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Implement alternate voltage divider equation

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This commit is contained in:
Leon Krieg
2024-09-20 01:07:31 +02:00
parent 682650dedb
commit db876151d6
2 changed files with 44 additions and 20 deletions
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2
src/bus/i2c.c
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@@ -525,7 +525,7 @@ word I2C_ADS1115_ReadRaw(int channel)
// 110 | ±0.256 V // 110 | ±0.256 V
// 111 | ±0.256 V // 111 | ±0.256 V
gain = 0x0000; // 6.144 V | 00000100 00000000 gain = 0x0000; // 6.144 V | 00000000 00000000
// Input multiplexer configuration: These bits // Input multiplexer configuration: These bits
// configure the input multiplexer. // configure the input multiplexer.

62
src/common/math.c
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@@ -2,41 +2,62 @@
#include <math.h> #include <math.h>
// https://www.sciencedirect.com/science/article/abs/pii/S0263224111002594
// https://en.wikipedia.org/wiki/Steinhart%E2%80%93Hart_equation // https://en.wikipedia.org/wiki/Steinhart%E2%80%93Hart_equation
// https://onlinelibrary.wiley.com/doi/abs/10.1002/andp.18280890511 // https://onlinelibrary.wiley.com/doi/abs/10.1002/andp.18280890511
// https://onlinelibrary.wiley.com/doi/abs/10.1002/andp.18441370202 // https://onlinelibrary.wiley.com/doi/abs/10.1002/andp.18441370202
// https://bmcnoldy.earth.miami.edu/Humidity.html // https://bmcnoldy.earth.miami.edu/Humidity.html
// TODO: Is resistance function semantically correct?
// TODO: Check results for humidity conversion functions.
// Thermistor general constants // Thermistor general constants
#define R1 10000.0f // Resistor ohm value #define VIN 5.0f // Source voltage (VCC)
#define VRES 0xFFFF // 16-bit ADC resolution #define GAIN 6.144f // PGA gain setting 0x00
#define AMAX 0x7FFF // 16-bit ADC resolution
#define R1 10000 // 10k Ohm resistor
// Thermistor coeficients // Thermistor coeficients
#define SC1 1.009249522e-3f #define SC1 1.009249522e-3f
#define SC2 2.378405444e-4f #define SC2 2.378405444e-4f
#define SC3 2.019202697e-7f #define SC3 2.019202697e-7f
// Humidity conversion constants // Conversion constants
#define TH1 100.0f #define TH1 100.0f
#define TH2 243.04f #define TH2 243.04f
#define TH3 17.625f #define TH3 17.625f
float Resistance(int adc_raw) float Resistance(int adc_raw)
{ {
float r2; float av, r2;
// TODO: Improve readability. // [GND] - [10K R] - [ADC IN] - [THERMISTOR] - [VCC]
// TODO: Make sure output is correct.
// [GND] - [10K RESISTOR] - [ADC INPUT] - [THERMISTOR] - [VCC] // The ADC measurement is taken between a 10k Ohm
// resistor and the thermistor. This is also called
// a voltage divider circuit which has the following
// equation:
// return ((VRES - adc_raw) * R1) / adc_raw; // Vout = Vin * (R2 / R1 + R2)
r2 = (float) adc_raw / 264.8f * 0.05f;
return R1 / (5.0f / r2 - 1.0f); // We can solve for resistance R2 by rearranging and
// simplifying the equation:
// R2 = R1 / (Vin / Vout - 1)
// If the ADC reference voltage and voltage divider
// source voltage (Vin) are the same, then the
// following is true:
// Vin / Vout = AMAX / AV
// This means the ratio of voltage divider input
// voltage to output voltage is the same as the ratio
// of the ADC full range value (AMAX) to the value
// returned by the ADC (AV). For a 16 bit ADC like
// the ADS1115 the AMAX is 0x7FFF.
av = adc_raw * (GAIN / VIN);
r2 = R1 / (AMAX / av - 1.0f);
return r2;
} }
float SteinhartHart(float res) float SteinhartHart(float res)
@@ -45,6 +66,9 @@ float SteinhartHart(float res)
// Convert resistance to temperature // Convert resistance to temperature
// When the temperature increases, NTC thermistor
// resistance will decrease:
// 25C = 10000 Ohms // 25C = 10000 Ohms
// 100C = 6744 Ohms // 100C = 6744 Ohms