Added temperature window to M109. This is needed for systems with no overshoot PID.

Marlin/Configuration.h

 // Actual temperature must be close to target for this long before M109 returns success
 #define TEMP_RESIDENCY_TIME 10	// (seconds)
 #define TEMP_HYSTERESIS 3       // (degC) range of +/- temperatures considered "close" to the target one
+#define TEMP_WINDOW     1       // (degC) Window around target to start the recidency timer x degC early.
 
 // The minimal temperature defines the temperature below which the heater will not be enabled It is used
 // to check that the wiring to the thermistor is not broken. 

Marlin/Marlin.pde

 // M240 - Trigger a camera to take a photograph
 // M301 - Set PID parameters P I and D
 // M302 - Allow cold extrudes
+// M303 - PID relay autotune S<temperature> sets the target temperature. (default target temperature = 150C)
 // M400 - Finish all moves
 // M500 - stores paramters in EEPROM
 // M501 - reads parameters from EEPROM (if you need reset them after you changed them temporarily).  
 // M502 - reverts to the default "factory settings".  You still need to store them in EEPROM afterwards if you want to.
 // M503 - print the current settings (from memory not from eeprom)
-// M303 - PID relay autotune S<temperature> sets the target temperature. (default target temperature = 150C)
 // M999 - Restart after being stopped by error
 
 //Stepper Movement Variables
         #ifdef TEMP_RESIDENCY_TIME
             /* start/restart the TEMP_RESIDENCY_TIME timer whenever we reach target temp for the first time
               or when current temp falls outside the hysteresis after target temp was reached */
-          if ((residencyStart == -1 &&  target_direction && !isHeatingHotend(tmp_extruder)) ||
-              (residencyStart == -1 && !target_direction && !isCoolingHotend(tmp_extruder)) ||
+          if ((residencyStart == -1 &&  target_direction && (degHotend(tmp_extruder) >= (degTargetHotend(tmp_extruder)-TEMP_WINDOW))) ||
+              (residencyStart == -1 && !target_direction && (degHotend(tmp_extruder) <= (degTargetHotend(tmp_extruder)+TEMP_WINDOW))) ||
               (residencyStart > -1 && labs(degHotend(tmp_extruder) - degTargetHotend(tmp_extruder)) > TEMP_HYSTERESIS) ) 
           {
             residencyStart = millis();