Temperature calculation improvements (#111)

* Improve temperature calculation

* Minor cleanup

examples/adc-continious-dma.rs

@@ -5,6 +5,7 @@ mod utils;
 
 use crate::hal::{
     adc::{
+        config,
         config::{Continuous, Dma as AdcDma, SampleTime, Sequence},
         AdcClaim, ClockSource, Temperature, Vref,
     },
@@ -13,13 +14,11 @@ use crate::hal::{
     gpio::GpioExt,
     pwr::PwrExt,
     rcc::{Config, RccExt},
+    signature::{VrefCal, VDDA_CALIB},
     stm32::Peripherals,
 };
 use stm32g4xx_hal as hal;
 
-use stm32g4xx_hal::adc::config;
-use stm32g4xx_hal::signature::{VrefCal, VDDA_CALIB};
-
 use cortex_m_rt::entry;
 use utils::logger::info;
 
@@ -92,8 +91,11 @@ fn main() -> ! {
         let vref =
             Vref::sample_to_millivolts_ext((b[2] + b[5]) / 2, vdda, config::Resolution::Twelve);
         info!("vref: {}mV", vref);
-        let raw_temp = (((b[1] + b[4]) / 2) as f32 * (vdda as f32 / 3000.0)) as u16;
-        let temp = Temperature::temperature_to_degrees_centigrade(raw_temp);
+        let temp = Temperature::temperature_to_degrees_centigrade(
+            (b[1] + b[4]) / 2,
+            vdda as f32 / 1000.,
+            config::Resolution::Twelve,
+        );
         info!("temp: {}°C", temp);
     }
 }

examples/adc-continious.rs

@@ -3,13 +3,14 @@
 
 use crate::hal::{
     adc::{
-        config::{Continuous, SampleTime, Sequence},
+        config::{Continuous, Resolution, SampleTime, Sequence},
         AdcClaim, ClockSource, Temperature, Vref,
     },
     delay::SYSTDelayExt,
     gpio::GpioExt,
     pwr::PwrExt,
     rcc::{Config, RccExt},
+    signature::{VrefCal, VDDA_CALIB},
     stm32::Peripherals,
 };
 use stm32g4xx_hal as hal;
@@ -65,11 +66,17 @@ fn main() -> ! {
         info!("pa3: {}mV", millivolts);
 
         adc = adc.wait_for_conversion_sequence().unwrap_active();
-        let millivolts = Vref::sample_to_millivolts(adc.current_sample());
+        let vref_sample = adc.current_sample();
+        let millivolts = Vref::sample_to_millivolts(vref_sample);
+        let vdda = VDDA_CALIB * VrefCal::get().read() as u32 / vref_sample as u32;
         info!("vref: {}mV", millivolts);
 
         adc = adc.wait_for_conversion_sequence().unwrap_active();
-        let millivolts = Temperature::temperature_to_degrees_centigrade(adc.current_sample());
-        info!("temp: {}℃C", millivolts); // Note: Temperature seems quite low...
+        let temp = Temperature::temperature_to_degrees_centigrade(
+            adc.current_sample(),
+            vdda as f32 / 1000.,
+            Resolution::Twelve,
+        );
+        info!("temp: {}°C", temp);
     }
 }

examples/adc-one-shot-dma.rs

@@ -5,7 +5,7 @@ use cortex_m_rt::entry;
 
 use crate::hal::{
     adc::{
-        config::{Continuous, Dma as AdcDma, SampleTime, Sequence},
+        config::{Continuous, Dma as AdcDma, Resolution, SampleTime, Sequence},
         AdcClaim, ClockSource, Temperature,
     },
     delay::SYSTDelayExt,
@@ -81,8 +81,11 @@ fn main() -> ! {
 
     let millivolts = adc.sample_to_millivolts(first_buffer[0]);
     info!("pa3: {}mV", millivolts);
-    let millivolts = Temperature::temperature_to_degrees_centigrade(first_buffer[1]);
-    info!("temp: {}℃C", millivolts); // Note: Temperature seems quite low...
+
+    // Assume vdda is 3.3V, see adc-continious.rs for an example of measuring VDDA using VREF
+    let temp =
+        Temperature::temperature_to_degrees_centigrade(first_buffer[1], 3.3, Resolution::Twelve);
+    info!("temp: {}°C", temp);
 
     #[allow(clippy::empty_loop)]
     loop {}

src/adc.rs

@@ -60,12 +60,76 @@ pub struct Vbat;
 /// Core temperature internal signal
 pub struct Temperature;
 impl Temperature {
+    /// Precompute the inverse of `VTEMP_CAL_VREFANALOG`, in volts,
+    /// for floating point calculations
+    const INV_VREFANALOG_VOLTS: f32 = 1000. / VDDA_CALIB as f32;
+    /// Temperature at which temperature sensor has been calibrated in production
+    /// for data into [`VtempCal30`] (tolerance: +-5 DegC) (unit: DegC).
+    const VTEMP_CAL_T30: u16 = 30;
+    /// Temperature at which temperature sensor has been calibrated in production
+    /// for data into [`VtempCal130`] (tolerance: +-5 DegC) (unit: DegC).
+    const VTEMP_CAL_T130: u16 = 130;
+
+    /// Convert a sample to 12 bits. Reference voltages were captured at 12 bits.
+    const fn to_12b(sample: u16, resolution: config::Resolution) -> u16 {
+        match resolution {
+            config::Resolution::Six => sample << 6,
+            config::Resolution::Eight => sample << 4,
+            config::Resolution::Ten => sample << 2,
+            config::Resolution::Twelve => sample,
+        }
+    }
+
+    /// Convert a raw sample from `Temperature` to deg C.
+    ///
+    /// ## Arguments
+    /// * `sample`: ADC sample taken on the [`Temperature`] channel.
+    /// * `vdda`: Analog reference voltage (vref+) when the temperature
+    /// sample was taken, in volts.
+    /// * `resolution`: Configured ADC resolution.
+    #[inline(always)]
+    pub fn temperature_to_degrees_centigrade(
+        sample: u16,
+        vdda: f32,
+        resolution: config::Resolution,
+    ) -> f32 {
+        // Reference measurements were taken at 12 bits
+        let sample_12b = Self::to_12b(sample, resolution);
+
+        // Normalize for the difference in VDDA
+        let sample_normalized = sample_12b as f32 * (vdda * Self::INV_VREFANALOG_VOLTS);
+
+        ((sample_normalized - VtempCal30::get().read() as f32)
+            * ((Self::VTEMP_CAL_T130 - Self::VTEMP_CAL_T30) as f32))
+            / ((VtempCal130::get().read() - VtempCal30::get().read()) as f32)
+            + Self::VTEMP_CAL_T30 as f32
+    }
+
     /// Convert a raw sample from `Temperature` to deg C
+    ///
+    /// ## Arguments
+    /// * `sample`: ADC sample taken on the [`Temperature`] channel.
+    /// * `vdda`: Analog reference voltage (vref+) when the temperature
+    /// sample was taken, in millivolts.
+    /// * `resolution`: Configured ADC resolution.
     #[inline(always)]
-    pub fn temperature_to_degrees_centigrade(sample: u16) -> f32 {
-        ((130.0 - 30.0) / (VtempCal130::get().read() as f32 - VtempCal30::get().read() as f32)
-            * (sample as f32 - VtempCal30::get().read() as f32))
-            + 30.0
+    pub fn temperature_to_degrees_centigrade_coarse(
+        sample: u16,
+        vdda: u32,
+        resolution: config::Resolution,
+    ) -> i16 {
+        // Reference measurements were taken at 12 bits
+        let sample_12b = Self::to_12b(sample, resolution);
+
+        // Normalize for the difference in VDDA
+        let sample_normalized = ((sample_12b as u32 * vdda) / VDDA_CALIB) as u16;
+
+        let t = ((sample_normalized as i32 - VtempCal30::get().read() as i32)
+            * ((Self::VTEMP_CAL_T130 - Self::VTEMP_CAL_T30) as i32))
+            / ((VtempCal130::get().read() - VtempCal30::get().read()) as i32)
+            + Self::VTEMP_CAL_T30 as i32;
+
+        t as i16
     }
 }