tera netxPM particulate sensor

Source/Meadow.Foundation.Peripherals/Sensors.Environmental.NextPm/Driver/CommandManager.cs

@@ -0,0 +1,54 @@
+using System;
+using System.Linq;
+
+namespace Meadow.Foundation.Sensors.Environmental
+{
+    internal static class CommandManager
+    {
+        public static (int commandLength, int expectedResponseLength) GenerateCommand(NextPm.CommandByte command, byte[] buffer, params byte[] payload)
+        {
+            buffer[0] = NextPm.SensorAddress;
+            buffer[1] = (byte)command;
+
+            var payloadLength = 2;
+
+            if (payload != null && payload.Length > 0)
+            {
+                Array.Copy(payload, 0, buffer, 2, payload.Length);
+                payloadLength += payload.Length;
+            }
+
+            buffer[payloadLength] = CalculateChecksum(buffer, 0, payloadLength);
+
+            switch (command)
+            {
+                case NextPm.CommandByte.ReadFirmware:
+                    return (3, 6);
+                case NextPm.CommandByte.ReadTempAndHumidity:
+                    return (3, 8);
+                case NextPm.CommandByte.ReadWriteFanSpeed:
+                    return (payloadLength + 1, 6);
+                case NextPm.CommandByte.SetPowerMode:
+                    return (payloadLength + 1, 4);
+                case NextPm.CommandByte.ReadSensorState:
+                    return (3, 4);
+                case NextPm.CommandByte.Read10sConcentrations:
+                case NextPm.CommandByte.Read60sConcentrations:
+                case NextPm.CommandByte.Read900sConcentrations:
+                    return (3, 16);
+                default: throw new NotImplementedException();
+            }
+        }
+
+        public static byte CalculateChecksum(byte[] data, int start, int length)
+        {
+            return (byte)(0x100 - (data.Skip(start).Take(length).Sum(d => d) % 256));
+        }
+
+        public static bool ValidateChecksum(byte[] data, int start, int length)
+        {
+            var expected = CalculateChecksum(data, start, length - 1);
+            return expected == data[start + length - 1];
+        }
+    }
+}

Source/Meadow.Foundation.Peripherals/Sensors.Environmental.NextPm/Driver/NextPm.Constants.cs

@@ -0,0 +1,7 @@
+namespace Meadow.Foundation.Sensors.Environmental
+{
+    public partial class NextPm
+    {
+        internal const byte SensorAddress = 0x81; // fixed address, per the data sheet
+    }
+}

Source/Meadow.Foundation.Peripherals/Sensors.Environmental.NextPm/Driver/NextPm.Enums.cs

@@ -0,0 +1,33 @@
+using System;
+
+namespace Meadow.Foundation.Sensors.Environmental
+{
+    public partial class NextPm
+    {
+        internal enum CommandByte : byte
+        {
+            Read10sConcentrations = 0x11,
+            Read60sConcentrations = 0x12,
+            Read900sConcentrations = 0x13,
+            ReadTempAndHumidity = 0x14,
+            SetPowerMode = 0x15,
+            ReadSensorState = 0x16,
+            ReadFirmware = 0x17,
+            ReadWriteFanSpeed = 0x21,
+            ReadWriteModbusAddress = 0x22
+        }
+
+        [Flags]
+        internal enum SensorStatus : byte
+        {
+            Sleep = 1 << 0,
+            Degraded = 1 << 1,
+            NotReady = 1 << 2,
+            HeaterError = 1 << 3,
+            TempHumidyError = 1 << 4,
+            FanError = 1 << 5,
+            MemoryError = 1 << 6,
+            LaserError = 1 << 7,
+        }
+    }
+}

Source/Meadow.Foundation.Peripherals/Sensors.Environmental.NextPm/Driver/NextPm.Serial.cs

@@ -0,0 +1,83 @@
+using Meadow.Hardware;
+using System;
+using System.Threading.Tasks;
+
+namespace Meadow.Foundation.Sensors.Environmental
+{
+    /// <summary>
+    /// Represents a TERA Sensor NextPM particulate matter sensor
+    /// </summary>
+    public partial class NextPm
+    {
+        private ISerialPort _port;
+        // dev note: these common buffers are used to minimize heap allocations during serial communications with the sensor
+        private byte[] _readBuffer = new byte[16];
+        private byte[] _writeBuffer = new byte[16];
+
+        private ISerialPort InitializePort(ISerialPort port)
+        {
+            if (port.IsOpen)
+            {
+                port.Close();
+            }
+
+            port.BaudRate = 115200;
+            port.DataBits = 8;
+            port.Parity = Parity.Even;
+            port.StopBits = StopBits.One;
+
+            return port;
+        }
+
+        private async Task SendCommand(CommandByte command, params byte[] payload)
+        {
+            var parameters = CommandManager.GenerateCommand(command, _writeBuffer, payload);
+
+            if (!_port.IsOpen)
+            {
+                _port.Open();
+            }
+
+            _port.Write(_writeBuffer, 0, parameters.commandLength);
+
+            var read = 0;
+            var expected = parameters.expectedResponseLength;
+            var timeoutMs = 3000;
+            var readDelay = 500;
+
+            await Task.Delay(100); // initial device response delay
+
+            Array.Clear(_readBuffer, 0, _readBuffer.Length);
+
+            do
+            {
+                read += _port.Read(_readBuffer, read, _readBuffer.Length - read);
+                if (read >= expected) break;
+
+                if (read > 2 && _readBuffer[1] != (byte)command && _readBuffer[1] == 0x16)
+                {
+                    var status = (SensorStatus)_readBuffer[2];
+                    // typically a "fail" due to the device being in sleep state
+                    throw new TeraException($"Device status: {status}");
+                }
+
+                await Task.Delay(readDelay);
+                timeoutMs -= readDelay;
+            } while (timeoutMs > 0);
+
+            if (read < expected)
+            {
+                throw new TeraException("Serial timeout");
+            }
+            else
+            {
+                var checksumIsValid = CommandManager.ValidateChecksum(_readBuffer, 0, parameters.expectedResponseLength);
+
+                if (!checksumIsValid)
+                {
+                    throw new TeraException("Invalid response checksum");
+                }
+            }
+        }
+    }
+}