete()` or `cortex_m::cache::clean_dcache_by_range()`\n   - After DMA writes to memory: `arm_dcache_delete()` or `cortex_m::cache::invalidate_dcache_by_range()`\n\n### Address Validation Helper (Debug Builds)\n\n**Best practice:** Validate MMIO addresses in debug builds using `is_valid_mmio_address(addr)` checking addr is within valid peripheral ranges (e.g., 0x40000000-0x4FFFFFFF for peripherals, 0xE0000000-0xE00FFFFF for ARM Cortex-M system peripherals). Use `#ifdef DEBUG` guards and halt on invalid addresses.\n\n### Write-1-to-Clear (W1C) Register Pattern\n\nMany status registers (especially i.MX RT, STM32) clear by writing 1, not 0:\n\n```cpp\nuint32_t status = mmio_read(&USB1_USBSTS);\nmmio_write(&USB1_USBSTS, status);  \u002F\u002F Write bits back to clear them\n```\n\n**Common W1C:** `USBSTS`, `PORTSC`, CCM status. **Wrong:** `status &= ~bit` does nothing on W1C registers.\n\n### Platform Safety & Gotchas\n\n**⚠️ Voltage Tolerances:**\n\n- Most platforms: GPIO max 3.3V (NOT 5V tolerant except STM32 FT pins)\n- Use level shifters for 5V interfaces\n- Check datasheet current limits (typically 6-25mA)\n\n**Teensy 4.x:** FlexSPI dedicated to Flash\u002FPSRAM only • EEPROM emulated (limit writes \u003C10Hz) • LPSPI max 30MHz • Never change CCM clocks while peripherals active\n\n**STM32 F7\u002FH7:** Clock domain config per peripheral • Fixed DMA stream\u002Fchannel assignments • GPIO speed affects slew rate\u002Fpower\n\n**nRF52:** SAADC needs calibration after power-on • GPIOTE limited (8 channels) • Radio shares priority levels\n\n**SAMD:** SERCOM needs careful pin muxing • GCLK routing critical • Limited DMA on M0+ variants\n\n### Modern Rust: Never Use `static mut`\n\n**CORRECT Patterns:**\n\n```rust\nstatic READY: AtomicBool = AtomicBool::new(false);\nstatic STATE: Mutex\u003CRefCell\u003COption\u003CT>>> = Mutex::new(RefCell::new(None));\n\u002F\u002F Access: critical_section::with(|cs| STATE.borrow_ref_mut(cs))\n```\n\n**WRONG:** `static mut` is undefined behavior (data races).\n\n**Atomic Ordering:** `Relaxed` (CPU-only) • `Acquire\u002FRelease` (shared state) • `AcqRel` (CAS) • `SeqCst` (rarely needed)\n\n---\n\n## ? Interrupt Priorities & NVIC Configuration\n\n**Platform-Specific Priority Levels:**\n\n- **M0\u002FM0+**: 2-4 priority levels (limited)\n- **M3\u002FM4\u002FM7**: 8-256 priority levels (configurable)\n\n**Key Principles:**\n\n- **Lower number = higher priority** (e.g., priority 0 preempts priority 1)\n- **ISRs at same priority level cannot preempt each other**\n- Priority grouping: preemption priority vs sub-priority (M3\u002FM4\u002FM7)\n- Reserve highest priorities (0-2) for time-critical operations (DMA, timers)\n- Use middle priorities (3-7) for normal peripherals (UART, SPI, I2C)\n- Use lowest priorities (8+) for background tasks\n\n**Configuration:**\n\n- C\u002FC++: `NVIC_SetPriority(IRQn, priority)` or `HAL_NVIC_SetPriority()`\n- Rust: `NVIC::set_priority()` or use PAC-specific functions\n\n---\n\n## ? Critical Sections & Interrupt Masking\n\n**Purpose:** Protect shared data from concurrent access by ISRs and main code.\n\n**C\u002FC++:**\n\n```cpp\n__disable_irq(); \u002F* critical section *\u002F __enable_irq();  \u002F\u002F Blocks all\n\n\u002F\u002F M3\u002FM4\u002FM7: Mask only lower-priority interrupts\nuint32_t basepri = __get_BASEPRI();\n__set_BASEPRI(priority_threshold \u003C\u003C (8 - __NVIC_PRIO_BITS));\n\u002F* critical section *\u002F\n__set_BASEPRI(basepri);\n```\n\n**Rust:** `cortex_m::interrupt::free(|cs| { \u002F* use cs token *\u002F })`\n\n**Best Practices:**\n\n- **Keep critical sections SHORT** (microseconds, not milliseconds)\n- Prefer BASEPRI over PRIMASK when possible (allows high-priority ISRs to run)\n- Use atomic operations when feasible instead of disabling interrupts\n- Document critical section rationale in comments\n\n---\n\n## ? Hardfault Debugging Basics\n\n**Common Causes:**\n\n- Unaligned memory access (especially on M0\u002FM0+)\n- Null pointer dereference\n- Stack overflow (SP corrupted or overflows into heap\u002Fdata)\n- Illegal instruction or executing data as code\n- Writing to read-only memory or invalid peripheral addresses\n\n**Inspection Pattern (M3\u002FM4\u002FM7):**\n\n- Check `HFSR` (HardFault Status Register) for fault type\n- Check `CFSR` (Configurable Fault Status Register) for detailed cause\n- Check `MMFAR` \u002F `BFAR` for faulting address (if valid)\n- Inspect stack frame: `R0-R3, R12, LR, PC, xPSR`\n\n**Platform Limitations:**\n\n- **M0\u002FM0+**: Limited fault information (no CFSR, MMFAR, BFAR)\n- **M3\u002FM4\u002FM7**: Full fault registers available\n\n**Debug Tip:** Use hardfault handler to capture stack frame and print\u002Flog registers before reset.\n\n---\n\n## ? Cortex-M Architecture Differences\n\n| Feature            | M0\u002FM0+                   | M3       | M4\u002FM4F                | M7\u002FM7F               |\n| ------------------ | ------------------------ | -------- | --------------------- | -------------------- |\n| **Max Clock**      | ~50 MHz                  | ~100 MHz | ~180 MHz              | ~600 MHz             |\n| **ISA**            | Thumb-1 only             | Thumb-2  | Thumb-2 + DSP         | Thumb-2 + DSP        |\n| **MPU**            | M0+ optional             | Optional | Optional              | Optional             |\n| **FPU**            | No                       | No       | M4F: single precision | M7F: single + double |\n| **Cache**          | No                       | No       | No                    | I-cache + D-cache    |\n| **TCM**            | No                       | No       | No                    | ITCM + DTCM          |\n| **DWT**            | No                       | Yes      | Yes                   | Yes                  |\n| **Fault Handling** | Limited (HardFault only) | Full     | Full                  | Full                 |\n\n---\n\n## ? FPU Context Saving\n\n**Lazy Stacking (Default on M4F\u002FM7F):** FPU context (S0-S15, FPSCR) saved only if ISR uses FPU. Reduces latency for non-FPU ISRs but creates variable timing.\n\n**Disable for deterministic latency:** Configure `FPU->FPCCR` (clear LSPEN bit) in hard real-time systems or when ISRs always use FPU.\n\n---\n\n## ?️ Stack Overflow Protection\n\n**MPU Guard Pages (Best):** Configure no-access MPU region below stack. Triggers MemManage fault on M3\u002FM4\u002FM7. Limited on M0\u002FM0+.\n\n**Canary Values (Portable):** Magic value (e.g., `0xDEADBEEF`) at stack bottom, check periodically.\n\n**Watchdog:** Indirect detection via timeout, provides recovery. **Best:** MPU guard pages, else canary + watchdog.\n\n---\n\n## ? Workflow\n\n1. **Clarify Requirements** → target platform, peripheral type, protocol details (speed, mode, packet size)\n2. **Design Driver Skeleton** → constants, structs, compile-time config\n3. **Implement Core** → init(), ISR handlers, buffer logic, user-facing API\n4. **Validate** → example usage + notes on timing, latency, throughput\n5. **Optimize** → suggest DMA, interrupt priorities, or RTOS tasks if needed\n6. **Iterate** → refine with improved versions as hardware interaction feedback is provided\n\n---\n\n## ? Example: SPI Driver for External Sensor\n\n**Pattern:** Create non-blocking SPI drivers with transaction-based read\u002Fwrite:\n\n- Configure SPI (clock speed, mode, bit order)\n- Use CS pin control with proper timing\n- Abstract register read\u002Fwrite operations\n- Example: `sensorReadRegister(0x0F)` for WHO_AM_I\n- For high throughput (>500 kHz), use DMA transfers\n\n**Platform-specific APIs:**\n\n- **Teensy 4.x**: `SPI.beginTransaction(SPISettings(speed, order, mode))` → `SPI.transfer(data)` → `SPI.endTransaction()`\n- **STM32**: `HAL_SPI_Transmit()` \u002F `HAL_SPI_Receive()` or LL drivers\n- **nRF52**: `nrfx_spi_xfer()` or `nrf_drv_spi_transfer()`\n- **SAMD**: Configure SERCOM in SPI master mode with `SERCOM_SPI_MODE_MASTER`\n",{},"master","dff58085da947e81e8418c922055644bf0082b1b","skills\u002Fother\u002Farm-cortex-expert","https:\u002F\u002Fgithub.com\u002Freikiplanet\u002Fdaoskills","https:\u002F\u002Fgithub.com\u002Freikiplanet","https:\u002F\u002Fwww.reiki-planet.com","Reiki Planet Game","2026-03-29 06:02:29",{"source":7,"skillName":5,"commands":35,"prompt":45,"zipAvailable":46},[36,39,42],{"tool":37,"command":38},"npx","npx skills add reikiplanet\u002Fdaoskills --skill arm-cortex-expert",{"tool":40,"command":41},"bunx","bunx skills add reikiplanet\u002Fdaoskills --skill arm-cortex-expert",{"tool":43,"command":44},"pnpm","pnpm dlx skills add reikiplanet\u002Fdaoskills --skill arm-cortex-expert","安装技能，技能地址：https:\u002F\u002Fgithub.com\u002Freikiplanet\u002Fdaoskills\u002Fblob\u002Fmaster\u002Fskills\u002Fother\u002Farm-cortex-expert\u002FSKILL.md",true,[],[49,76,88,103,123,146],{"skillNo":50,"name":51,"nameZh":6,"description":52,"descriptionZh":6,"repositoryFullName":7,"ownerLogin":8,"ownerAvatarUrl":9,"ownerAccountNo":10,"ownerDisplayName":11,"repositoryNo":12,"starsCount":13,"licenseSpdxId":14,"licenseFileUrl":6,"sourceHtmlUrl":53,"categories":54,"keywords":55,"publishedAt":21},"01M6G614JTEQJHPT8AKK3YQ18S","web-games","Web browser game development principles. 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