push script to generate NTC lut

fw/main.c

@@ -28,11 +28,12 @@
 // constants
 // LUT for converting NTC readings to degrees kelvin
 // Nominal: 1kOhm, Beta: 3380, Step: 64
-// TODO: Since the board temperature is almost always in the 300-200K range add
-// more steps in order to better represent that interval
 const uint8_t ntc_step_size = 64;
-const int16_t ntc_table[64] = {
-    1180, 197, 155, 133, 119, 108, 100, 93, 87, 82, 77, 73, 69, 66, 63, 60, 57, 54, 52, 50, 47, 45, 43, 41, 39, 37, 35, 34, 32, 30, 28, 27, 25, 23, 22, 20, 19, 17, 15, 14, 12, 11, 9, 7, 6, 4, 2, 0, -1, -3, -5, -7, -9, -11, -14, -16, -19, -22, -25, -28, -33, -38, -44, -55
+const int16_t ntc_lut[] = {
+	1316, 197, 155, 133, 119, 108, 100, 93, 87, 82, 77, 73, 69, 66, 63, 60,
+	57, 54, 52, 50, 47, 45, 43, 41, 39, 37, 35, 34, 32, 30, 28, 27,
+	25, 23, 22, 20, 19, 17, 15, 14, 12, 11, 9, 7, 6, 4, 2, 0,
+	-1, -3, -5, -7, -9, -11, -14, -16, -19, -22, -25, -28, -32, -38, -44, -55
 };
 
 
@@ -50,8 +51,8 @@ static inline int16_t get_temp_k(uint16_t adc_reading)
 	if (adc_reading > 4095) return 0;
 	uint8_t index = adc_reading / ntc_step_size;
 	uint8_t remainder = adc_reading % ntc_step_size;
-	int16_t temp_base = index < 64 ? ntc_table[index] : 0;
-    int16_t temp_next = index < 63 ? ntc_table[index + 1] : temp_base;
+	int16_t temp_base = index < 64 ? ntc_lut[index] : 0;
+    int16_t temp_next = index < 63 ? ntc_lut[index + 1] : temp_base;
     return temp_base + ((temp_next - temp_base) * remainder)/ntc_step_size;
 }
 

fw/script/ntc_lut.py

@@ -0,0 +1,59 @@
+#!/usr/bin/env python3
+
+import math
+
+# Script to generate NTC lookup table
+#   ^ Vcc
+#   |
+# [RES]
+#   |
+#   +-- Vadc
+#   |
+# [NTC]
+#   |
+#  _|_ GND
+#  ///
+
+adc_res = 12
+ntc_value = 10_000 # at 25°C
+resistor_value = 10_000
+ntc_beta = 3380 # at 25°C
+steps = 64
+
+# NTC resistance to temperature (°C) conversion
+def resistance_to_t(ntc_r):
+	# Standard B-parameter equation
+    # T = 1 / (1/T0 + 1/B * ln(R/R0))
+	inv_t = 1/(25 + 273.15) + math.log(ntc_r/ntc_value)/ntc_beta
+	return 1/inv_t - 273.15
+
+# Resistor divider percentage to NTC resistance conversion
+# x = NTC / (NTC + RES)  => NTC = RES * x / (1 - x)
+def x_to_r(x):
+	x = max(0.0001, min(0.9999, x))
+	return resistor_value * x / (1 - x)
+
+# Use 4096 for cleaner divisions
+adc_max = 2**adc_res
+step_size = adc_max // steps
+
+values = []
+for i in range(steps):
+	adc_val = i * step_size
+	x = adc_val / adc_max
+	r = x_to_r(adc_val / adc_max)
+	t = resistance_to_t(r)
+	values.append(int(round(t)))
+
+def to_c_array(values, ctype="float", name="table", formatter=str, colcount=8):
+    # apply formatting to each element
+    values = [formatter(v) for v in values]
+    # split into rows with up to `colcount` elements per row
+    rows = [values[i:i+colcount] for i in range(0, len(values), colcount)]
+    # separate elements with commas, separate rows with newlines
+    body = ',\n    '.join([', '.join(r) for r in rows])
+    # assemble components into the complete string
+    return '{} {}[] = {{\n    {}}};'.format(ctype, name, body)
+
+print("uint8_t ntc_step_size = {};".format(int(step_size)))
+print(to_c_array(values, ctype="int16_t", name="ntc_lut", formatter=str, colcount=16))