Make ADC sensor reading frequency adjustable

Marlin/temperature.h

@@ -50,6 +50,49 @@
   #define EXTRUDER_IDX  active_extruder
 #endif
 
+/**
+ * States for ADC reading in the ISR
+ */
+enum ADCSensorState {
+  #if HAS_TEMP_0
+    PrepareTemp_0,
+    MeasureTemp_0,
+  #endif
+  #if HAS_TEMP_1
+    PrepareTemp_1,
+    MeasureTemp_1,
+  #endif
+  #if HAS_TEMP_2
+    PrepareTemp_2,
+    MeasureTemp_2,
+  #endif
+  #if HAS_TEMP_3
+    PrepareTemp_3,
+    MeasureTemp_3,
+  #endif
+  #if HAS_TEMP_4
+    PrepareTemp_4,
+    MeasureTemp_4,
+  #endif
+  #if HAS_TEMP_BED
+    PrepareTemp_BED,
+    MeasureTemp_BED,
+  #endif
+  #if ENABLED(FILAMENT_WIDTH_SENSOR)
+    Prepare_FILWIDTH,
+    Measure_FILWIDTH,
+  #endif
+  SensorsReady, // Temperatures ready. Delay the next round of readings to let ADC pins settle.
+  StartupDelay  // Startup, delay initial temp reading a tiny bit so the hardware can settle
+};
+
+// Minimum number of Temperature::ISR loops between sensor readings.
+// Multiplied by 16 (OVERSAMPLENR) to obtain the total time to
+// get all oversampled sensor readings
+#define MIN_ADC_ISR_LOOPS 10
+
+#define ACTUAL_ADC_SAMPLES max(int(MIN_ADC_ISR_LOOPS), int(SensorsReady))
+
 class Temperature {
 
   public:
@@ -74,7 +117,7 @@ class Temperature {
     #endif
 
     #if ENABLED(PIDTEMP) || ENABLED(PIDTEMPBED)
-      #define PID_dT ((OVERSAMPLENR * 12.0)/(F_CPU / 64.0 / 256.0))
+      #define PID_dT ((OVERSAMPLENR * float(ACTUAL_ADC_SAMPLES)) / (F_CPU / 64.0 / 256.0))
     #endif
 
     #if ENABLED(PIDTEMP)