spi_temperature: Limit maximum temperature in MAX31865.calc_adc() to …

…melting point of platinum (Klipper3d#6320)

Limit the maximum temperature in MAX31865.calc_adc() to the melting
point of platinum. Above this temperature the Callendar-Van Dusem
formula does not make sense. The default value for max_temp is
99999999.9 and at this temperature the result of this formula is
negative. This sets max_sample_value to 0 which causes the mcu
to shutdown.

Set max adc value to (1<<15)-1 .  This is needed because the max value of the adc register
of the MAX31865 is 0b1111 1111 1111 1110 which represents
32767 and not 32768.

Signed-off-by: David van 't Wout <[email protected]>

klippy/extras/spi_temperature.py

@@ -320,9 +320,10 @@ def calc_temp(self, adc):
     def calc_adc(self, temp):
         # Calculate relative resistance via Callendar-Van Dusen formula:
         #  resistance = rtd_nominal_r * (1 + CVD_A * temp + CVD_B * temp**2)
+        temp = min(temp, 1768.3)  # Melting point of platinum
         R_div_nominal = 1. + CVD_A * temp + CVD_B * temp * temp
         adc = int(R_div_nominal / self.adc_to_resist_div_nominal + 0.5)
-        adc = max(0, min(MAX31865_ADC_MAX, adc))
+        adc = max(0, min(MAX31865_ADC_MAX - 1, adc))
         adc = adc << 1 # Add fault bit
         return adc
     def build_spi_init(self, config):