🐛 Resolve DUE Servo pulse issue (#24305)

Co-authored-by: sjasonsmith <20053467+sjasonsmith@users.noreply.github.com>
Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>

Marlin/src/HAL/AVR/Servo.cpp

@@ -67,26 +67,25 @@ static volatile int8_t Channel[_Nbr_16timers];              // counter for the s
 /************ static functions common to all instances ***********************/
 
 static inline void handle_interrupts(const timer16_Sequence_t timer, volatile uint16_t* TCNTn, volatile uint16_t* OCRnA) {
-  if (Channel[timer] < 0)
-    *TCNTn = 0; // channel set to -1 indicated that refresh interval completed so reset the timer
-  else {
-    if (SERVO_INDEX(timer, Channel[timer]) < ServoCount && SERVO(timer, Channel[timer]).Pin.isActive)
-      extDigitalWrite(SERVO(timer, Channel[timer]).Pin.nbr, LOW); // pulse this channel low if activated
-  }
-
-  Channel[timer]++;    // increment to the next channel
-  if (SERVO_INDEX(timer, Channel[timer]) < ServoCount && Channel[timer] < SERVOS_PER_TIMER) {
-    *OCRnA = *TCNTn + SERVO(timer, Channel[timer]).ticks;
-    if (SERVO(timer, Channel[timer]).Pin.isActive)    // check if activated
-      extDigitalWrite(SERVO(timer, Channel[timer]).Pin.nbr, HIGH); // it's an active channel so pulse it high
+  int8_t cho = Channel[timer];                                        // Handle the prior Channel[timer] first
+  if (cho < 0)                                                        // Channel -1 indicates the refresh interval completed...
+    *TCNTn = 0;                                                       // ...so reset the timer
+  else if (SERVO_INDEX(timer, cho) < ServoCount)                      // prior channel handled?
+    extDigitalWrite(SERVO(timer, cho).Pin.nbr, LOW);                  // pulse the prior channel LOW
+
+  Channel[timer] = ++cho;                                             // Handle the next channel (or 0)
+  if (cho < SERVOS_PER_TIMER && SERVO_INDEX(timer, cho) < ServoCount) {
+    *OCRnA = *TCNTn + SERVO(timer, cho).ticks;                        // set compare to current ticks plus duration
+    if (SERVO(timer, cho).Pin.isActive)                               // activated?
+      extDigitalWrite(SERVO(timer, cho).Pin.nbr, HIGH);               // yes: pulse HIGH
   }
   else {
     // finished all channels so wait for the refresh period to expire before starting over
-    if (((unsigned)*TCNTn) + 4 < usToTicks(REFRESH_INTERVAL))    // allow a few ticks to ensure the next OCR1A not missed
-      *OCRnA = (unsigned int)usToTicks(REFRESH_INTERVAL);
-    else
-      *OCRnA = *TCNTn + 4;  // at least REFRESH_INTERVAL has elapsed
-    Channel[timer] = -1; // this will get incremented at the end of the refresh period to start again at the first channel
+    const unsigned int cval = ((unsigned)*TCNTn) + 32 / (SERVO_TIMER_PRESCALER), // allow 32 cycles to ensure the next OCR1A not missed
+                       ival = (unsigned int)usToTicks(REFRESH_INTERVAL); // at least REFRESH_INTERVAL has elapsed
+    *OCRnA = max(cval, ival);
+
+    Channel[timer] = -1;                                              // reset the timer counter to 0 on the next call
   }
 }
 
@@ -123,91 +122,102 @@ static inline void handle_interrupts(const timer16_Sequence_t timer, volatile ui
 
 /****************** end of static functions ******************************/
 
-void initISR(timer16_Sequence_t timer) {
-  #ifdef _useTimer1
-    if (timer == _timer1) {
-      TCCR1A = 0;             // normal counting mode
-      TCCR1B = _BV(CS11);     // set prescaler of 8
-      TCNT1 = 0;              // clear the timer count
-      #if defined(__AVR_ATmega8__) || defined(__AVR_ATmega128__)
-        SBI(TIFR, OCF1A);      // clear any pending interrupts;
-        SBI(TIMSK, OCIE1A);    // enable the output compare interrupt
-      #else
-        // here if not ATmega8 or ATmega128
-        SBI(TIFR1, OCF1A);     // clear any pending interrupts;
-        SBI(TIMSK1, OCIE1A);   // enable the output compare interrupt
-      #endif
-      #ifdef WIRING
-        timerAttach(TIMER1OUTCOMPAREA_INT, Timer1Service);
-      #endif
-    }
-  #endif
-
-  #ifdef _useTimer3
-    if (timer == _timer3) {
-      TCCR3A = 0;             // normal counting mode
-      TCCR3B = _BV(CS31);     // set prescaler of 8
-      TCNT3 = 0;              // clear the timer count
-      #ifdef __AVR_ATmega128__
-        SBI(TIFR, OCF3A);     // clear any pending interrupts;
-        SBI(ETIMSK, OCIE3A);  // enable the output compare interrupt
-      #else
-        SBI(TIFR3, OCF3A);   // clear any pending interrupts;
-        SBI(TIMSK3, OCIE3A); // enable the output compare interrupt
-      #endif
-      #ifdef WIRING
-        timerAttach(TIMER3OUTCOMPAREA_INT, Timer3Service);  // for Wiring platform only
-      #endif
-    }
-  #endif
-
-  #ifdef _useTimer4
-    if (timer == _timer4) {
-      TCCR4A = 0;             // normal counting mode
-      TCCR4B = _BV(CS41);     // set prescaler of 8
-      TCNT4 = 0;              // clear the timer count
-      TIFR4 = _BV(OCF4A);     // clear any pending interrupts;
-      TIMSK4 = _BV(OCIE4A);   // enable the output compare interrupt
-    }
-  #endif
-
-  #ifdef _useTimer5
-    if (timer == _timer5) {
-      TCCR5A = 0;             // normal counting mode
-      TCCR5B = _BV(CS51);     // set prescaler of 8
-      TCNT5 = 0;              // clear the timer count
-      TIFR5 = _BV(OCF5A);     // clear any pending interrupts;
-      TIMSK5 = _BV(OCIE5A);   // enable the output compare interrupt
-    }
-  #endif
-}
-
-void finISR(timer16_Sequence_t timer) {
-  // Disable use of the given timer
-  #ifdef WIRING
-    if (timer == _timer1) {
-      CBI(
-        #if defined(__AVR_ATmega1281__) || defined(__AVR_ATmega2561__)
-          TIMSK1
+void initISR(const timer16_Sequence_t timer_index) {
+  switch (timer_index) {
+    default: break;
+
+    #ifdef _useTimer1
+      case _timer1:
+        TCCR1A = 0;             // normal counting mode
+        TCCR1B = _BV(CS11);     // set prescaler of 8
+        TCNT1 = 0;              // clear the timer count
+        #if defined(__AVR_ATmega8__) || defined(__AVR_ATmega128__)
+          SBI(TIFR, OCF1A);      // clear any pending interrupts;
+          SBI(TIMSK, OCIE1A);    // enable the output compare interrupt
         #else
-          TIMSK
+          // here if not ATmega8 or ATmega128
+          SBI(TIFR1, OCF1A);     // clear any pending interrupts;
+          SBI(TIMSK1, OCIE1A);   // enable the output compare interrupt
         #endif
-          , OCIE1A);    // disable timer 1 output compare interrupt
-      timerDetach(TIMER1OUTCOMPAREA_INT);
-    }
-    else if (timer == _timer3) {
-      CBI(
-        #if defined(__AVR_ATmega1281__) || defined(__AVR_ATmega2561__)
-          TIMSK3
+        #ifdef WIRING
+          timerAttach(TIMER1OUTCOMPAREA_INT, Timer1Service);
+        #endif
+        break;
+    #endif
+
+    #ifdef _useTimer3
+      case _timer3:
+        TCCR3A = 0;             // normal counting mode
+        TCCR3B = _BV(CS31);     // set prescaler of 8
+        TCNT3 = 0;              // clear the timer count
+        #ifdef __AVR_ATmega128__
+          SBI(TIFR, OCF3A);     // clear any pending interrupts;
+          SBI(ETIMSK, OCIE3A);  // enable the output compare interrupt
         #else
-          ETIMSK
+          SBI(TIFR3, OCF3A);   // clear any pending interrupts;
+          SBI(TIMSK3, OCIE3A); // enable the output compare interrupt
+        #endif
+        #ifdef WIRING
+          timerAttach(TIMER3OUTCOMPAREA_INT, Timer3Service);  // for Wiring platform only
         #endif
-          , OCIE3A);    // disable the timer3 output compare A interrupt
-      timerDetach(TIMER3OUTCOMPAREA_INT);
+        break;
+    #endif
+
+    #ifdef _useTimer4
+      case _timer4:
+        TCCR4A = 0;             // normal counting mode
+        TCCR4B = _BV(CS41);     // set prescaler of 8
+        TCNT4 = 0;              // clear the timer count
+        TIFR4 = _BV(OCF4A);     // clear any pending interrupts;
+        TIMSK4 = _BV(OCIE4A);   // enable the output compare interrupt
+        break;
+    #endif
+
+    #ifdef _useTimer5
+      case _timer5:
+        TCCR5A = 0;             // normal counting mode
+        TCCR5B = _BV(CS51);     // set prescaler of 8
+        TCNT5 = 0;              // clear the timer count
+        TIFR5 = _BV(OCF5A);     // clear any pending interrupts;
+        TIMSK5 = _BV(OCIE5A);   // enable the output compare interrupt
+        break;
+    #endif
+  }
+}
+
+void finISR(const timer16_Sequence_t timer_index) {
+  // Disable use of the given timer
+  #ifdef WIRING
+    switch (timer_index) {
+      default: break;
+
+      case _timer1:
+        CBI(
+          #if defined(__AVR_ATmega1281__) || defined(__AVR_ATmega2561__)
+            TIMSK1
+          #else
+            TIMSK
+          #endif
+          , OCIE1A    // disable timer 1 output compare interrupt
+        );
+        timerDetach(TIMER1OUTCOMPAREA_INT);
+        break;
+
+      case _timer3:
+        CBI(
+          #if defined(__AVR_ATmega1281__) || defined(__AVR_ATmega2561__)
+            TIMSK3
+          #else
+            ETIMSK
+          #endif
+          , OCIE3A    // disable the timer3 output compare A interrupt
+        );
+        timerDetach(TIMER3OUTCOMPAREA_INT);
+        break;
     }
   #else // !WIRING
     // For arduino - in future: call here to a currently undefined function to reset the timer
-    UNUSED(timer);
+    UNUSED(timer_index);
   #endif
 }
 

Marlin/src/HAL/DUE/Servo.cpp

@@ -47,7 +47,7 @@
 #include "../shared/servo.h"
 #include "../shared/servo_private.h"
 
-static volatile int8_t Channel[_Nbr_16timers];              // counter for the servo being pulsed for each timer (or -1 if refresh interval)
+static Flags<_Nbr_16timers> DisablePending; // ISR should disable the timer at the next timer reset
 
 // ------------------------
 /// Interrupt handler for the TC0 channel 1.
@@ -71,82 +71,91 @@ void Servo_Handler(const timer16_Sequence_t, Tc*, const uint8_t);
 #endif
 
 void Servo_Handler(const timer16_Sequence_t timer, Tc *tc, const uint8_t channel) {
-  // clear interrupt
-  tc->TC_CHANNEL[channel].TC_SR;
-  if (Channel[timer] < 0)
-    tc->TC_CHANNEL[channel].TC_CCR |= TC_CCR_SWTRG; // channel set to -1 indicated that refresh interval completed so reset the timer
-  else if (SERVO_INDEX(timer, Channel[timer]) < ServoCount && SERVO(timer, Channel[timer]).Pin.isActive)
-    extDigitalWrite(SERVO(timer, Channel[timer]).Pin.nbr, LOW); // pulse this channel low if activated
-
-  Channel[timer]++;    // increment to the next channel
-  if (SERVO_INDEX(timer, Channel[timer]) < ServoCount && Channel[timer] < SERVOS_PER_TIMER) {
-    tc->TC_CHANNEL[channel].TC_RA = tc->TC_CHANNEL[channel].TC_CV + SERVO(timer,Channel[timer]).ticks;
-    if (SERVO(timer,Channel[timer]).Pin.isActive)    // check if activated
-      extDigitalWrite(SERVO(timer, Channel[timer]).Pin.nbr, HIGH); // its an active channel so pulse it high
+  static int8_t Channel[_Nbr_16timers];                               // Servo counters to pulse (or -1 for refresh interval)
+  int8_t cho = Channel[timer];                                        // Handle the prior Channel[timer] first
+  if (cho < 0) {                                                      // Channel -1 indicates the refresh interval completed...
+    tc->TC_CHANNEL[channel].TC_CCR |= TC_CCR_SWTRG;                   // ...so reset the timer
+    if (DisablePending[timer]) {
+      // Disabling only after the full servo period expires prevents
+      // pulses being too close together if immediately re-enabled.
+      DisablePending.clear(timer);
+      TC_Stop(tc, channel);
+      tc->TC_CHANNEL[channel].TC_SR;                                  // clear interrupt
+      return;
+    }
+  }
+  else if (SERVO_INDEX(timer, cho) < ServoCount)                      // prior channel handled?
+    extDigitalWrite(SERVO(timer, cho).Pin.nbr, LOW);                  // pulse the prior channel LOW
+
+  Channel[timer] = ++cho;                                             // go to the next channel (or 0)
+  if (cho < SERVOS_PER_TIMER && SERVO_INDEX(timer, cho) < ServoCount) {
+    tc->TC_CHANNEL[channel].TC_RA = tc->TC_CHANNEL[channel].TC_CV + SERVO(timer, cho).ticks;
+    if (SERVO(timer, cho).Pin.isActive)                               // activated?
+      extDigitalWrite(SERVO(timer, cho).Pin.nbr, HIGH);               // yes: pulse HIGH
   }
   else {
     // finished all channels so wait for the refresh period to expire before starting over
-    tc->TC_CHANNEL[channel].TC_RA =
-      tc->TC_CHANNEL[channel].TC_CV < usToTicks(REFRESH_INTERVAL) - 4
-        ? (unsigned int)usToTicks(REFRESH_INTERVAL) // allow a few ticks to ensure the next OCR1A not missed
-        : tc->TC_CHANNEL[channel].TC_CV + 4;        // at least REFRESH_INTERVAL has elapsed
-    Channel[timer] = -1; // this will get incremented at the end of the refresh period to start again at the first channel
+    const unsigned int cval = tc->TC_CHANNEL[channel].TC_CV + 128 / (SERVO_TIMER_PRESCALER), // allow 128 cycles to ensure the next CV not missed
+                       ival = (unsigned int)usToTicks(REFRESH_INTERVAL); // at least REFRESH_INTERVAL has elapsed
+    tc->TC_CHANNEL[channel].TC_RA = max(cval, ival);
+
+    Channel[timer] = -1;                                              // reset the timer CCR on the next call
   }
+
+  tc->TC_CHANNEL[channel].TC_SR;                                      // clear interrupt
 }
 
 static void _initISR(Tc *tc, uint32_t channel, uint32_t id, IRQn_Type irqn) {
   pmc_enable_periph_clk(id);
   TC_Configure(tc, channel,
-    TC_CMR_TCCLKS_TIMER_CLOCK3 | // MCK/32
-    TC_CMR_WAVE |                // Waveform mode
-    TC_CMR_WAVSEL_UP_RC );       // Counter running up and reset when equals to RC
-
-  /* 84MHz, MCK/32, for 1.5ms: 3937 */
-  TC_SetRA(tc, channel, 2625); // 1ms
-
-  /* Configure and enable interrupt */
+      TC_CMR_WAVE                   // Waveform mode
+    | TC_CMR_WAVSEL_UP_RC           // Counter running up and reset when equal to RC
+    | (SERVO_TIMER_PRESCALER ==   2 ? TC_CMR_TCCLKS_TIMER_CLOCK1 : 0) // MCK/2
+    | (SERVO_TIMER_PRESCALER ==   8 ? TC_CMR_TCCLKS_TIMER_CLOCK2 : 0) // MCK/8
+    | (SERVO_TIMER_PRESCALER ==  32 ? TC_CMR_TCCLKS_TIMER_CLOCK3 : 0) // MCK/32
+    | (SERVO_TIMER_PRESCALER == 128 ? TC_CMR_TCCLKS_TIMER_CLOCK4 : 0) // MCK/128
+  );
+
+  // Wait 1ms before the first ISR
+  TC_SetRA(tc, channel, (F_CPU) / (SERVO_TIMER_PRESCALER) / 1000UL); // 1ms
+
+  // Configure and enable interrupt
   NVIC_EnableIRQ(irqn);
-  // TC_IER_CPAS: RA Compare
-  tc->TC_CHANNEL[channel].TC_IER = TC_IER_CPAS;
+  tc->TC_CHANNEL[channel].TC_IER = TC_IER_CPAS; // TC_IER_CPAS: RA Compare
 
   // Enables the timer clock and performs a software reset to start the counting
   TC_Start(tc, channel);
 }
 
-void initISR(const timer16_Sequence_t timer) {
-  #ifdef _useTimer1
-    if (timer == _timer1) _initISR(TC_FOR_TIMER1, CHANNEL_FOR_TIMER1, ID_TC_FOR_TIMER1, IRQn_FOR_TIMER1);
-  #endif
-  #ifdef _useTimer2
-    if (timer == _timer2) _initISR(TC_FOR_TIMER2, CHANNEL_FOR_TIMER2, ID_TC_FOR_TIMER2, IRQn_FOR_TIMER2);
-  #endif
-  #ifdef _useTimer3
-    if (timer == _timer3) _initISR(TC_FOR_TIMER3, CHANNEL_FOR_TIMER3, ID_TC_FOR_TIMER3, IRQn_FOR_TIMER3);
-  #endif
-  #ifdef _useTimer4
-    if (timer == _timer4) _initISR(TC_FOR_TIMER4, CHANNEL_FOR_TIMER4, ID_TC_FOR_TIMER4, IRQn_FOR_TIMER4);
-  #endif
-  #ifdef _useTimer5
-    if (timer == _timer5) _initISR(TC_FOR_TIMER5, CHANNEL_FOR_TIMER5, ID_TC_FOR_TIMER5, IRQn_FOR_TIMER5);
-  #endif
+void initISR(const timer16_Sequence_t timer_index) {
+  CRITICAL_SECTION_START();
+  const bool disable_soon = DisablePending[timer_index];
+  DisablePending.clear(timer_index);
+  CRITICAL_SECTION_END();
+
+  if (!disable_soon) switch (timer_index) {
+    default: break;
+    #ifdef _useTimer1
+      case _timer1: return _initISR(TC_FOR_TIMER1, CHANNEL_FOR_TIMER1, ID_TC_FOR_TIMER1, IRQn_FOR_TIMER1);
+    #endif
+    #ifdef _useTimer2
+      case _timer2: return _initISR(TC_FOR_TIMER2, CHANNEL_FOR_TIMER2, ID_TC_FOR_TIMER2, IRQn_FOR_TIMER2);
+    #endif
+    #ifdef _useTimer3
+      case _timer3: return _initISR(TC_FOR_TIMER3, CHANNEL_FOR_TIMER3, ID_TC_FOR_TIMER3, IRQn_FOR_TIMER3);
+    #endif
+    #ifdef _useTimer4
+      case _timer4: return _initISR(TC_FOR_TIMER4, CHANNEL_FOR_TIMER4, ID_TC_FOR_TIMER4, IRQn_FOR_TIMER4);
+    #endif
+    #ifdef _useTimer5
+      case _timer5: return _initISR(TC_FOR_TIMER5, CHANNEL_FOR_TIMER5, ID_TC_FOR_TIMER5, IRQn_FOR_TIMER5);
+    #endif
+  }
 }
 
-void finISR(timer16_Sequence_t) {
-  #ifdef _useTimer1
-    TC_Stop(TC_FOR_TIMER1, CHANNEL_FOR_TIMER1);
-  #endif
-  #ifdef _useTimer2
-    TC_Stop(TC_FOR_TIMER2, CHANNEL_FOR_TIMER2);
-  #endif
-  #ifdef _useTimer3
-    TC_Stop(TC_FOR_TIMER3, CHANNEL_FOR_TIMER3);
-  #endif
-  #ifdef _useTimer4
-    TC_Stop(TC_FOR_TIMER4, CHANNEL_FOR_TIMER4);
-  #endif
-  #ifdef _useTimer5
-    TC_Stop(TC_FOR_TIMER5, CHANNEL_FOR_TIMER5);
-  #endif
+void finISR(const timer16_Sequence_t timer_index) {
+  // Timer is disabled from the ISR, to ensure proper final pulse length.
+  DisablePending.set(timer_index);
 }
 
 #endif // HAS_SERVOS

Marlin/src/HAL/DUE/ServoTimers.h

@@ -37,7 +37,7 @@
 #define _useTimer5
 
 #define TRIM_DURATION             2   // compensation ticks to trim adjust for digitalWrite delays
-#define SERVO_TIMER_PRESCALER     32  // timer prescaler
+#define SERVO_TIMER_PRESCALER     2   // timer prescaler
 
 /*
   TC0, chan 0 => TC0_Handler

Marlin/src/HAL/DUE/timers.cpp

@@ -89,10 +89,17 @@ void HAL_timer_start(const uint8_t timer_num, const uint32_t frequency) {
   NVIC_SetPriority(irq, timer_config[timer_num].priority);
 
   // wave mode, reset counter on match with RC,
-  TC_Configure(tc, channel, TC_CMR_WAVE | TC_CMR_WAVSEL_UP_RC | TC_CMR_TCCLKS_TIMER_CLOCK1);
+  TC_Configure(tc, channel,
+      TC_CMR_WAVE
+    | TC_CMR_WAVSEL_UP_RC
+    | (HAL_TIMER_PRESCALER ==   2 ? TC_CMR_TCCLKS_TIMER_CLOCK1 : 0)
+    | (HAL_TIMER_PRESCALER ==   8 ? TC_CMR_TCCLKS_TIMER_CLOCK2 : 0)
+    | (HAL_TIMER_PRESCALER ==  32 ? TC_CMR_TCCLKS_TIMER_CLOCK3 : 0)
+    | (HAL_TIMER_PRESCALER == 128 ? TC_CMR_TCCLKS_TIMER_CLOCK4 : 0)
+  );
 
   // Set compare value
-  TC_SetRC(tc, channel, VARIANT_MCK / 2 / frequency);
+  TC_SetRC(tc, channel, VARIANT_MCK / (HAL_TIMER_PRESCALER) / frequency);
 
   // And start timer
   TC_Start(tc, channel);

Marlin/src/HAL/DUE/timers.h

@@ -35,7 +35,8 @@
 typedef uint32_t hal_timer_t;
 #define HAL_TIMER_TYPE_MAX 0xFFFFFFFF
 
-#define HAL_TIMER_RATE         ((F_CPU) / 2)    // frequency of timers peripherals
+#define HAL_TIMER_PRESCALER    2
+#define HAL_TIMER_RATE         ((F_CPU) / (HAL_TIMER_PRESCALER))  // frequency of timers peripherals
 
 #ifndef MF_TIMER_STEP
   #define MF_TIMER_STEP         2  // Timer Index for Stepper

Marlin/src/HAL/SAMD51/Servo.cpp

@@ -77,7 +77,8 @@ HAL_SERVO_TIMER_ISR() {
   ;
   const uint8_t tcChannel = TIMER_TCCHANNEL(timer);
 
-  if (currentServoIndex[timer] < 0) {
+  int8_t cho = currentServoIndex[timer];                // Handle the prior servo first
+  if (cho < 0) {                                        // Servo -1 indicates the refresh interval completed...
     #if defined(_useTimer1) && defined(_useTimer2)
       if (currentServoIndex[timer ^ 1] >= 0) {
         // Wait for both channels
@@ -86,41 +87,33 @@ HAL_SERVO_TIMER_ISR() {
         return;
       }
     #endif
-    tc->COUNT16.COUNT.reg = TC_COUNTER_START_VAL;
+    tc->COUNT16.COUNT.reg = TC_COUNTER_START_VAL;       // ...so reset the timer
     SYNC(tc->COUNT16.SYNCBUSY.bit.COUNT);
   }
-  else if (SERVO_INDEX(timer, currentServoIndex[timer]) < ServoCount && SERVO(timer, currentServoIndex[timer]).Pin.isActive)
-    digitalWrite(SERVO(timer, currentServoIndex[timer]).Pin.nbr, LOW);      // pulse this channel low if activated
-
-  // Select the next servo controlled by this timer
-  currentServoIndex[timer]++;
+  else if (SERVO_INDEX(timer, cho) < ServoCount)        // prior channel handled?
+    digitalWrite(SERVO(timer, cho).Pin.nbr, LOW);       // pulse the prior channel LOW
 
-  if (SERVO_INDEX(timer, currentServoIndex[timer]) < ServoCount && currentServoIndex[timer] < SERVOS_PER_TIMER) {
-    if (SERVO(timer, currentServoIndex[timer]).Pin.isActive)                // check if activated
-      digitalWrite(SERVO(timer, currentServoIndex[timer]).Pin.nbr, HIGH);   // it's an active channel so pulse it high
+  currentServoIndex[timer] = ++cho;                     // go to the next channel (or 0)
+  if (cho < SERVOS_PER_TIMER && SERVO_INDEX(timer, cho) < ServoCount) {
+    if (SERVO(timer, cho).Pin.isActive)                 // activated?
+      digitalWrite(SERVO(timer, cho).Pin.nbr, HIGH);    // yes: pulse HIGH
 
-    tc->COUNT16.CC[tcChannel].reg = getTimerCount() - (uint16_t)SERVO(timer, currentServoIndex[timer]).ticks;
+    tc->COUNT16.CC[tcChannel].reg = getTimerCount() - (uint16_t)SERVO(timer, cho).ticks;
   }
   else {
     // finished all channels so wait for the refresh period to expire before starting over
-    currentServoIndex[timer] = -1;   // this will get incremented at the end of the refresh period to start again at the first channel
-
-    const uint16_t tcCounterValue = getTimerCount();
-
-    if ((TC_COUNTER_START_VAL - tcCounterValue) + 4UL < usToTicks(REFRESH_INTERVAL))  // allow a few ticks to ensure the next OCR1A not missed
-      tc->COUNT16.CC[tcChannel].reg = TC_COUNTER_START_VAL - (uint16_t)usToTicks(REFRESH_INTERVAL);
-    else
-      tc->COUNT16.CC[tcChannel].reg = (uint16_t)(tcCounterValue - 4UL);               // at least REFRESH_INTERVAL has elapsed
+    currentServoIndex[timer] = -1;                                          // reset the timer COUNT.reg on the next call
+    const uint16_t cval = getTimerCount() - 256 / (SERVO_TIMER_PRESCALER),  // allow 256 cycles to ensure the next CV not missed
+                   ival = (TC_COUNTER_START_VAL) - (uint16_t)usToTicks(REFRESH_INTERVAL); // at least REFRESH_INTERVAL has elapsed
+    tc->COUNT16.CC[tcChannel].reg = min(cval, ival);
   }
   if (tcChannel == 0) {
     SYNC(tc->COUNT16.SYNCBUSY.bit.CC0);
-    // Clear the interrupt
-    tc->COUNT16.INTFLAG.reg = TC_INTFLAG_MC0;
+    tc->COUNT16.INTFLAG.reg = TC_INTFLAG_MC0; // Clear the interrupt
   }
   else {
     SYNC(tc->COUNT16.SYNCBUSY.bit.CC1);
-    // Clear the interrupt
-    tc->COUNT16.INTFLAG.reg = TC_INTFLAG_MC1;
+    tc->COUNT16.INTFLAG.reg = TC_INTFLAG_MC1; // Clear the interrupt
   }
 }
 
@@ -201,9 +194,9 @@ void initISR(const timer16_Sequence_t timer) {
   }
 }
 
-void finISR(const timer16_Sequence_t timer) {
+void finISR(const timer16_Sequence_t timer_index) {
   Tc * const tc = timer_config[SERVO_TC].pTc;
-  const uint8_t tcChannel = TIMER_TCCHANNEL(timer);
+  const uint8_t tcChannel = TIMER_TCCHANNEL(timer_index);
 
   // Disable the match channel interrupt request
   tc->COUNT16.INTENCLR.reg = (tcChannel == 0) ? TC_INTENCLR_MC0 : TC_INTENCLR_MC1;

Marlin/src/HAL/shared/servo_private.h

@@ -94,5 +94,5 @@ extern ServoInfo_t servo_info[MAX_SERVOS];
 
 // Public functions
 
-extern void initISR(const timer16_Sequence_t timer);
-extern void finISR(const timer16_Sequence_t timer);
+void initISR(const timer16_Sequence_t timer_index);
+void finISR(const timer16_Sequence_t timer_index);

Marlin/src/core/serial.cpp

@@ -72,8 +72,8 @@ void serial_print_P(PGM_P str) {
   while (const char c = pgm_read_byte(str++)) SERIAL_CHAR(c);
 }
 
-void serial_echo_start()  { static PGMSTR(echomagic, "echo:"); serial_print_P(echomagic); }
-void serial_error_start() { static PGMSTR(errormagic, "Error:"); serial_print_P(errormagic); }
+void serial_echo_start()  { serial_print(F("echo:")); }
+void serial_error_start() { serial_print(F("Error:")); }
 
 void serial_spaces(uint8_t count) { count *= (PROPORTIONAL_FONT_RATIO); while (count--) SERIAL_CHAR(' '); }
 

Marlin/src/core/types.h

@@ -99,8 +99,8 @@ struct Flags {
   void set(const int n)                    { b |=  (bits_t)_BV(n); }
   void clear(const int n)                  { b &= ~(bits_t)_BV(n); }
   bool test(const int n) const             { return TEST(b, n); }
-  bool operator[](const int n)             { return test(n); }
-  bool operator[](const int n) const       { return test(n); }
+  const bool operator[](const int n)       { return test(n); }
+  const bool operator[](const int n) const { return test(n); }
   int size() const                         { return sizeof(b); }
 };
 
@@ -113,8 +113,8 @@ struct Flags<1> {
   void set(const int)                     { b = true; }
   void clear(const int)                   { b = false; }
   bool test(const int) const              { return b; }
-  bool operator[](const int)              { return b; }
-  bool operator[](const int) const        { return b; }
+  bool& operator[](const int)             { return b; }
+  bool  operator[](const int) const       { return b; }
   int size() const                        { return sizeof(b); }
 };