Disable feedback current readout on flawed HW Revs

Thermostats v2.2 and below have a noisy and offset feedback current
`tec_i` caused by missing hardware on 2 MAX1968 TEC driver pins:

1. A missing RC filter on the ITEC pin that would have isolated CPU
sampling pulses from the signal; and
2. Some missing buffering on the VREF pin that would have avoided
loading the VREF signal, preventing voltage drops from the nominal 1.5V.

Since the resulting signal `tec_i` derived from these two signals can
have an error of around +/- 100mA, and readback may affect the stability
performance of the Thermostat, disable current readback entirely on
affected hardware revisions for now.

See https://github.com/sinara-hw/Thermostat/issues/117 and
https://github.com/sinara-hw/Thermostat/issues/120.

On hardware revisions v3.x and above, this would be fixed.

README.md

@@ -271,6 +271,8 @@ with the following keys.
 | `tec_u_meas`   | Volts           | Measurement of the voltage across the TEC            |
 | `pid_output`   | Amperes         | PID control output                                   |
 
+Note: With Thermostat v2 and below, the voltage and current readouts `i_tec` and `tec_i` are disabled and null due to faulty hardware that introduces a lot of noise in the signal.
+
 ## PID Tuning
 
 The thermostat implements a PID control loop for each of the TEC channels, more details on setting up the PID control loop can be found [here](./doc/PID%20tuning.md).

src/channels.rs

@@ -20,6 +20,7 @@ use crate::{
     command_handler::JsonBuffer,
     pins,
     steinhart_hart,
+    hw_rev,
 };
 
 pub const CHANNELS: usize = 2;
@@ -28,17 +29,18 @@ pub const R_SENSE: f64 = 0.05;
 const DAC_OUT_V_MAX: f64 = 3.0;
 
 // TODO: -pub
-pub struct Channels {
+pub struct Channels<'a> {
     channel0: Channel<Channel0>,
     channel1: Channel<Channel1>,
     pub adc: ad7172::Adc<pins::AdcSpi, pins::AdcNss>,
     /// stm32f4 integrated adc
     pins_adc: pins::PinsAdc,
     pub pwm: pins::PwmPins,
+    hwrev: &'a hw_rev::HWRev,
 }
 
-impl Channels {
+impl<'a> Channels<'a> {
-    pub fn new(pins: pins::Pins) -> Self {
+    pub fn new(pins: pins::Pins, hwrev: &'a hw_rev::HWRev) -> Self {
         let mut adc = ad7172::Adc::new(pins.adc_spi, pins.adc_nss).unwrap();
         // Feature not used
         adc.set_sync_enable(false).unwrap();
@@ -56,7 +58,7 @@ impl Channels {
         let channel1 = Channel::new(pins.channel1, adc_calibration1);
         let pins_adc = pins.pins_adc;
         let pwm = pins.pwm;
-        let mut channels = Channels { channel0, channel1, adc, pins_adc, pwm };
+        let mut channels = Channels { channel0, channel1, adc, pins_adc, pwm, hwrev };
         for channel in 0..CHANNELS {
             channels.calibrate_dac_value(channel);
             channels.set_i(channel, ElectricCurrent::new::<ampere>(0.0));
@@ -429,8 +431,8 @@ impl Channels {
 
     fn report(&mut self, channel: usize) -> Report {
         let i_set = self.get_i(channel);
-        let i_tec = self.read_itec(channel);
+        let i_tec = if self.hwrev.major > 2 {Some(self.read_itec(channel))} else {None};
-        let tec_i = self.get_tec_i(channel);
+        let tec_i = if self.hwrev.major > 2 {Some(self.get_tec_i(channel))} else {None};
         let dac_value = self.get_dac(channel);
         let state = self.channel_state(channel);
         let pid_output = ElectricCurrent::new::<ampere>(state.pid.y1);
@@ -543,8 +545,8 @@ pub struct Report {
     i_set: ElectricCurrent,
     dac_value: ElectricPotential,
     dac_feedback: ElectricPotential,
-    i_tec: ElectricPotential,
+    i_tec: Option<ElectricPotential>,
-    tec_i: ElectricCurrent,
+    tec_i: Option<ElectricCurrent>,
     tec_u_meas: ElectricPotential,
     pid_output: ElectricCurrent,
 }

src/main.rs

@@ -138,7 +138,7 @@ fn main() -> ! {
 
     let mut store = flash_store::store(dp.FLASH);
 
-    let mut channels = Channels::new(pins);
+    let mut channels = Channels::new(pins, &hwrev);
     for c in 0..CHANNELS {
         match store.read_value::<ChannelConfig>(CHANNEL_CONFIG_KEY[c]) {
             Ok(Some(config)) =>