Project Context (Auto-Injected)

Target configuration: !cat .cargo/config.toml 2>/dev/null || echo "No .cargo/config.toml found"

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Embedded Domain

Layer 3: Domain Constraints

Domain Constraints → Design Implications

Domain Rule	Design Constraint	Rust Implication
No heap	Stack allocation	heapless, no Box/Vec
No std	Core only	#![no_std]
Real-time	Predictable timing	No dynamic alloc
Resource limited	Minimal memory	Static buffers
Hardware safety	Safe peripheral access	HAL + ownership
Interrupt safe	No blocking in ISR	Atomic, critical sections

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Critical Constraints

No Dynamic Allocation

RULE: Cannot use heap (no allocator)
WHY: Deterministic memory, no OOM
RUST: heapless::Vec<T, N>, arrays

Interrupt Safety

RULE: Shared state must be interrupt-safe
WHY: ISR can preempt at any time
RUST: Mutex<RefCell<T>> + critical section

Hardware Ownership

RULE: Peripherals must have clear ownership
WHY: Prevent conflicting access
RUST: HAL takes ownership, singletons

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Trace Down ↓

From constraints to design (Layer 2):

"Need no_std compatible data structures"
    ↓ m02-resource: heapless collections
    ↓ Static sizing: heapless::Vec<T, N>

"Need interrupt-safe state"
    ↓ m03-mutability: Mutex<RefCell<Option<T>>>
    ↓ m07-concurrency: Critical sections

"Need peripheral ownership"
    ↓ m01-ownership: Singleton pattern
    ↓ m12-lifecycle: RAII for hardware

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Layer Stack

Layer	Examples	Purpose
PAC	stm32f4, esp32c3	Register access
HAL	stm32f4xx-hal	Hardware abstraction
Framework	RTIC, Embassy	Concurrency
Traits	embedded-hal	Portable drivers

Framework Comparison

Framework	Style	Best For
RTIC	Priority-based	Interrupt-driven apps
Embassy	Async	Complex state machines
Bare metal	Manual	Simple apps

Key Crates

Purpose	Crate
Runtime (ARM)	cortex-m-rt
Panic handler	panic-halt, panic-probe
Collections	heapless
HAL traits	embedded-hal
Logging	defmt
Flash/debug	probe-run

Design Patterns

Pattern	Purpose	Implementation
no_std setup	Bare metal	#![no_std] + #![no_main]
Entry point	Startup	#[entry] or embassy
Static state	ISR access	Mutex<RefCell<Option<T>>>
Fixed buffers	No heap	heapless::Vec<T, N>

Code Pattern: Static Peripheral

#![no_std]
#![no_main]

use cortex_m::interrupt::{self, Mutex};
use core::cell::RefCell;

static LED: Mutex<RefCell<Option<Led>>> = Mutex::new(RefCell::new(None));

interrupt::free(|cs| {
    LED.borrow(cs).replace(Some(led));
});

loop {
    interrupt::free(|cs| {
        if let Some(led) = LED.borrow(cs).borrow_mut().as_mut() {
            led.toggle();
        }
    });
}

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## Common Mistakes

| Mistake | Domain Violation | Fix |
|---------|-----------------|-----|
| Using Vec | Heap allocation | heapless::Vec |
| No critical section | Race with ISR | Mutex + interrupt::free |
| Blocking in ISR | Missed interrupts | Defer to main loop |
| Unsafe peripheral | Hardware conflict | HAL ownership |

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## Trace to Layer 1

| Constraint | Layer 2 Pattern | Layer 1 Implementation |
|------------|-----------------|------------------------|
| No heap | Static collections | heapless::Vec<T, N> |
| ISR safety | Critical sections | Mutex<RefCell<T>> |
| Hardware ownership | Singleton | take().unwrap() |
| no_std | Core-only | #![no_std], #![no_main] |

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## Related Skills

| When | See |
|------|-----|
| Static memory | m02-resource |
| Interior mutability | m03-mutability |
| Interrupt patterns | m07-concurrency |
| Unsafe for hardware | unsafe-checker |