temp: Actually calculate temperature values

Do the monster math to convert ADC readings into appropriate temperature
values.  The absolute result is way off but it seems to respond sanely
in relative terms.

Additionally, apply a low pass filter with f=1Hz to smoothen the noise a
tiny bit.

Misc/Temp-Monitor/firmware.py

@@ -1,20 +1,40 @@
-import time
+import math
 from cs1237 import CS1237
 from machine import Pin
 
 clock = Pin(2)
 data = Pin(4)
 
-cs1237 = CS1237(clock, data)
-cs1237.config(gain=1)
+ADC_RATE = 40
 
+cs1237 = CS1237(clock, data)
+cs1237.config(gain=1, rate=ADC_RATE)
+
+print(repr(cs1237.get_config()))
+
+THERMISTOR_BETA = 3950.0
+THERMISTOR_R25 = 10e3
+
+THERMISTOR_RINF = THERMISTOR_R25 * math.exp(-THERMISTOR_BETA / 298.15)
+
+# Filter constant in seconds
+FILTER_RC = 1
+FILTER_A = 1 / ADC_RATE / (FILTER_RC / 1 / ADC_RATE)
+
+temperature_filtered = 25
 while True:
     raw_value = cs1237.read()
-    value = float(raw_value) / (2**23 - 1)
+    value = float(raw_value) / (2**24 - 1)
+
     if value > 0.9999:
         value = 0
     else:
-        value = 1 / (1 / value - 1)
-    temperature = 25 - value
-    if abs(temperature) < 50:
-        print(temperature)
+        value = 10e3 * 1 / (1 / value - 1)
+
+    temperature = THERMISTOR_BETA / math.log(value / THERMISTOR_RINF) - 273.15
+
+    temperature_filtered = (
+        temperature_filtered * (1 - FILTER_A) + temperature * FILTER_A
+    )
+
+    print(temperature_filtered)