diff --git a/Marlin/src/libs/MAX31865.cpp b/Marlin/src/libs/MAX31865.cpp
index c042504cf87..0d709b1b0b4 100644
--- a/Marlin/src/libs/MAX31865.cpp
+++ b/Marlin/src/libs/MAX31865.cpp
@@ -133,13 +133,13 @@ SPISettings MAX31865::spiConfig = SPISettings(
/**
* Initialize the SPI interface and set the number of RTD wires used
*
- * @param wires The number of wires in enum format. Can be MAX31865_2WIRE, MAX31865_3WIRE, or MAX31865_4WIRE.
- * @param zero The resistance of the RTD at 0 degC, in ohms.
- * @param ref The resistance of the reference resistor, in ohms.
- * @param wire The resistance of the wire connecting the sensor to the RTD, in ohms.
+ * @param wires The number of wires as an enum: MAX31865_2WIRE, MAX31865_3WIRE, or MAX31865_4WIRE.
+ * @param zero_res The resistance of the RTD at 0°C, in ohms.
+ * @param ref_res The resistance of the reference resistor, in ohms.
+ * @param wire_res The resistance of the wire connecting the sensor to the RTD, in ohms.
*/
-void MAX31865::begin(max31865_numwires_t wires, float zero_res, float ref_res, float wire_res) {
- zeroRes = zero_res;
+void MAX31865::begin(max31865_numwires_t wires, const_float_t zero_res, const_float_t ref_res, const_float_t wire_res) {
+ resNormalizer = 100.0f / zero_res; // reciprocal of resistance, scaled by 100
refRes = ref_res;
wireRes = wire_res;
@@ -437,42 +437,61 @@ float MAX31865::temperature() {
*
* @return Temperature in C
*/
-float MAX31865::temperature(uint16_t adc_val) {
+float MAX31865::temperature(const uint16_t adc_val) {
return temperature(((adc_val) * RECIPROCAL(32768.0f)) * refRes - wireRes);
}
/**
* Calculate the temperature in C from the RTD resistance.
- * Uses the technique outlined in this PDF:
- * http://www.analog.com/media/en/technical-documentation/application-notes/AN709_0.pdf
*
* @param rtd_res the resistance value in ohms
- * @return the temperature in degC
+ * @return the temperature in °C
*/
float MAX31865::temperature(float rtd_res) {
+
+ rtd_res *= resNormalizer; // normalize to 100 ohm
+
+ // Constants for calculating temperature from the measured RTD resistance.
+ // http://www.analog.com/media/en/technical-documentation/application-notes/AN709_0.pdf
+ constexpr float RTD_Z1 = -0.0039083,
+ RTD_Z2 = +1.758480889e-5,
+ RTD_Z3 = -2.31e-8,
+ RTD_Z4 = -1.155e-6;
+
+ // Callender-Van Dusen equation
float temp = (RTD_Z1 + sqrt(RTD_Z2 + (RTD_Z3 * rtd_res))) * RECIPROCAL(RTD_Z4);
- // From the PDF...
//
// The previous equation is valid only for temperatures of 0°C and above.
// The equation for RRTD(t) that defines negative temperature behavior is a
// fourth-order polynomial (after expanding the third term) and is quite
// impractical to solve for a single expression of temperature as a function
- // of resistance.
+ // of resistance. So here we use a Linear Approximation instead.
//
if (temp < 0) {
- rtd_res = (rtd_res / zeroRes) * 100; // normalize to 100 ohm
- float rpoly = rtd_res;
+ #ifndef MAX31865_APPROX
+ #define MAX31865_APPROX 5
+ #endif
- temp = -242.02 + (2.2228 * rpoly);
- rpoly *= rtd_res; // square
- temp += 2.5859e-3 * rpoly;
- rpoly *= rtd_res; // ^3
- temp -= 4.8260e-6 * rpoly;
- rpoly *= rtd_res; // ^4
- temp -= 2.8183e-8 * rpoly;
- rpoly *= rtd_res; // ^5
- temp += 1.5243e-10 * rpoly;
+ constexpr float RTD_C[] = {
+ #if MAX31865_APPROX == 5
+ -242.02, +2.2228, +2.5859e-3, -4.8260e-6, -2.8183e-8, +1.5243e-10
+ #elif MAX31865_APPROX == 4
+ -241.96, +2.2163, +2.8541e-3, -9.9121e-6, -1.7152e-8
+ #elif MAX31865_APPROX == 3
+ -242.09, +2.2276, +2.5178e-3, -5.8620e-6
+ #else
+ -242.97, +2.2838, +1.4727e-3
+ #endif
+ };
+
+ float rpoly = rtd_res;
+ temp = RTD_C[0];
+ temp += rpoly * RTD_C[1];
+ rpoly *= rtd_res; temp += rpoly * RTD_C[2];
+ if (MAX31865_APPROX >= 3) rpoly *= rtd_res; temp += rpoly * RTD_C[3];
+ if (MAX31865_APPROX >= 4) rpoly *= rtd_res; temp += rpoly * RTD_C[4];
+ if (MAX31865_APPROX >= 5) rpoly *= rtd_res; temp += rpoly * RTD_C[5];
}
return temp;
diff --git a/Marlin/src/libs/MAX31865.h b/Marlin/src/libs/MAX31865.h
index baea455485d..95bde756cee 100644
--- a/Marlin/src/libs/MAX31865.h
+++ b/Marlin/src/libs/MAX31865.h
@@ -73,13 +73,6 @@
#define MAX31865_FAULT_RTDINLOW 0x08 // D3
#define MAX31865_FAULT_OVUV 0x04 // D2
-// http://www.analog.com/media/en/technical-documentation/application-notes/AN709_0.pdf
-// constants for calculating temperature from the measured RTD resistance.
-#define RTD_Z1 -0.0039083
-#define RTD_Z2 0.00001758480889
-#define RTD_Z3 -0.0000000231
-#define RTD_Z4 -0.000001155
-
typedef enum max31865_numwires {
MAX31865_2WIRE = 0,
MAX31865_3WIRE = 1,
@@ -103,7 +96,7 @@ private:
uint16_t spiDelay;
- float zeroRes, refRes, wireRes;
+ float resNormalizer, refRes, wireRes;
#if ENABLED(MAX31865_USE_READ_ERROR_DETECTION)
millis_t lastReadStamp = 0;
@@ -160,7 +153,7 @@ public:
int8_t spi_clk);
#endif
- void begin(max31865_numwires_t wires, float zero_res, float ref_res, float wire_res);
+ void begin(max31865_numwires_t wires, const_float_t zero_res, const_float_t ref_res, const_float_t wire_res);
uint8_t readFault();
void clearFault();
@@ -168,6 +161,6 @@ public:
uint16_t readRaw();
float readResistance();
float temperature();
- float temperature(uint16_t adc_val);
+ float temperature(const uint16_t adc_val);
float temperature(float rtd_res);
};