I am having a play with a Raspberry Pi 3 running Windows IoT. I have wired up a DS18B20 sensor and I am able to communicate to it fine via a UWP app.
I now wanted to make this app into a BackgroundTask app. I am using this code for the OneWire coms
class WireSearchResult
{
public byte[] id = new byte[8];
public int lastForkPoint = 0;
}
public class OneWire
{
private SerialDevice serialPort = null;
DataWriter dataWriteObject = null;
DataReader dataReaderObject = null;
public async Task<string> GetFirstSerialPort()
{
try
{
string aqs = SerialDevice.GetDeviceSelector("UART0");
var dis = await DeviceInformation.FindAllAsync(aqs);
if (dis.Count > 0)
{
var deviceInfo = dis.First();
return deviceInfo.Id;
}
}
catch (Exception ex)
{
Debug.WriteLine("Unable to get serial device: " + ex.Message);
}
return null;
}
public void shutdown()
{
if (serialPort != null)
{
serialPort.Dispose();
serialPort = null;
}
}
async Task<bool> onewireReset(string deviceId)
{
try
{
if (serialPort != null)
serialPort.Dispose();
serialPort = await SerialDevice.FromIdAsync(deviceId);
// Configure serial settings
serialPort.WriteTimeout = TimeSpan.FromMilliseconds(1000);
serialPort.ReadTimeout = TimeSpan.FromMilliseconds(1000);
serialPort.BaudRate = 9600;
serialPort.Parity = SerialParity.None;
serialPort.StopBits = SerialStopBitCount.One;
serialPort.DataBits = 8;
serialPort.Handshake = SerialHandshake.None;
dataWriteObject = new DataWriter(serialPort.OutputStream);
dataWriteObject.WriteByte(0xF0);
await dataWriteObject.StoreAsync();
dataReaderObject = new DataReader(serialPort.InputStream);
await dataReaderObject.LoadAsync(1);
byte resp = dataReaderObject.ReadByte();
if (resp == 0xFF)
{
System.Diagnostics.Debug.WriteLine("Nothing connected to UART");
return false;
}
else if (resp == 0xF0)
{
System.Diagnostics.Debug.WriteLine("No 1-wire devices are present");
return false;
}
else
{
//System.Diagnostics.Debug.WriteLine("Response: " + resp);
serialPort.Dispose();
serialPort = await SerialDevice.FromIdAsync(deviceId);
// Configure serial settings
serialPort.WriteTimeout = TimeSpan.FromMilliseconds(1000);
serialPort.ReadTimeout = TimeSpan.FromMilliseconds(1000);
serialPort.BaudRate = 115200;
serialPort.Parity = SerialParity.None;
serialPort.StopBits = SerialStopBitCount.One;
serialPort.DataBits = 8;
serialPort.Handshake = SerialHandshake.None;
dataWriteObject = new DataWriter(serialPort.OutputStream);
dataReaderObject = new DataReader(serialPort.InputStream);
return true;
}
}
catch (Exception ex)
{
System.Diagnostics.Debug.WriteLine("Exception: " + ex.Message);
return false;
}
}
public async Task onewireWriteByte(byte b)
{
for (byte i = 0; i < 8; i++, b = (byte)(b >> 1))
{
// Run through the bits in the byte, extracting the
// LSB (bit 0) and sending it to the bus
await onewireBit((byte)(b & 0x01));
}
}
async Task<byte> onewireBit(byte b)
{
var bit = b > 0 ? 0xFF : 0x00;
dataWriteObject.WriteByte((byte)bit);
await dataWriteObject.StoreAsync();
await dataReaderObject.LoadAsync(1);
var data = dataReaderObject.ReadByte();
return (byte)(data & 0xFF);
}
async Task<byte> onewireReadByte()
{
byte b = 0;
for (byte i = 0; i < 8; i++)
{
// Build up byte bit by bit, LSB first
b = (byte)((b >> 1) + 0x80 * await onewireBit(1));
}
// System.Diagnostics.Debug.WriteLine("onewireReadByte result: " + b);
return b;
}
public async Task<double> getTemperature(string deviceId)
{
double tempCelsius = -200;
if (await onewireReset(deviceId))
{
await onewireWriteByte(0xCC); //1-Wire SKIP ROM command (ignore device id)
await onewireWriteByte(0x44); //DS18B20 convert T command
// (initiate single temperature conversion)
// thermal data is stored in 2-byte temperature
// register in scratchpad memory
// Wait for at least 750ms for data to be collated
await Task.Delay(750);
// Get the data
await onewireReset(deviceId);
await onewireWriteByte(0xCC); //1-Wire Skip ROM command (ignore device id)
await onewireWriteByte(0xBE); //DS18B20 read scratchpad command
// DS18B20 will transmit 9 bytes to master (us)
// starting with the LSB
byte tempLSB = await onewireReadByte(); //read lsb
byte tempMSB = await onewireReadByte(); //read msb
// Reset bus to stop sensor sending unwanted data
await onewireReset(deviceId);
// Log the Celsius temperature
tempCelsius = ((tempMSB * 256) + tempLSB) / 16.0;
var temp2 = ((tempMSB << 8) + tempLSB) * 0.0625; //just another way of calculating it
System.Diagnostics.Debug.WriteLine("Temperature: " + tempCelsius + " degrees C " + temp2);
}
return tempCelsius;
}
}
And finally the StartupTask
public sealed class StartupTask : IBackgroundTask
{
private BackgroundTaskDeferral deferral;
private OneWire onewire;
private string deviceId = string.Empty;
private bool inprog = false;
private Timer timer;
public void Run(IBackgroundTaskInstance taskInstance)
{
deferral = taskInstance.GetDeferral();
onewire = new OneWire();
deviceId = await onewire.GetFirstSerialPort();
if(deviceId != null)
await onewire.getTemperature(deviceId));
BackgroundTaskDeferral.Complete();
}
}
The problem I have is that when I run this code it hangs on one of the lines that disposes of the SerialDevice in OneWire class.
I have read in a few places that its related to the BackgroundTask and using Async/Await
I'm using the same onewire code in a background task to talk to the DS18B20 sensor and was experiencing exactly the same behaviour as you.
What I found is that if I put a delay of 100 milliseconds before calling the serial port dispose method it works
await Task.Delay(100)
I've tried less than 100 milliseconds but it just kept hanging.
This stackoverflow questions first answer explains the issue with serial ports in the .Net Framework Why Thread.Sleep() before SerialPort.Open and Close?