payload.c

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#include <stdint.h>

typedef enum : uintptr_t {
  k_payload_probe_sentinel = 0x22010000U, 
  k_payload_probe_counter  = 0x22010004U, 
} payload_probe_t;

typedef enum : uint32_t {
  k_payload_sentinel_value = 0x9710C0DEU, 
} payload_sentinel_t;

extern uint32_t g_ra8_ls_stack_top;

[[noreturn]] void payload_reset(void);
 
void payload_reset(void)
{
  *(volatile uint32_t*)(uintptr_t)k_payload_probe_sentinel = (uint32_t)k_payload_sentinel_value;
  volatile uint32_t* const counter = (volatile uint32_t*)(uintptr_t)k_payload_probe_counter;
  *counter                         = 0U;
  /* Heartbeat: advance forever so a J-Link probe sees liveness. Intentional
   * terminal loop (this is a leaf demo payload, not reusable firmware). */
  for (;;) {
    *counter = *counter + 1U;
  }
}

[[gnu::section(".vectors"), gnu::used]] static const uintptr_t s_vectors[2] = {
  (uintptr_t)&g_ra8_ls_stack_top, /* initial MSP                              */
  (uintptr_t)&payload_reset,      /* reset vector (Thumb bit from the symbol) */
};