🚸 Temperature Variance Monitor tweaks (#23571)

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

Marlin/Configuration_adv.h

@@ -368,10 +368,35 @@
 
 #if ANY(THERMAL_PROTECTION_HOTENDS, THERMAL_PROTECTION_BED, THERMAL_PROTECTION_CHAMBER, THERMAL_PROTECTION_COOLER)
   /**
-   * Thermal Protection Variance Monitor - EXPERIMENTAL.
-   * Kill the machine on a stuck temperature sensor. Disable if you get false positives.
+   * Thermal Protection Variance Monitor - EXPERIMENTAL
+   * Kill the machine on a stuck temperature sensor.
+   *
+   * This feature may cause some thermally-stable systems to halt. Be sure to test it throughly under
+   * a variety of conditions. Disable if you get false positives.
+   *
+   * This feature ensures that temperature sensors are updating regularly. If sensors die or get "stuck",
+   * or if Marlin stops reading them, temperatures will remain constant while heaters may still be powered!
+   * This feature only monitors temperature changes so it should catch any issue, hardware or software.
+   *
+   * By default it uses the THERMAL_PROTECTION_*_PERIOD constants (above) for the time window, within which
+   * at least one temperature change must occur, to indicate that sensor polling is working. If any monitored
+   * heater's temperature remains totally constant (without even a fractional change) during this period, a
+   * thermal malfunction error occurs and the printer is halted.
+   *
+   * A very stable heater might produce a false positive and halt the printer. In this case, try increasing
+   * the corresponding THERMAL_PROTECTION_*_PERIOD constant a bit. Keep in mind that uncontrolled heating
+   * shouldn't be allowed to persist for more than a minite or two.
+   *
+   * Be careful to distinguish false positives from real sensor issues before disabling this feature. If the
+   * heater's temperature appears even slightly higher than expected after restarting, you may have a real
+   * thermal malfunction. Check the temperature graph in your host for any unusual bumps.
    */
-  //#define THERMAL_PROTECTION_VARIANCE_MONITOR   // Detect a sensor malfunction preventing temperature updates
+  //#define THERMAL_PROTECTION_VARIANCE_MONITOR
+  #if ENABLED(THERMAL_PROTECTION_VARIANCE_MONITOR)
+    // Variance detection window to override the THERMAL_PROTECTION...PERIOD settings above.
+    // Keep in mind that some heaters heat up faster than others.
+    //#define THERMAL_PROTECTION_VARIANCE_MONITOR_PERIOD 30  // (s) Override all watch periods
+  #endif
 #endif
 
 #if ENABLED(PIDTEMP)

Marlin/src/HAL/LPC1768/HAL.cpp

@@ -28,6 +28,7 @@
 DefaultSerial1 USBSerial(false, UsbSerial);
 
 uint32_t MarlinHAL::adc_result = 0;
+pin_t MarlinHAL::adc_pin = 0;
 
 // U8glib required functions
 extern "C" {

Marlin/src/HAL/LPC1768/HAL.h

@@ -241,15 +241,18 @@ public:
 
   // Begin ADC sampling on the given pin. Called from Temperature::isr!
   static uint32_t adc_result;
-  static void adc_start(const pin_t pin) {
-    adc_result = FilteredADC::read(pin) >> (16 - HAL_ADC_RESOLUTION); // returns 16bit value, reduce to required bits
-  }
+  static pin_t adc_pin;
+
+  static void adc_start(const pin_t pin) { adc_pin = pin; }
 
   // Is the ADC ready for reading?
-  static bool adc_ready() { return true; }
+  static bool adc_ready() { return LPC176x::adc_hardware.done(LPC176x::pin_get_adc_channel(adc_pin)); }
 
   // The current value of the ADC register
-  static uint16_t adc_value() { return uint16_t(adc_result); }
+  static uint16_t adc_value() { 
+    adc_result = FilteredADC::read(adc_pin) >> (16 - HAL_ADC_RESOLUTION); // returns 16bit value, reduce to required bits
+    return uint16_t(adc_result); 
+  }
 
   /**
    * Set the PWM duty cycle for the pin to the given value.

Marlin/src/inc/Warnings.cpp

@@ -63,6 +63,11 @@
   #warning "Warning! Don't use dummy thermistors (998/999) for final build!"
 #endif
 
+#if ANY(THERMAL_PROTECTION_HOTENDS, THERMAL_PROTECTION_BED, THERMAL_PROTECTION_CHAMBER, THERMAL_PROTECTION_COOLER) \
+  && NONE(THERMAL_PROTECTION_VARIANCE_MONITOR, NO_VARIANCE_MONITOR_WARNING)
+  #warning "THERMAL_PROTECTION_VARIANCE_MONITOR is recommended. See Configuration_adv.h for details. (Define NO_VARIANCE_MONITOR_WARNING to suppress this.)"
+#endif
+
 #if NONE(HAS_RESUME_CONTINUE, HOST_PROMPT_SUPPORT)
   #warning "Your Configuration provides no method to acquire user feedback!"
 #endif

Marlin/src/module/temperature.cpp

@@ -2915,11 +2915,18 @@ void Temperature::init() {
     */
 
     #if ENABLED(THERMAL_PROTECTION_VARIANCE_MONITOR)
+
+      #ifdef THERMAL_PROTECTION_VARIANCE_MONITOR_PERIOD
+        #define VARIANCE_WINDOW THERMAL_PROTECTION_VARIANCE_MONITOR_PERIOD
+      #else
+        #define VARIANCE_WINDOW period_seconds
+      #endif
+
       if (state == TRMalfunction) { // temperature invariance may continue, regardless of heater state
         variance += ABS(current - last_temp); // no need for detection window now, a single change in variance is enough
         last_temp = current;
         if (!NEAR_ZERO(variance)) {
-          variance_timer = millis() + SEC_TO_MS(period_seconds);
+          variance_timer = millis() + SEC_TO_MS(VARIANCE_WINDOW);
           variance = 0.0;
           state = TRStable; // resume from where we detected the problem
         }
@@ -2980,7 +2987,7 @@ void Temperature::init() {
               state = TRMalfunction;
               break;
             }
-            variance_timer = now + SEC_TO_MS(period_seconds);
+            variance_timer = now + SEC_TO_MS(VARIANCE_WINDOW);
             variance = 0.0;
             last_temp = current;
           }