XIAO requires a delay() to initialize while MKR Zero does not

Hi All,

I have this task switching code which works on MKR Zero as is but on XIAO it only works if there’s a delay of about 500ms in the beginning of setup(); Is there any way to avoid it? Or how to best handle it in a library? I want to share a multitasking library and I want to hide this delay() from the user but so far it looks like the delay() has to be the first line in the sketch’s setup()… Any suggestions how to work around this? is this a known issue? Below is the code you can try in Arduino IDE, uncomment delay() in setup() for it to work (blink) on XIAO.

#define SSIZE 1024

void task() {
  while (1) {
    digitalWrite(LED_BUILTIN, HIGH);
    digitalWrite(LED_BUILTIN, LOW);

struct Ctx {
  uint32_t r8;
  uint32_t r9;
  uint32_t r10;
  uint32_t r11;
  uint32_t r4;
  uint32_t r5;
  uint32_t r6;
  uint32_t r7;
  uint32_t r0;
  uint32_t r1;
  uint32_t r2;
  uint32_t r3;
  uint32_t r12;
  uint32_t lr;
  uint32_t pc;
  uint32_t psr;

struct TaskInfo {
  uint8_t* sp;
  uint8_t stack[SSIZE];

TaskInfo* _tasks[2];
volatile int _current_task = 0;
volatile bool _initialized = false;

void init_task(struct TaskInfo* taskInfo) {
  taskInfo->sp = &taskInfo->stack[SSIZE - 1];
  // 8 bytes align per ARM Cortex+ requirement when entering interrupt
  taskInfo->sp = (uint8_t*)((uintptr_t)taskInfo->sp & ~0x7);

  // clear registers
  for (unsigned i = 0; i < sizeof(Ctx); ++i) {
    *--taskInfo->sp = 0;

  auto ctx = (Ctx*)taskInfo->sp;
  // compiler/architecture specific
  ctx->psr = 0x01000000;
  ctx->pc = (uintptr_t)task;

uint8_t* swap_stack(uint8_t* sp) {
  if (_initialized) {
    _tasks[_current_task]->sp = sp;
    _current_task = (_current_task + 1) % 2;
    sp = _tasks[_current_task]->sp;
  return sp;

extern "C" {
  void __attribute__((naked)) PendSV_Handler() {
    asm volatile("push {r4-r7}");
    asm volatile("mov r4,r8");
    asm volatile("mov r5,r9");
    asm volatile("mov r6,r10");
    asm volatile("mov r7,r11");
    asm volatile("push {r4-r7}");

    asm volatile("mov r0, sp");
    asm volatile("push {lr}");
    asm volatile("blx %0"
                 : "r"(swap_stack)
                 : "r0");
    asm volatile("mov r12, r0");
    asm volatile("pop {r0}");
    asm volatile("mov lr, r0");

    asm volatile("mov sp, r12");
    asm volatile("pop {r4-r7}");
    asm volatile("mov r8,r4");
    asm volatile("mov r9,r5");
    asm volatile("mov r10,r6");
    asm volatile("mov r11,r7");
    asm volatile("pop {r4-r7}");
    asm volatile("bx lr");

  int sysTickHook() {
    return 0;

void setup() {
  // uncomment for XIAO to work (blink led)
  _tasks[0] = new TaskInfo();
  _tasks[1] = new TaskInfo();
  _initialized = true;

void loop() {
  // nothing here