fix for PT1000 negative temp calculation

Author: ladyada

Adafruit_MAX31865.cpp

@@ -122,7 +122,7 @@ float  Adafruit_MAX31865::temperature(float RTDnominal, float refResistor) {
   Rt /= 32768;
   Rt *= refResistor;
 
-  //Serial.print("Resistance: "); Serial.println(Rt, 8);
+  // Serial.print("\nResistance: "); Serial.println(Rt, 8);
 
   Z1 = -RTD_A;
   Z2 = RTD_A * RTD_A - (4 * RTD_B);
@@ -135,6 +135,9 @@ float  Adafruit_MAX31865::temperature(float RTDnominal, float refResistor) {
   if (temp >= 0) return temp;
 
   // ugh.
+  Rt /= RTDnominal;
+  Rt *= 100;      // normalize to 100 ohm
+
   float rpoly = Rt;
 
   temp = -242.02;

examples/max31865/max31865.ino

@@ -21,9 +21,11 @@ Adafruit_MAX31865 max = Adafruit_MAX31865(10, 11, 12, 13);
 // use hardware SPI, just pass in the CS pin
 //Adafruit_MAX31865 max = Adafruit_MAX31865(10);
 
-// The value of the Rref resistor. Use 430.0!
-#define RREF 430.0
-
+// The value of the Rref resistor. Use 430.0 for PT100 and 4300.0 for PT1000
+#define RREF      430.0
+// The 'nominal' 0-degrees-C resistance of the sensor
+// 100.0 for PT100, 1000.0 for PT1000
+#define RNOMINAL  100.0
 
 void setup() {
   Serial.begin(115200);
@@ -41,7 +43,7 @@ void loop() {
   ratio /= 32768;
   Serial.print("Ratio = "); Serial.println(ratio,8);
   Serial.print("Resistance = "); Serial.println(RREF*ratio,8);
-  Serial.print("Temperature = "); Serial.println(max.temperature(100, RREF));
+  Serial.print("Temperature = "); Serial.println(max.temperature(RNOMINAL, RREF));
 
   // Check and print any faults
   uint8_t fault = max.readFault();

library.properties

@@ -1,5 +1,5 @@
 name=Adafruit MAX31865 library
-version=1.0.0
+version=1.0.1
 author=Adafruit
 maintainer=Adafruit <[email protected]>
 sentence=Library for the Adafruit RTD Amplifier breakout with MAX31865