🐛 Fix PID upon entering PID_FUNCTIONAL_RANGE (MarlinFirmware#26926)

The PID algorithm did not cache the last seen temperature until it entered the PID_FUNCTIONAL_RANGE. This caused an incorrect output power to be calculated temporarily while the algorithm caught up.

This has likely always been a problem for bed and chamber PID. For the hotend this error was introduced in refactoring in commit 54e7b93.

Marlin/src/module/temperature.h

@@ -204,31 +204,35 @@ typedef struct { float p, i, d, c, f; } raw_pidcf_t;
 
       float get_pid_output(const float target, const float current) {
         const float pid_error = target - current;
+        float output_pow;
         if (!target || pid_error < -(PID_FUNCTIONAL_RANGE)) {
           pid_reset = true;
-          return 0;
+          output_pow = 0;
         }
         else if (pid_error > PID_FUNCTIONAL_RANGE) {
           pid_reset = true;
-          return MAX_POW;
+          output_pow = MAX_POW;
         }
+        else {
+          if (pid_reset) {
+            pid_reset = false;
+            temp_iState = 0.0;
+            work_d = 0.0;
+          }
 
-        if (pid_reset) {
-          pid_reset = false;
-          temp_iState = 0.0;
-          work_d = 0.0;
-        }
+          const float max_power_over_i_gain = float(MAX_POW) / Ki - float(MIN_POW);
+          temp_iState = constrain(temp_iState + pid_error, 0, max_power_over_i_gain);
 
-        const float max_power_over_i_gain = float(MAX_POW) / Ki - float(MIN_POW);
-        temp_iState = constrain(temp_iState + pid_error, 0, max_power_over_i_gain);
+          work_p = Kp * pid_error;
+          work_i = Ki * temp_iState;
+          work_d = work_d + PID_K2 * (Kd * (temp_dState - current) - work_d);
 
-        work_p = Kp * pid_error;
-        work_i = Ki * temp_iState;
-        work_d = work_d + PID_K2 * (Kd * (temp_dState - current) - work_d);
+          output_pow = constrain(work_p + work_i + work_d + float(MIN_POW), 0, MAX_POW);
+        }
 
         temp_dState = current;
 
-        return constrain(work_p + work_i + work_d + float(MIN_POW), 0, MAX_POW);
+        return output_pow;
       }
 
   };