Fixed a couple of incorrect equations in compensation calculations

Author: Adam Sasine

src/lib.rs

@@ -203,6 +203,14 @@ impl From<Address> for u8 {
     }
 }
 
+/// Output from the BMP390 consists of ADC outputs.
+///
+/// These must be compensated using formulas from the datasheet to obtain the actual temperature and pressure values,
+/// using coefficients stored in non-volatile memory (NVM).
+///
+/// # Datasheet
+/// - Section 3.11 Output compensation.
+/// - Appendix A: Computation formulae reference implementation.
 #[derive(Debug, Clone, Copy, Format)]
 struct CalibrationCoefficients {
     par_t1: f32,
@@ -222,6 +230,8 @@ struct CalibrationCoefficients {
 }
 
 impl CalibrationCoefficients {
+    /// Read the calibration coefficients from the BMP390's NVM registers and convert them to into a set of
+    /// floating-point calibration coefficients for the formulas implemented in the compensation functions.
     async fn try_from_i2c<I: I2c>(address: Address, i2c: &mut I) -> Result<Self, Error<I::Error>> {
         let mut calibration_coefficient_regs = [0; 21];
         i2c.write_read(
@@ -238,7 +248,8 @@ impl CalibrationCoefficients {
     /// Calculate the calibration coefficients from the raw register data in registers [`Register::NVM_PAR_T1_0`] to
     /// [`Register::NVM_PAR_P11`].
     ///
-    /// See: Datasheet Apendix A, Section 8.4
+    /// # Datasheet
+    /// Apendix A, Section 8.4
     fn from_registers(data: &[u8; 21]) -> Self {
         trace!("NVM_PAR: {=[u8]:#04x}", *data);
         let nvm_par_t1: u16 = (data[1] as u16) << 8 | data[0] as u16;
@@ -263,50 +274,54 @@ impl CalibrationCoefficients {
             par_p1: ((nvm_par_p1 as f32) - 16_384.0) / 1_048_576.0, // 2^14 / 2^20
             par_p2: ((nvm_par_p2 as f32) - 16_384.0) / 536_870_912.0, // 2^14 / 2^29
             par_p3: (nvm_par_p3 as f32) / 4_294_967_296.0, // 2^32
-            par_p4: (nvm_par_p4 as f32) / 137_438_953_427.0, // 2^37
+            par_p4: (nvm_par_p4 as f32) / 137_438_953_472.0, // 2^37
             par_p5: (nvm_par_p5 as f32) / 0.125,           // 2^-3
             par_p6: (nvm_par_p6 as f32) / 64.0,            // 2^6
             par_p7: (nvm_par_p7 as f32) / 256.0,           // 2^8
             par_p8: (nvm_par_p8 as f32) / 32768.0,         // 2^15
-            par_p9: (nvm_par_p9 as f32) / 81_474_976_710_656.0, //2^48
-            par_p10: (nvm_par_p10 as f32) / 81_474_976_710_656.0, // 2^48
+            par_p9: (nvm_par_p9 as f32) / 281_474_976_710_656.0, //2^48
+            par_p10: (nvm_par_p10 as f32) / 281_474_976_710_656.0, // 2^48
             par_p11: (nvm_par_p11 as f32) / 36_893_488_147_419_103_232.0, // 2^65
         }
     }
 
     /// Compensate a temperature reading according to calibration coefficients.
     ///
-    /// See: Datasheet Apendix A, Section 8.5
-    fn compensate_temperature(&self, temperature_uncompensated: i32) -> ThermodynamicTemperature {
+    /// # Datasheet
+    /// Apendix A, Section 8.5
+    fn compensate_temperature(&self, temperature_uncompensated: u32) -> ThermodynamicTemperature {
+        // This could be done in fewer expressions, but it's broken down for clarity and to match the datasheet
         let uncompensated = temperature_uncompensated as f32;
-        let partial_1 = uncompensated - self.par_t1;
-        let partial_2 = partial_1 * self.par_t2;
-        let temperature = partial_2 + (partial_1 * partial_1) * self.par_t3;
+        let partial_data1 = uncompensated - self.par_t1;
+        let partial_data2 = partial_data1 * self.par_t2;
+        let temperature = partial_data2 + (partial_data1 * partial_data1) * self.par_t3;
         ThermodynamicTemperature::new::<degree_celsius>(temperature)
     }
 
     /// Compensate a pressure reading according to calibration coefficients.
     ///
-    /// See: Datasheet Apendix A, Section 8.6
-    fn compensate_pressure(&self, temperature: ThermodynamicTemperature, pressure_uncompensated: i32) -> Pressure {
+    /// # Datasheet
+    /// Apendix A, Section 8.6
+    fn compensate_pressure(&self, temperature: ThermodynamicTemperature, pressure_uncompensated: u32) -> Pressure {
+        // This could be done in fewer expressions, but it's broken down for clarity and to match the datasheet
         let uncompensated = pressure_uncompensated as f32;
         let temperature = temperature.get::<degree_celsius>();
-        let partial_1 = self.par_p6 * temperature;
-        let partial_2 = self.par_p7 * temperature * temperature;
-        let partial_3 = self.par_p8 * temperature * temperature * temperature;
-        let partial_out1 = self.par_p5 + partial_1 + partial_2 + partial_3;
-
-        let partial_1 = self.par_p2 * temperature;
-        let partial_2 = self.par_p3 * temperature * temperature;
-        let partial_3 = self.par_p4 * temperature * temperature * temperature;
-        let partial_out2 = uncompensated * (self.par_p1 + partial_1 + partial_2 + partial_3);
-
-        let partial_1 = uncompensated * uncompensated;
-        let partial_2 = self.par_p9 + self.par_p10 * temperature;
-        let partial_3 = partial_1 * partial_2;
-        let partial_4 = partial_3 + temperature * temperature * temperature * temperature * self.par_p11;
-
-        let pressure = partial_out1 + partial_out2 + partial_4;
+        let partial_data1 = self.par_p6 * temperature;
+        let partial_data2 = self.par_p7 * temperature * temperature;
+        let partial_data3 = self.par_p8 * temperature * temperature * temperature;
+        let partial_out1 = self.par_p5 + partial_data1 + partial_data2 + partial_data3;
+
+        let partial_data1 = self.par_p2 * temperature;
+        let partial_data2 = self.par_p3 * temperature * temperature;
+        let partial_data3 = self.par_p4 * temperature * temperature * temperature;
+        let partial_out2 = uncompensated * (self.par_p1 + partial_data1 + partial_data2 + partial_data3);
+
+        let partial_data1 = uncompensated * uncompensated;
+        let partial_data2 = self.par_p9 + self.par_p10 * temperature;
+        let partial_data3 = partial_data1 * partial_data2;
+        let partial_data4 = partial_data3 + uncompensated * uncompensated * uncompensated * self.par_p11;
+
+        let pressure = partial_out1 + partial_out2 + partial_data4;
         Pressure::new::<pascal>(pressure)
     }
 
@@ -475,11 +490,8 @@ where
             .map_err(Error::I2c)?;
 
         // DATA_3 is the LSB, DATA_5 is the MSB
-        let temperature = i32::from(read[0]) | i32::from(read[1]) << 8 | i32::from(read[2]) << 16;
-        let temperature = self
-            .coefficients
-            .compensate_temperature(temperature);
-
+        let temperature = u32::from(read[0]) | u32::from(read[1]) << 8 | u32::from(read[2]) << 16;
+        let temperature = self.coefficients.compensate_temperature(temperature);
         Ok(temperature)
     }
 
@@ -502,10 +514,10 @@ where
         trace!("DATA = {=[u8]:#04x}", read);
 
         // pressure is 0:2 (XLSB, LSB, MSB), temperature is 3:5 (XLSB, LSB, MSB)
-        let temperature = i32::from(read[3]) | i32::from(read[4]) << 8 | i32::from(read[5]) << 16;
+        let temperature = u32::from(read[3]) | u32::from(read[4]) << 8 | u32::from(read[5]) << 16;
         let temperature = self.coefficients.compensate_temperature(temperature);
 
-        let pressure = i32::from(read[0]) | i32::from(read[1]) << 8 | i32::from(read[2]) << 16;
+        let pressure = u32::from(read[0]) | u32::from(read[1]) << 8 | u32::from(read[2]) << 16;
         let pressure = self.coefficients.compensate_pressure(temperature, pressure);
 
         Ok(Measurement {
@@ -541,7 +553,7 @@ mod tests {
 
     /// The [`Measurement::pressure`] value for [`PRESSURE_TEMPERATURE_BYTES`] when compensated by [`CalibrationCoefficients::default()`].
     fn expected_pressure() -> Pressure {
-        Pressure::new::<pascal>(100_207.97)
+        Pressure::new::<pascal>(98370.55)
     }
 
     /// Bytes for the DATA registers (0x07 .. 0x09) for a temperature measurement.
@@ -553,7 +565,7 @@ mod tests {
     }
 
     fn expected_altitude() -> Length {
-        Length::new::<meter>(93.36266)
+        Length::new::<meter>(248.78754)
     }
 
     impl Default for CalibrationCoefficients {