_____  __  __  _  _   _   _   _   _   _   _ 
  / ____||  \/  || || | | | | | | \ | | | | | |
 | |     | \  / || || |_| | | | |  \| | | |_| |
 | |     | |\/| ||__   _| | | | | . ` | |  _  |
 | |____ | |  | |   | | | |_| | | |\  | | | | |
  \_____||_|  |_|   |_|  \___/  |_| \_| |_| |_|

        C O R T E X - M 4   U T I L I T I E S

cm4u – Cortex‑M4 Core Utilities

Tiny, header‑only, not‑a-HAL core utilities for Cortex‑M4.

Prefix is cm4u_ (Cortex‑M4 Utilities).
Focus is on register‑level goodies you always end up re‑writing:

• DWT‑based cycle counter + delays (us / ms)
• Check if you’re in Thread or Handler mode
• CONTROL / PRIMASK / BASEPRI / FAULTMASK helpers
• MSP / PSP helpers
• Barriers (DMB/DSB/ISB) and NOP
• PendSV + SVC triggers
• Light NVIC helpers
• Tiny profiling helpers

No drivers, no peripherals, no HAL/BSP bloat.
Just the core‑core bits.

---

Files

• cm4u_core.h – header‑only CM4 utilities (all static inline).
• example_main.c – tiny usage example.

Drop cm4u_core.h next to your CMSIS device headers and go.

---

Quick Start

#include "stm32f4xx.h"   // or your device
#include "core_cm4.h"
#include "cm4u_core.h"

int main(void)
{
    const uint32_t core_hz = 168000000u;   // your core clock

    // Enable DWT cycle counter once at startup
    cm4u_dwt_init();

    // Simple 10 us delay
    cm4u_delay_us(10u, core_hz);

    while (1) {
        uint32_t start = cm4u_profile_cycles_start();

        // Your low‑jitter, real‑time magic here
        __NOP();

        uint32_t spent_cycles = cm4u_profile_cycles_end(start);
        (void)spent_cycles;
    }
}

---

Thread vs Handler Mode

void debug_where_am_i(void)
{
    if (cm4u_in_thread_mode()) {
        // main / task / normal code
    } else if (cm4u_in_handler_mode()) {
        uint32_t exc = cm4u_get_exception_number();
        (void)exc; // e.g. log or blink a different LED
    }
}

---

Critical Sections

uint32_t primask = cm4u_critical_enter();
/* non‑interruptible section */
cm4u_critical_exit(primask);

This uses PRIMASK so you can safely nest different critical sections (if each stores and restores its own previous PRIMASK).

---

DWT‑Based Delays

void delay_example(uint32_t core_hz)
{
    cm4u_dwt_init();              // once is enough, but cheap if called again

    cm4u_delay_us(5, core_hz);    // 5 microseconds
    cm4u_delay_ms(1, core_hz);    // 1 millisecond
}

This is busy‑wait and fully synchronous:

• jitter is basically just the loop and NOP
• ideal for short, deterministic waits
• not for multi‑millisecond “sleep the system” style delays

---

System Tricks

// Trigger PendSV (e.g. your own context switcher)
cm4u_trigger_pendsv();

// Trigger SVC #0 (supervisor call)
cm4u_trigger_svc(0);

// Software reset
cm4u_system_reset();

---

License

MIT