Port TEC DAC calibration procedure from Thermostat

- Needs clean up
- To be evaluated and rethink the calibration procedure

src/thermostat/max1968.rs

@@ -35,7 +35,7 @@ pub const R_SENSE: ElectricalResistance = ElectricalResistance {
 // Rev 0_2: DAC Chip connects 3V3 reference voltage and thus provide 0-3.3V output range
 // TODO: Rev 0_3: DAC Chip connects 3V3 reference voltage,
 //      which is then passed through a resistor divider to provide 0-3V output range
-const DAC_OUT_V_MAX: f64 = 3.3;
+pub const DAC_OUT_V_MAX: f64 = 3.3;
 const TEC_VSEC_BIAS_V: ElectricPotential = ElectricPotential {
     dimension: PhantomData,
     units: PhantomData,
@@ -258,17 +258,15 @@ impl MAX1968 {
     }
 
     pub fn set_i(&mut self, i_tec: ElectricCurrent) -> ElectricCurrent {
-        let center_point = self.phy.center_pt;
-        let r_sense = R_SENSE;
-        let voltage = i_tec * 10.0 * r_sense + center_point;
+        let voltage = i_tec * 10.0 * R_SENSE + self.phy.center_pt;
         let voltage = self.set_dac(voltage);
-        let i_tec = (voltage - center_point) / (10.0 * r_sense);
+        let i_tec = (voltage - self.phy.center_pt) / (10.0 * R_SENSE);
         i_tec
     }
 
     // AN4073: ADC Reading Dispersion can be reduced through Averaging
     // Upon test, 16 Point Averaging = +-3 LSB Dispersion
-    fn adc_read(&mut self, adc_read_target: AdcReadTarget, avg_pt: u16) -> ElectricPotential {
+    pub fn adc_read(&mut self, adc_read_target: AdcReadTarget, avg_pt: u16) -> ElectricPotential {
         let mut sample: u32 = 0;
         sample = match adc_read_target {
             AdcReadTarget::VREF => {
@@ -317,7 +315,7 @@ impl MAX1968 {
     }
 
     pub fn get_dac_vfb(&mut self) -> ElectricPotential {
-        self.adc_read(AdcReadTarget:: DacVfb, 1)
+        self.adc_read(AdcReadTarget:: DacVfb, 16)
     }
 
     pub fn get_tec_i(&mut self) -> ElectricCurrent {

src/thermostat/mod.rs

@@ -1,2 +1,3 @@
 pub mod ad5680;
 pub mod max1968;
+pub mod thermostat;

src/thermostat/thermostat.rs

@@ -1,5 +1,14 @@
+use crate::sys_timer;
 use crate::thermostat::ad5680;
-use crate::thermostat::MAX1968;
+use crate::thermostat::max1968::{MAX1968, AdcReadTarget, DAC_OUT_V_MAX};
+use log::info;
+use uom::si::{
+    electric_current::ampere,
+    electric_potential::{millivolt, volt},
+    electrical_resistance::ohm,
+    f64::{ElectricCurrent, ElectricPotential, ElectricalResistance},
+    ratio::ratio,
+};
 
 pub struct Thermostat {
     max1968: MAX1968,
@@ -15,4 +24,65 @@ impl Thermostat{
     pub fn setup(&mut self){
         self.max1968.setup();
     }
+
+    /// Calibrates the DAC output to match vref of the MAX driver to reduce zero-current offset of the MAX driver output.
+    ///
+    /// The thermostat DAC applies a control voltage signal to the CTLI pin of MAX driver chip to control its output current.
+    /// The CTLI input signal is centered around VREF of the MAX chip. Applying VREF to CTLI sets the output current to 0.
+    /// 
+    /// This calibration routine measures the VREF voltage and the DAC output with the STM32 ADC, and uses a breadth-first     
+    /// search to find the DAC setting that will produce a DAC output voltage closest to VREF. This DAC output voltage will 
+    /// be stored and used in subsequent i_set routines to bias the current control signal to the measured VREF, reducing 
+    /// the offset error of the current control signal.
+    ///
+    /// The input offset of the STM32 ADC is eliminated by using the same ADC for the measurements, and by only using the
+    /// difference in VREF and DAC output for the calibration.
+    /// 
+    /// This routine should be called only once after boot, repeated reading of the vref signal and changing of the stored 
+    /// VREF measurement can introduce significant noise at the current output, degrading the stabilily performance of the
+    /// thermostat. 
+    pub fn calibrate_dac_value(&mut self) {
+        let samples = 50;
+        let mut target_voltage = ElectricPotential::new::<volt>(0.0);
+        for _ in 0..samples {
+            target_voltage = target_voltage + self.max1968.adc_read(AdcReadTarget::VREF, 1);
+        }
+        target_voltage = target_voltage / samples as f64;
+        let mut start_value = 1;
+        let mut best_error = ElectricPotential::new::<volt>(100.0);
+        let before_cal = self.max1968.phy.center_pt;
+        for step in (0..18).rev() {
+            info!("Step: {} Calibrating", step);
+            let mut prev_value = start_value;
+            for value in (start_value..=ad5680::MAX_VALUE).step_by(1 << step) {
+                //info!("Calibrating");
+                self.max1968.phy.dac.set(value).unwrap();
+                sys_timer::sleep(5);
+
+                let dac_feedback = self.max1968.adc_read(AdcReadTarget::DacVfb, 64);
+                let error = target_voltage - dac_feedback;
+                if error < ElectricPotential::new::<volt>(0.0) {
+                    break;
+                } else if error < best_error {
+                    best_error = error;
+                    start_value = prev_value;
+
+                    let vref = (value as f64 / ad5680::MAX_VALUE as f64) * ElectricPotential::new::<volt>(DAC_OUT_V_MAX);
+                    self.max1968.set_center_point(vref);
+
+                }
+                prev_value = value;
+            }
+        }
+        
+        loop {
+            info!("Before Calibration, VREF = {:?}", before_cal);
+            info!("After Calibration, VREF = {:?}", self.max1968.phy.center_pt);
+            self.max1968.set_i(ElectricCurrent::new::<ampere>(0.0));
+            info!("VREF Value {:?}", self.max1968.adc_read(AdcReadTarget::VREF, 64));
+            info!("DAC VFB Value {:?}", self.max1968.adc_read(AdcReadTarget::DacVfb, 64));
+            sys_timer::sleep(100);
+        }
+    }
+    
 }