diff --git a/Marlin/src/HAL/HAL_AVR/HAL_AVR.h b/Marlin/src/HAL/HAL_AVR/HAL_AVR.h
index 8ea43c815..c3b7403e3 100644
--- a/Marlin/src/HAL/HAL_AVR/HAL_AVR.h
+++ b/Marlin/src/HAL/HAL_AVR/HAL_AVR.h
@@ -139,6 +139,7 @@ extern "C" {
 
 #define ENABLE_STEPPER_DRIVER_INTERRUPT()  SBI(TIMSK1, OCIE1A)
 #define DISABLE_STEPPER_DRIVER_INTERRUPT() CBI(TIMSK1, OCIE1A)
+#define STEPPER_ISR_ENABLED() TEST(TIMSK1, OCIE1A)
 
 #define ENABLE_TEMPERATURE_INTERRUPT()  SBI(TIMSK0, OCIE0B)
 #define DISABLE_TEMPERATURE_INTERRUPT() CBI(TIMSK0, OCIE0B)
diff --git a/Marlin/src/HAL/HAL_DUE/HAL_timers_Due.cpp b/Marlin/src/HAL/HAL_DUE/HAL_timers_Due.cpp
index dbeb28541..a604f6806 100644
--- a/Marlin/src/HAL/HAL_DUE/HAL_timers_Due.cpp
+++ b/Marlin/src/HAL/HAL_DUE/HAL_timers_Due.cpp
@@ -112,23 +112,28 @@ void HAL_timer_start(const uint8_t timer_num, const uint32_t frequency) {
 }
 
 void HAL_timer_enable_interrupt(const uint8_t timer_num) {
-  const tTimerConfig *pConfig = &TimerConfig[timer_num];
+  const tTimerConfig * const pConfig = &TimerConfig[timer_num];
   pConfig->pTimerRegs->TC_CHANNEL[pConfig->channel].TC_IER = TC_IER_CPCS;
 }
 
 void HAL_timer_disable_interrupt(const uint8_t timer_num) {
-  const tTimerConfig *pConfig = &TimerConfig[timer_num];
+  const tTimerConfig * const pConfig = &TimerConfig[timer_num];
   pConfig->pTimerRegs->TC_CHANNEL[pConfig->channel].TC_IDR = TC_IDR_CPCS;
 }
 
+void HAL_timer_interrupt_enabled(const uint8_t timer_num) {
+  const tTimerConfig * const pConfig = &TimerConfig[timer_num];
+  return pConfig->pTimerRegs->TC_CHANNEL[pConfig->channel].TC_IER == TC_IER_CPCS;
+}
+
 #if 0
   void HAL_timer_set_count(const uint8_t timer_num, const uint32_t count) {
-    const tTimerConfig *pConfig = &TimerConfig[timer_num];
+    const tTimerConfig * const pConfig = &TimerConfig[timer_num];
     TC_SetRC(pConfig->pTimerRegs, pConfig->channel, count);
   }
 
   void HAL_timer_isr_prologue(const uint8_t timer_num) {
-    const tTimerConfig *pConfig = &TimerConfig[timer_num];
+    const tTimerConfig * const pConfig = &TimerConfig[timer_num];
     TC_GetStatus(pConfig->pTimerRegs, pConfig->channel);
   }
 #endif
diff --git a/Marlin/src/HAL/HAL_DUE/HAL_timers_Due.h b/Marlin/src/HAL/HAL_DUE/HAL_timers_Due.h
index b93e80b7d..d4d4a6e51 100644
--- a/Marlin/src/HAL/HAL_DUE/HAL_timers_Due.h
+++ b/Marlin/src/HAL/HAL_DUE/HAL_timers_Due.h
@@ -55,6 +55,7 @@ typedef uint32_t hal_timer_t;
 
 #define ENABLE_STEPPER_DRIVER_INTERRUPT() HAL_timer_enable_interrupt(STEP_TIMER_NUM)
 #define DISABLE_STEPPER_DRIVER_INTERRUPT()  HAL_timer_disable_interrupt(STEP_TIMER_NUM)
+#define STEPPER_ISR_ENABLED() HAL_timer_interrupt_enabled(STEP_TIMER_NUM)
 
 #define ENABLE_TEMPERATURE_INTERRUPT()  HAL_timer_enable_interrupt(TEMP_TIMER_NUM)
 #define DISABLE_TEMPERATURE_INTERRUPT() HAL_timer_disable_interrupt(TEMP_TIMER_NUM)
@@ -91,32 +92,33 @@ extern const tTimerConfig TimerConfig[];
 void HAL_timer_start(const uint8_t timer_num, const uint32_t frequency);
 
 FORCE_INLINE static void HAL_timer_set_count(const uint8_t timer_num, const hal_timer_t count) {
-  const tTimerConfig *pConfig = &TimerConfig[timer_num];
+  const tTimerConfig * const pConfig = &TimerConfig[timer_num];
   pConfig->pTimerRegs->TC_CHANNEL[pConfig->channel].TC_RC = count;
 }
 
 FORCE_INLINE static hal_timer_t HAL_timer_get_count(const uint8_t timer_num) {
-  const tTimerConfig *pConfig = &TimerConfig[timer_num];
+  const tTimerConfig * const pConfig = &TimerConfig[timer_num];
   return pConfig->pTimerRegs->TC_CHANNEL[pConfig->channel].TC_RC;
 }
 
 FORCE_INLINE static void HAL_timer_set_current_count(const uint8_t timer_num, const hal_timer_t count) {
-  const tTimerConfig *pConfig = &TimerConfig[timer_num];
+  const tTimerConfig * const pConfig = &TimerConfig[timer_num];
   pConfig->pTimerRegs->TC_CHANNEL[pConfig->channel].TC_CV = count;
 }
 
 FORCE_INLINE static hal_timer_t HAL_timer_get_current_count(const uint8_t timer_num) {
-  const tTimerConfig *pConfig = &TimerConfig[timer_num];
+  const tTimerConfig * const pConfig = &TimerConfig[timer_num];
   return pConfig->pTimerRegs->TC_CHANNEL[pConfig->channel].TC_CV;
 }
 
 void HAL_timer_enable_interrupt(const uint8_t timer_num);
 void HAL_timer_disable_interrupt(const uint8_t timer_num);
+bool HAL_timer_interrupt_enabled(const uint8_t timer_num);
 
 //void HAL_timer_isr_prologue(const uint8_t timer_num);
 
 FORCE_INLINE static void HAL_timer_isr_prologue(const uint8_t timer_num) {
-  const tTimerConfig *pConfig = &TimerConfig[timer_num];
+  const tTimerConfig * const pConfig = &TimerConfig[timer_num];
   // Reading the status register clears the interrupt flag
   pConfig->pTimerRegs->TC_CHANNEL[pConfig->channel].TC_SR;
 }
diff --git a/Marlin/src/HAL/HAL_LPC1768/HAL_timers.cpp b/Marlin/src/HAL/HAL_LPC1768/HAL_timers.cpp
index c9e25693e..3f7507aa5 100644
--- a/Marlin/src/HAL/HAL_LPC1768/HAL_timers.cpp
+++ b/Marlin/src/HAL/HAL_LPC1768/HAL_timers.cpp
@@ -75,6 +75,14 @@ void HAL_timer_disable_interrupt(const uint8_t timer_num) {
   }
 }
 
+bool HAL_timer_interrupt_enabled(const uint8_t timer_num) {
+  switch (timer_num) {
+    case 0: return NVIC_GetActive(TIMER0_IRQn);
+    case 1: return NVIC_GetActive(TIMER1_IRQn);
+  }
+  return false;
+}
+
 void HAL_timer_isr_prologue(const uint8_t timer_num) {
   switch (timer_num) {
     case 0: SBI(LPC_TIM0->IR, 0); break; // Clear the Interrupt
diff --git a/Marlin/src/HAL/HAL_LPC1768/HAL_timers.h b/Marlin/src/HAL/HAL_LPC1768/HAL_timers.h
index 34b3b24b9..8b4c222af 100644
--- a/Marlin/src/HAL/HAL_LPC1768/HAL_timers.h
+++ b/Marlin/src/HAL/HAL_LPC1768/HAL_timers.h
@@ -58,6 +58,8 @@ typedef uint32_t hal_timer_t;
 
 #define ENABLE_STEPPER_DRIVER_INTERRUPT() HAL_timer_enable_interrupt(STEP_TIMER_NUM)
 #define DISABLE_STEPPER_DRIVER_INTERRUPT() HAL_timer_disable_interrupt(STEP_TIMER_NUM)
+#define STEPPER_ISR_ENABLED() HAL_timer_interrupt_enabled(STEP_TIMER_NUM)
+
 #define ENABLE_TEMPERATURE_INTERRUPT() HAL_timer_enable_interrupt(TEMP_TIMER_NUM)
 #define DISABLE_TEMPERATURE_INTERRUPT() HAL_timer_disable_interrupt(TEMP_TIMER_NUM)
 
@@ -125,6 +127,7 @@ FORCE_INLINE static hal_timer_t HAL_timer_get_current_count(const uint8_t timer_
 
 void HAL_timer_enable_interrupt(const uint8_t timer_num);
 void HAL_timer_disable_interrupt(const uint8_t timer_num);
+bool HAL_timer_interrupt_enabled(const uint8_t timer_num);
 void HAL_timer_isr_prologue(const uint8_t timer_num);
 
 #endif // _HAL_TIMERS_DUE_H
diff --git a/Marlin/src/HAL/HAL_STM32F1/HAL_timers_Stm32f1.cpp b/Marlin/src/HAL/HAL_STM32F1/HAL_timers_Stm32f1.cpp
index 45ed20df1..0d64e86f5 100644
--- a/Marlin/src/HAL/HAL_STM32F1/HAL_timers_Stm32f1.cpp
+++ b/Marlin/src/HAL/HAL_STM32F1/HAL_timers_Stm32f1.cpp
@@ -93,7 +93,7 @@ const tTimerConfig TimerConfig [NUM_HARDWARE_TIMERS] = {
  * TODO: Calculate Timer prescale value, so we get the 32bit to adjust
  */
 
-void HAL_timer_start(uint8_t timer_num, uint32_t frequency) {
+void HAL_timer_start(const uint8_t timer_num, const uint32_t frequency) {
   nvic_irq_num irq_num;
   switch (timer_num) {
     case 1: irq_num = NVIC_TIMER1_CC; break;
@@ -135,7 +135,7 @@ void HAL_timer_start(uint8_t timer_num, uint32_t frequency) {
   }
 }
 
-void HAL_timer_enable_interrupt(uint8_t timer_num) {
+void HAL_timer_enable_interrupt(const uint8_t timer_num) {
   switch (timer_num) {
     case STEP_TIMER_NUM:
       timer_enable_irq(STEP_TIMER_DEV, STEP_TIMER_CHAN);
@@ -148,7 +148,7 @@ void HAL_timer_enable_interrupt(uint8_t timer_num) {
   }
 }
 
-void HAL_timer_disable_interrupt(uint8_t timer_num) {
+void HAL_timer_disable_interrupt(const uint8_t timer_num) {
   switch (timer_num) {
     case STEP_TIMER_NUM:
       timer_disable_irq(STEP_TIMER_DEV, STEP_TIMER_CHAN);
@@ -161,4 +161,12 @@ void HAL_timer_disable_interrupt(uint8_t timer_num) {
   }
 }
 
+bool HAL_timer_interrupt_enabled(const uint8_t timer_num) {
+  switch (timer_num) {
+    case STEP_TIMER_NUM: return bool(TIM_DIER(STEP_TIMER_DEV) & STEP_TIMER_CHAN);
+    case TEMP_TIMER_NUM: return bool(TIM_DIER(TEMP_TIMER_DEV) & TEMP_TIMER_CHAN);
+  }
+  return false;
+}
+
 #endif // __STM32F1__
diff --git a/Marlin/src/HAL/HAL_STM32F1/HAL_timers_Stm32f1.h b/Marlin/src/HAL/HAL_STM32F1/HAL_timers_Stm32f1.h
index bdc083e2a..79c9408ca 100644
--- a/Marlin/src/HAL/HAL_STM32F1/HAL_timers_Stm32f1.h
+++ b/Marlin/src/HAL/HAL_STM32F1/HAL_timers_Stm32f1.h
@@ -62,8 +62,6 @@ typedef uint16_t hal_timer_t;
 #define STEP_TIMER_DEV TIMER_DEV(STEP_TIMER_NUM)
 #define TEMP_TIMER_DEV TIMER_DEV(TEMP_TIMER_NUM)
 
-
-
 //STM32_HAVE_TIMER(n);
 
 #define HAL_TIMER_RATE         (F_CPU)  // frequency of timers peripherals
@@ -79,6 +77,7 @@ typedef uint16_t hal_timer_t;
 
 #define ENABLE_STEPPER_DRIVER_INTERRUPT() timer_enable_irq(STEP_TIMER_DEV, STEP_TIMER_CHAN)
 #define DISABLE_STEPPER_DRIVER_INTERRUPT() timer_disable_irq(STEP_TIMER_DEV, STEP_TIMER_CHAN)
+#define STEPPER_ISR_ENABLED() HAL_timer_interrupt_enabled(STEP_TIMER_NUM)
 
 #define ENABLE_TEMPERATURE_INTERRUPT() timer_enable_irq(TEMP_TIMER_DEV, TEMP_TIMER_CHAN)
 #define DISABLE_TEMPERATURE_INTERRUPT() timer_disable_irq(TEMP_TIMER_DEV, TEMP_TIMER_CHAN)
@@ -113,9 +112,10 @@ static HardwareTimer TempTimer(TEMP_TIMER_NUM);
 // Public functions
 // --------------------------------------------------------------------------
 
-void HAL_timer_start(uint8_t timer_num, uint32_t frequency);
-void HAL_timer_enable_interrupt(uint8_t timer_num);
-void HAL_timer_disable_interrupt(uint8_t timer_num);
+void HAL_timer_start(const uint8_t timer_num, const uint32_t frequency);
+void HAL_timer_enable_interrupt(const uint8_t timer_num);
+void HAL_timer_disable_interrupt(const uint8_t timer_num);
+bool HAL_timer_interrupt_enabled(const uint8_t timer_num);
 
 /**
  * NOTE: By default libmaple sets ARPE = 1, which means the Auto reload register is preloaded (will only update with an update event)
diff --git a/Marlin/src/HAL/HAL_TEENSY35_36/HAL_timers_Teensy.cpp b/Marlin/src/HAL/HAL_TEENSY35_36/HAL_timers_Teensy.cpp
index 6a2d39234..03bf9dadd 100644
--- a/Marlin/src/HAL/HAL_TEENSY35_36/HAL_timers_Teensy.cpp
+++ b/Marlin/src/HAL/HAL_TEENSY35_36/HAL_timers_Teensy.cpp
@@ -67,6 +67,14 @@ void HAL_timer_disable_interrupt(const uint8_t timer_num) {
   }
 }
 
+bool HAL_timer_interrupt_enabled(const uint8_t timer_num) {
+  switch (timer_num) {
+    case 0: return NVIC_IS_ENABLED(IRQ_FTM0);
+    case 1: return NVIC_IS_ENABLED(IRQ_FTM1);
+  }
+  return false;
+}
+
 void HAL_timer_isr_prologue(const uint8_t timer_num) {
   switch(timer_num) {
     case 0:
diff --git a/Marlin/src/HAL/HAL_TEENSY35_36/HAL_timers_Teensy.h b/Marlin/src/HAL/HAL_TEENSY35_36/HAL_timers_Teensy.h
index 3675ea60b..9482bee5b 100644
--- a/Marlin/src/HAL/HAL_TEENSY35_36/HAL_timers_Teensy.h
+++ b/Marlin/src/HAL/HAL_TEENSY35_36/HAL_timers_Teensy.h
@@ -68,6 +68,8 @@ typedef uint32_t hal_timer_t;
 
 #define ENABLE_STEPPER_DRIVER_INTERRUPT() HAL_timer_enable_interrupt(STEP_TIMER_NUM)
 #define DISABLE_STEPPER_DRIVER_INTERRUPT() HAL_timer_disable_interrupt(STEP_TIMER_NUM)
+#define STEPPER_ISR_ENABLED() HAL_timer_interrupt_enabled(STEP_TIMER_NUM)
+
 #define ENABLE_TEMPERATURE_INTERRUPT() HAL_timer_enable_interrupt(TEMP_TIMER_NUM)
 #define DISABLE_TEMPERATURE_INTERRUPT() HAL_timer_disable_interrupt(TEMP_TIMER_NUM)
 
@@ -110,8 +112,8 @@ FORCE_INLINE static hal_timer_t HAL_timer_get_current_count(const uint8_t timer_
 
 void HAL_timer_enable_interrupt(const uint8_t timer_num);
 void HAL_timer_disable_interrupt(const uint8_t timer_num);
+bool HAL_timer_interrupt_enabled(const uint8_t timer_num);
 
 void HAL_timer_isr_prologue(const uint8_t timer_num);
 
 #endif // _HAL_TIMERS_TEENSY_H
-
diff --git a/Marlin/src/core/serial.h b/Marlin/src/core/serial.h
index 5f90cd0e7..5606f4772 100644
--- a/Marlin/src/core/serial.h
+++ b/Marlin/src/core/serial.h
@@ -61,25 +61,6 @@ enum DebugFlags {
   };
 #endif
 
-//todo: HAL: breaks encapsulation
-// For AVR only, define a serial interface based on configuration
-#ifdef __AVR__
-  #ifdef USBCON
-    #include <HardwareSerial.h>
-    #if ENABLED(BLUETOOTH)
-      #define MYSERIAL0 bluetoothSerial
-    #else
-      #define MYSERIAL0 Serial
-    #endif // BLUETOOTH
-  #else
-    #include "../HAL/HAL_AVR/MarlinSerial.h"
-    #define MYSERIAL0 customizedSerial
-  #endif
-#elif defined(ARDUINO_ARCH_SAM)
-  // To pull the Serial port definitions and overrides
-  #include "../HAL/HAL_DUE/MarlinSerial_Due.h"
-#endif
-
 extern uint8_t marlin_debug_flags;
 #define DEBUGGING(F) (marlin_debug_flags & (DEBUG_## F))
 
diff --git a/Marlin/src/module/planner.cpp b/Marlin/src/module/planner.cpp
index 701fe0e36..2d0e4ce38 100644
--- a/Marlin/src/module/planner.cpp
+++ b/Marlin/src/module/planner.cpp
@@ -1352,7 +1352,9 @@ void Planner::_buffer_steps(const int32_t (&target)[XYZE], float fr_mm_s, const
 
   // Initialize block entry speed. Compute based on deceleration to user-defined MINIMUM_PLANNER_SPEED.
   const float v_allowable = max_allowable_speed(-block->acceleration, MINIMUM_PLANNER_SPEED, block->millimeters);
-  block->entry_speed = min(vmax_junction, v_allowable);
+  // If stepper ISR is disabled, this indicates buffer_segment wants to add a split block.
+  // In this case start with the max. allowed speed to avoid an interrupted first move.
+  block->entry_speed = STEPPER_ISR_ENABLED() ? MINIMUM_PLANNER_SPEED : min(vmax_junction, v_allowable);
 
   // Initialize planner efficiency flags
   // Set flag if block will always reach maximum junction speed regardless of entry/exit speeds.
@@ -1362,7 +1364,7 @@ void Planner::_buffer_steps(const int32_t (&target)[XYZE], float fr_mm_s, const
   // block nominal speed limits both the current and next maximum junction speeds. Hence, in both
   // the reverse and forward planners, the corresponding block junction speed will always be at the
   // the maximum junction speed and may always be ignored for any speed reduction checks.
-  block->flag |= BLOCK_FLAG_RECALCULATE | (block->nominal_speed <= v_allowable ? BLOCK_FLAG_NOMINAL_LENGTH : 0);
+  block->flag |= block->nominal_speed <= v_allowable ? BLOCK_FLAG_RECALCULATE | BLOCK_FLAG_NOMINAL_LENGTH : BLOCK_FLAG_RECALCULATE;
 
   // Update previous path unit_vector and nominal speed
   COPY(previous_speed, current_speed);
@@ -1382,7 +1384,7 @@ void Planner::_buffer_steps(const int32_t (&target)[XYZE], float fr_mm_s, const
      *                                      In that case, the retract and move will be executed together.
      *                                      This leads to too many advance steps due to a huge e_acceleration.
      *                                      The math is good, but we must avoid retract moves with advance!
-     * lin_dist_e > 0                     : Extruder is running forward (e.g., for "Wipe while retracting" (Slic3r) or "Combing" (Cura) moves)
+     * lin_dist_e > 0                       : Extruder is running forward (e.g., for "Wipe while retracting" (Slic3r) or "Combing" (Cura) moves)
      */
     block->use_advance_lead =  esteps && (block->steps[X_AXIS] || block->steps[Y_AXIS])
                             && extruder_advance_k
@@ -1398,9 +1400,6 @@ void Planner::_buffer_steps(const int32_t (&target)[XYZE], float fr_mm_s, const
 
   #endif // LIN_ADVANCE
 
-  const float bnsr = 1.0 / block->nominal_speed;
-  calculate_trapezoid_for_block(block, block->entry_speed * bnsr, safe_speed * bnsr);
-
   // Move buffer head
   block_buffer_head = next_buffer_head;
 
diff --git a/Marlin/src/module/planner.h b/Marlin/src/module/planner.h
index efdd4c7be..a904caacb 100644
--- a/Marlin/src/module/planner.h
+++ b/Marlin/src/module/planner.h
@@ -534,6 +534,16 @@ class Planner {
     static block_t* get_current_block() {
       if (blocks_queued()) {
         block_t * const block = &block_buffer[block_buffer_tail];
+
+        // If the block has no trapezoid calculated, it's unsafe to execute.
+        if (movesplanned() > 1) {
+          const block_t * const next = &block_buffer[next_block_index(block_buffer_tail)];
+          if (TEST(block->flag, BLOCK_BIT_RECALCULATE) || TEST(next->flag, BLOCK_BIT_RECALCULATE))
+            return NULL;
+        }
+        else if (TEST(block->flag, BLOCK_BIT_RECALCULATE))
+          return NULL;
+
         #if ENABLED(ULTRA_LCD)
           block_buffer_runtime_us -= block->segment_time_us; // We can't be sure how long an active block will take, so don't count it.
         #endif