From 12b5c8f7f45c8e20a6935330d8723c28a731df74 Mon Sep 17 00:00:00 2001
From: Thomas Kolb <cfr34k-git@tkolb.de>
Date: Fri, 29 Sep 2023 20:50:16 +0200
Subject: [PATCH] Fixed the NTC temperature calculation

---
 src/temp_sensor.c | 25 ++++++++++++-------------
 1 file changed, 12 insertions(+), 13 deletions(-)

diff --git a/src/temp_sensor.c b/src/temp_sensor.c
index 2990ef7..0df956e 100644
--- a/src/temp_sensor.c
+++ b/src/temp_sensor.c
@@ -46,10 +46,11 @@ static fxp_t adc_val_to_pin_voltage(uint16_t adc_val)
 
 static fxp_t calc_temperature_ntc(uint16_t adc_val)
 {
-	static const fxp_t ln_r_ntc_nom = 705030; // ln(47kΩ) converted to fxp_t with 16 fractional bits
-	static const fxp_t r1 = FXP_FROM_INT(10000);
+	// note: all resistor values in kΩ! The factor 1000 is removed from the numbers!
+	static const fxp_t ln_r_ntc_nom = 252323; // ln(47 kΩ) converted to fxp_t with 16 fractional bits
+	static const fxp_t r1 = FXP_FROM_INT(10);
 	static const fxp_t b_constant = FXP_FROM_INT(4125);
-	static const fxp_t ntc_temp_nom = 19539558; // (273.15+25) * 2**16
+	static const fxp_t ntc_temp_nom_inv = 220; // 1/(273.15+25) * 2**16
 	static const fxp_t celsius2kelvin = 17901158; // (273.15) * 2**16
 
 	fxp_t v_r1 = adc_val_to_pin_voltage(adc_val);
@@ -59,16 +60,14 @@ static fxp_t calc_temperature_ntc(uint16_t adc_val)
 
 	fxp_t ln_r_ntc = fxp_from_float(logf(fxp_to_float(r_ntc)));
 
-	fxp_t temp_k = fxp_div(FXP_FROM_INT(1.0),
-			fxp_div(
-				fxp_add(
-					fxp_div(
-						fxp_sub(
-							ln_r_ntc,
-							ln_r_ntc_nom),
-						b_constant),
-					FXP_FROM_INT(1)),
-				ntc_temp_nom));
+	fxp_t temp_k = fxp_div(FXP_FROM_INT(1),
+			fxp_add(
+				fxp_div(
+					fxp_sub(
+						ln_r_ntc,
+						ln_r_ntc_nom),
+					b_constant),
+				ntc_temp_nom_inv));
 
 	return fxp_sub(temp_k, celsius2kelvin);
 }