Step timing cleanup and rounding fix (#16258)

Marlin/src/module/stepper.cpp

 #if DISABLED(MIXING_EXTRUDER)
   #define E_APPLY_STEP(v,Q) E_STEP_WRITE(stepper_extruder, v)
 #endif
+
+#define CYCLES_TO_NS(CYC) (1000UL * (CYC) / ((F_CPU) / 1000000))
+constexpr uint32_t NS_PER_PULSE_TIMER_TICK = 1000000000UL / (STEPPER_TIMER_RATE);
+
+// Round up when converting from ns to timer ticks
+constexpr uint32_t NS_TO_PULSE_TIMER_TICKS(uint32_t NS) { return (NS + (NS_PER_PULSE_TIMER_TICK) / 2) / (NS_PER_PULSE_TIMER_TICK); }
+
 #define TIMER_SETUP_NS (CYCLES_TO_NS(TIMER_READ_ADD_AND_STORE_CYCLES))
 
 #define PULSE_HIGH_TICK_COUNT hal_timer_t(NS_TO_PULSE_TIMER_TICKS(_MIN_PULSE_HIGH_NS - _MIN(_MIN_PULSE_HIGH_NS, TIMER_SETUP_NS)))
   // Take multiple steps per interrupt (For high speed moves)
   bool firstStep = true;
   xyze_bool_t step_needed{0};
-  hal_timer_t end_tick_count;
+  hal_timer_t end_tick_count = 0;
 
   do {
     #define _APPLY_STEP(AXIS) AXIS ##_APPLY_STEP
 
     // Step E stepper if we have steps
     bool firstStep = true;
-    hal_timer_t end_tick_count;
+    hal_timer_t end_tick_count = 0;
 
     while (LA_steps) {
       #if (MINIMUM_STEPPER_PULSE || MAXIMUM_STEPPER_RATE) && DISABLED(I2S_STEPPER_STREAM)

Marlin/src/module/stepper.h

 // Estimate the amount of time the Stepper ISR will take to execute
 //
 
+/**
+ * The method of calculating these cycle-constants is unclear.
+ * Most of them are no longer used directly for pulse timing, and exist
+ * only to estimate a maximum step rate based on the user's configuration.
+ * As 32-bit processors continue to diverge, maintaining cycle counts
+ * will become increasingly difficult and error-prone.
+ */
+
 #ifdef CPU_32_BIT
+  /**
+   * Duration of START_TIMED_PULSE
+   * 
+   * ...as measured on an LPC1768 with a scope and converted to cycles.
+   * Not applicable to other 32-bit processors, but as long as others
+   * take longer, pulses will be longer. For example the SKR Pro
+   * (stm32f407zgt6) requires ~60 cyles.
+   */
   #define TIMER_READ_ADD_AND_STORE_CYCLES 34UL
 
   // The base ISR takes 792 cycles
   #define ISR_STEPPER_CYCLES 16UL
 
 #else
+  // Cycles to perform actions in START_TIMED_PULSE
   #define TIMER_READ_ADD_AND_STORE_CYCLES 13UL
 
   // The base ISR takes 752 cycles
   #define MIN_STEPPER_PULSE_CYCLES _MIN_STEPPER_PULSE_CYCLES(1UL)
 #endif
 
-// Calculate the minimum ticks of the PULSE timer that must elapse with the step pulse enabled
-// adding the "start stepper pulse" code section execution cycles to account for that not all
-// pulses start at the beginning of the loop, so an extra time must be added to compensate so
-// the last generated pulse (usually the extruder stepper) has the right length
+// Calculate the minimum pulse times (high and low)
 #if MINIMUM_STEPPER_PULSE && MAXIMUM_STEPPER_RATE
   constexpr uint32_t _MIN_STEP_PERIOD_NS = 1000000000UL / MAXIMUM_STEPPER_RATE;
   constexpr uint32_t _MIN_PULSE_HIGH_NS = 1000UL * MINIMUM_STEPPER_PULSE;
   constexpr uint32_t _MIN_PULSE_LOW_NS = _MAX((_MIN_STEP_PERIOD_NS - _MIN(_MIN_STEP_PERIOD_NS, _MIN_PULSE_HIGH_NS)), _MIN_PULSE_HIGH_NS);
 #elif MINIMUM_STEPPER_PULSE
   // Assume 50% duty cycle
-  constexpr uint32_t _MIN_STEP_PERIOD_NS = 1000000000UL / MAXIMUM_STEPPER_RATE;
   constexpr uint32_t _MIN_PULSE_HIGH_NS = 1000UL * MINIMUM_STEPPER_PULSE;
   constexpr uint32_t _MIN_PULSE_LOW_NS = _MIN_PULSE_HIGH_NS;
 #elif MAXIMUM_STEPPER_RATE
   #error "Expected at least one of MINIMUM_STEPPER_PULSE or MAXIMUM_STEPPER_RATE to be defined"
 #endif
 
-// TODO: NS_TO_PULSE_TIMER_TICKS has some rounding issues:
-//   1. PULSE_TIMER_TICKS_PER_US rounds to an integer, which loses 20% of the count for a 2.5 MHz pulse tick (such as for LPC1768)
-//   2. The math currently rounds down to the closes tick. Perhaps should round up.
-constexpr uint32_t NS_TO_PULSE_TIMER_TICKS(uint32_t NS) { return PULSE_TIMER_TICKS_PER_US * (NS) / 1000UL; }
-#define CYCLES_TO_NS(CYC) (1000UL * (CYC) / ((F_CPU) / 1000000))
 
 // But the user could be enforcing a minimum time, so the loop time is
 #define ISR_LOOP_CYCLES (ISR_LOOP_BASE_CYCLES + _MAX(MIN_STEPPER_PULSE_CYCLES, MIN_ISR_LOOP_CYCLES))