Review remarks. Timeout handling for Mhz19b changed.

src/devices/Ahtxx/AhtBase.cs

@@ -14,29 +14,15 @@ namespace Iot.Device.Ahtxx
     /// Note: has been tested with AHT20 only, but should work with AHT10 and AHT15 as well.
     /// Up to now all functions are contained in the base class, though there might be differences between the sensors types.
     /// </summary>
-    public class AhtBase : IDisposable
+    public abstract class AhtBase : IDisposable
     {
         /// <summary>
         /// Address of AHTxx device (0x38). This address is fix and cannot be changed.
         /// This implies that only one device can be attached to a single I2C bus at a time.
         /// </summary>
-        public const int DeviceAddress = 0x38;
+        public const int DefaultI2cAddress = 0x38;
 
-        // datasheet version 1.1, table 10
-        private enum StatusBit : byte
-        {
-            Calibrated = 0x08,
-            Busy = 0x80
-        }
-
-        private enum Command : byte
-        {
-            Calibrate = 0xbe,
-            SoftRest = 0xba,
-            Measure = 0xac
-        }
-
-        private I2cDevice _i2cDevice = null;
+        private I2cDevice _i2cDevice;
         private double _temperature;
         private double _humidity;
 
@@ -93,6 +79,7 @@ namespace Iot.Device.Ahtxx
                 0x33,
                 0x00
             };
+
             _i2cDevice.Write(buffer);
 
             // According to the datasheet the measurement takes 80 ms and completion is indicated by the status bit.
@@ -122,7 +109,7 @@ namespace Iot.Device.Ahtxx
         /// </summary>
         private void SoftReset()
         {
-            _i2cDevice.WriteByte((byte)Command.SoftRest);
+            _i2cDevice.WriteByte((byte)Command.SoftReset);
             // reset requires 20ms at most, c.f. datasheet version 1.1, ch. 5.5
             Thread.Sleep(20);
         }
@@ -138,6 +125,7 @@ namespace Iot.Device.Ahtxx
                 0x08, // command parameters c.f. datasheet, version 1.1, ch. 5.4
                 0x00
             };
+
             _i2cDevice.Write(buffer);
             // wait 10ms c.f. datasheet, version 1.1, ch. 5.4
             Thread.Sleep(10);
@@ -171,5 +159,19 @@ namespace Iot.Device.Ahtxx
             _i2cDevice?.Dispose();
             _i2cDevice = null;
         }
+
+        // datasheet version 1.1, table 10
+        private enum StatusBit : byte
+        {
+            Calibrated = 0x08,
+            Busy = 0x80
+        }
+
+        private enum Command : byte
+        {
+            Calibrate = 0xbe,
+            SoftReset = 0xba,
+            Measure = 0xac
+        }
     }
 }

src/devices/Ahtxx/samples/Ahtxx.Sample.cs

@@ -19,7 +19,7 @@ namespace Iot.Device.Ahtxx.Samples
         public static void Main(string[] args)
         {
             const int I2cBus = 1;
-            I2cConnectionSettings i2cSettings = new I2cConnectionSettings(I2cBus, Aht20.DeviceAddress);
+            I2cConnectionSettings i2cSettings = new I2cConnectionSettings(I2cBus, Aht20.DefaultI2cAddress);
             I2cDevice i2cDevice = I2cDevice.Create(i2cSettings);
             Aht20 aht20Sensor = new Aht20(i2cDevice);
 

src/devices/Mhz19b/Mhz19b.cs

@@ -16,28 +16,7 @@ namespace Iot.Device.Mhz19b
     /// </summary>
     public sealed class Mhz19b : IDisposable
     {
-        private enum Command : byte
-        {
-            ReadCo2Concentration = 0x86,
-            CalibrateZeroPoint = 0x87,
-            CalibrateSpanPoint = 0x88,
-            AutoCalibrationSwitch = 0x79,
-            DetectionRangeSetting = 0x99
-        }
-
-        private enum MessageFormat
-        {
-            Start = 0x00,
-            SensorNum = 0x01,
-            Command = 0x02,
-            DataHighRequest = 0x03,
-            DataLowRequest = 0x04,
-            DataHighResponse = 0x02,
-            DataLowResponse = 0x03,
-            Checksum = 0x08
-        }
-
-        private const int MessageSize = 9;
+        private const int MessageBytes = 9;
         private bool _shouldDispose = false;
         private SerialPort _serialPort = null;
         private Stream _serialPortStream = null;
@@ -72,6 +51,7 @@ namespace Iot.Device.Mhz19b
                 ReadTimeout = 1000,
                 WriteTimeout = 1000
             };
+
             _serialPort.Open();
             _serialPortStream = _serialPort.BaseStream;
             _shouldDispose = true;
@@ -79,63 +59,52 @@ namespace Iot.Device.Mhz19b
 
         /// <summary>
         /// Gets the current CO2 concentration from the sensor.
-        /// The validity is true if the current concentration was successfully read.
-        /// If the serial communication timed out or the checksum was invalid the validity is false.
-        /// If the validity is false the ratio is set to 0.
         /// </summary>
-        /// <returns>CO2 concentration in ppm and validity</returns>
-        /// <exception cref="System.IO.IOException">Communication with sensor failed</exception>
+        /// <returns>CO2 volume concentration</returns>
+        /// <exception cref="IOException">Communication with sensor failed</exception>
+        /// <exception cref="TimeoutException">A timeout occurred while communicating with the sensor</exception>
         public VolumeConcentration GetCo2Reading()
         {
-            try
-            {
-                // send read command request
-                var request = CreateRequest(Command.ReadCo2Concentration);
-                request[(int)MessageFormat.Checksum] = Checksum(request);
-                _serialPortStream.Write(request, 0, request.Length);
+            // send read command request
+            var request = CreateRequest(Command.ReadCo2Concentration);
+            request[(int)MessageFormat.Checksum] = Checksum(request);
+            _serialPortStream.Write(request, 0, request.Length);
 
-                // read complete response (9 bytes expected)
-                byte[] response = new byte[MessageSize];
-                int timeout = 100;
-                int bytesRead = 0;
-                while (timeout > 0 && bytesRead < MessageSize)
-                {
-                    bytesRead += _serialPortStream.Read(response, bytesRead, response.Length - bytesRead);
-                    Thread.Sleep(1);
-                    timeout--;
-                }
+            // read complete response (9 bytes expected)
+            byte[] response = new byte[MessageBytes];
 
-                if (timeout == 0)
-                {
-                    throw new IOException("Timeout");
-                }
+            long endTicks = DateTime.Now.AddMilliseconds(250).Ticks;
+            int bytesRead = 0;
+            while (DateTime.Now.Ticks < endTicks && bytesRead < MessageBytes)
+            {
+                bytesRead += _serialPortStream.Read(response, bytesRead, response.Length - bytesRead);
+                Thread.Sleep(1);
+            }
 
-                // check response and return calculated concentration if valid
-                if (response[(int)MessageFormat.Checksum] == Checksum(response))
-                {
-                    return VolumeConcentration.FromPartsPerMillion((int)response[(int)MessageFormat.DataHighResponse] * 256 + (int)response[(int)MessageFormat.DataLowResponse]);
-                }
-                else
-                {
-                    throw new IOException("Invalid response message received from sensor");
-                }
+            if (bytesRead < MessageBytes)
+            {
+                throw new TimeoutException($"Communication with sensor failed.");
             }
-            catch (Exception e)
+
+            // check response and return calculated concentration if valid
+            if (response[(int)MessageFormat.Checksum] == Checksum(response))
             {
-                throw new IOException("Sensor communication failed", e);
+                return VolumeConcentration.FromPartsPerMillion((int)response[(int)MessageFormat.DataHighResponse] * 256 + (int)response[(int)MessageFormat.DataLowResponse]);
+            }
+            else
+            {
+                throw new IOException("Invalid response message received from sensor");
             }
         }
 
         /// <summary>
         /// Initiates a zero point calibration.
-        /// The sensor doesn't respond anything, so this is fire and forget.
         /// </summary>
         /// <exception cref="System.IO.IOException">Communication with sensor failed</exception>
         public void PerformZeroPointCalibration() => SendRequest(CreateRequest(Command.CalibrateZeroPoint));
 
         /// <summary>
         /// Initiate a span point calibration.
-        /// The sensor doesn't respond anything, so this is fire and forget.
         /// </summary>
         /// <param name="span">span value, between 1000[ppm] and 5000[ppm]. The typical value is 2000[ppm].</param>
         /// <exception cref="System.ArgumentException">Thrown when span value is out of range</exception>
@@ -157,7 +126,6 @@ namespace Iot.Device.Mhz19b
 
         /// <summary>
         /// Switches the autmatic baseline correction on and off.
-        /// The sensor doesn't respond anything, so this is fire and forget.
         /// </summary>
         /// <param name="state">State of automatic correction</param>
         /// <exception cref="System.IO.IOException">Communication with sensor failed</exception>
@@ -172,7 +140,6 @@ namespace Iot.Device.Mhz19b
 
         /// <summary>
         /// Set the sensor detection range.
-        /// The sensor doesn't respond anything, so this is fire and forget
         /// </summary>
         /// <param name="detectionRange">Detection range of the sensor</param>
         /// <exception cref="System.IO.IOException">Communication with sensor failed</exception>
@@ -252,5 +219,26 @@ namespace Iot.Device.Mhz19b
                 _serialPort = null;
             }
         }
+
+        private enum Command : byte
+        {
+            ReadCo2Concentration = 0x86,
+            CalibrateZeroPoint = 0x87,
+            CalibrateSpanPoint = 0x88,
+            AutoCalibrationSwitch = 0x79,
+            DetectionRangeSetting = 0x99
+        }
+
+        private enum MessageFormat
+        {
+            Start = 0x00,
+            SensorNum = 0x01,
+            Command = 0x02,
+            DataHighRequest = 0x03,
+            DataLowRequest = 0x04,
+            DataHighResponse = 0x02,
+            DataLowResponse = 0x03,
+            Checksum = 0x08
+        }
     }
 }
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