diff --git a/inc/uf2.h b/inc/uf2.h
index d6d0c37..8ac7173 100644
--- a/inc/uf2.h
+++ b/inc/uf2.h
@@ -290,6 +290,12 @@ void delay(uint32_t ms);
 void hidHandoverLoop(int ep);
 void handoverPrep(void);
 
+// Useful for debugging.
+#ifdef BLINK_DEBUG
+void blink_n(uint32_t pin, uint32_t n, uint32_t interval);
+void blink_n_forever(uint32_t pin, uint32_t n, uint32_t interval);
+#endif
+
 #define CONCAT_1(a, b) a##b
 #define CONCAT_0(a, b) CONCAT_1(a, b)
 #define STATIC_ASSERT(e) enum { CONCAT_0(_static_assert_, __LINE__) = 1 / ((e) ? 1 : 0) }
diff --git a/src/init_samd21.c b/src/init_samd21.c
index 3a7c2af..dd4ace9 100644
--- a/src/init_samd21.c
+++ b/src/init_samd21.c
@@ -7,7 +7,7 @@
 
 volatile bool g_interrupt_enabled = true;
 
-// SAMD51 starts at 1MHz by default.
+// SAMD21 starts at 1MHz by default.
 uint32_t current_cpu_frequency_MHz = 1;
 
 static void gclk_sync(void) {
diff --git a/src/init_samd51.c b/src/init_samd51.c
index 3ec3c9c..a4525ff 100644
--- a/src/init_samd51.c
+++ b/src/init_samd51.c
@@ -82,7 +82,8 @@ void system_init(void) {
     MCLK->CPUDIV.reg = MCLK_CPUDIV_DIV_DIV1;
 
     SysTick_Config(1000);
-    current_cpu_frequency_MHz = 48;
+    // No change from initial frequency.
+    // current_cpu_frequency_MHz = 48;
 }
 
 void SysTick_Handler(void) { LED_TICK(); }
diff --git a/src/main.c b/src/main.c
index ec69af5..0df49ea 100644
--- a/src/main.c
+++ b/src/main.c
@@ -29,11 +29,11 @@
 
 /**
  * --------------------
- * SAM-BA Implementation on SAMD21
+ * SAM-BA Implementation on SAMD21 and SAMD51
  * --------------------
  * Requirements to use SAM-BA :
  *
- * Supported communication interfaces :
+ * Supported communication interfaces (SAMD21):
  * --------------------
  *
  * SERCOM5 : RX:PB23 TX:PB22
@@ -68,8 +68,6 @@
  *
  * Applications compiled to be executed along with the bootloader will start at
  * 0x2000 (samd21) or 0x4000 (samd51)
- * The bootloader doesn't changes the VTOR register, application code is
- * taking care of this.
  *
  */
 
@@ -110,7 +108,7 @@ static void check_start_application(void) {
         if (RESET_CONTROLLER->RCAUSE.bit.POR || *DBL_TAP_PTR != DBL_TAP_MAGIC_QUICK_BOOT) {
             // the second tap on reset will go into app
             *DBL_TAP_PTR = DBL_TAP_MAGIC_QUICK_BOOT;
-            // this will be cleared after succesful USB enumeration
+            // this will be cleared after successful USB enumeration
             // this is around 1.5s
             resetHorizon = timerHigh + 50;
             return;
@@ -150,7 +148,7 @@ extern char _etext;
 extern char _end;
 
 /**
- *  \brief SAMD21 SAM-BA Main loop.
+ *  \brief  SAM-BA Main loop.
  *  \return Unused (ANSI-C compatibility).
  */
 int main(void) {
@@ -181,8 +179,17 @@ int main(void) {
     while (SUPC->STATUS.bit.BOD33DET) {
         // Wait for voltage to rise above BOD33 value.
     }
-    // Wait 100ms for voltage to stabilize.
-    delay(100);
+
+    // If we are starting from a power-on or a brownout,
+    // wait for the voltage to stabilize. Don't do this on an
+    // external reset because it interferes with the timing of double-click.
+    // "BODVDD" means BOD33.
+    if (RSTC->RCAUSE.bit.POR || RSTC->RCAUSE.bit.BODVDD) {
+        do {
+            // Check again in 100ms.
+            delay(100);
+        } while (SUPC->STATUS.bit.BOD33DET);
+    }
 
     // Now enable reset if voltage falls below minimum.
     SUPC->BOD33.bit.ENABLE = 0;
@@ -190,7 +197,6 @@ int main(void) {
     SUPC->BOD33.bit.ENABLE = 1;
 #endif
 
-
 #if USB_VID == 0x239a && USB_PID == 0x0013     // Adafruit Metro M0
     // Delay a bit so SWD programmer can have time to attach.
     delay(15);
diff --git a/src/utils.c b/src/utils.c
index c8e5775..dce02a2 100644
--- a/src/utils.c
+++ b/src/utils.c
@@ -4,27 +4,48 @@
 static uint32_t timerLow;
 uint32_t timerHigh, resetHorizon;
 
-void delay(uint32_t ms) {
-// These multipliers were determined empirically and are only approximate.
-// SAMD21 starts up at 1MHz by default.
+void __attribute__ ((noinline)) delay(uint32_t ms) {
+    // These multipliers were determined empirically and are only approximate.
+    // After the pulsing LED is enabled (led_tick_on), the multipliers need to change
+    // due to the interrupt overhead of the pulsing.
+    // SAMD21 starts up at 1MHz by default.
 #ifdef SAMD21
-    uint32_t count = ms * 167 * (current_cpu_frequency_MHz);
+    uint32_t count = ms * (current_cpu_frequency_MHz) * (led_tick_on ? 149: 167);
 #endif
 #ifdef SAMD51
     // SAMD51 starts up at 48MHz by default, and we set the clock to
     // 48MHz, so we don't need to adjust for current_cpu_frequency_MHz.
-    uint32_t count = ms * 6000;
-    // On SAMD51, before the 1ms SysTick interrupts are set up, the
-    // timing loop will run about twice as fast, so double the count.
-    if (led_tick_on) {
-        count *= 2;
-    }
+    uint32_t count = ms * (led_tick_on ? 6353 : 6826);
 #endif
-    for (int i = 1; i < count; ++i) {
+    for (uint32_t i = 1; i < count; ++i) {
         asm volatile("");
     }
 }
 
+// Useful for debugging.
+// PIN_PA19 is D12 on Metro M0, D11 on Metro M4
+#ifdef BLINK_DEBUG
+void blink_n(uint32_t pin, uint32_t n, uint32_t interval) {
+    // Start out off.
+    PINOP(pin, DIRSET);
+    PINOP(pin, OUTCLR);
+    delay(interval);
+    for (int i = 0; i < n; ++i) {
+        PINOP(pin, OUTSET);
+        delay(interval);
+        PINOP(pin, OUTCLR);
+        delay(interval);
+    }
+}
+
+void blink_n_forever(uint32_t pin, uint32_t n, uint32_t interval) {
+    while(1) {
+        blink_n(pin, n, interval);
+        delay(interval*5);
+    }
+}
+#endif
+
 void timerTick(void) {
     if (timerLow-- == 0) {
         timerLow = TIMER_STEP;