A couple of years ago I wrote a .NET Core library for the Sensirion SHT20 temperature and humidity(Waterproof) sensor from DFRobot. This .NET nanoFramework version was “inspired” by the .NET Core library version, though I have added some message validation functionality.
My test setup is a simple .NET nanoFramework console application running on an STM32F7691 Discovery board.
The SH20DeviceI2C application has lots of magic numbers from the SHT20 datasheet and was just a tool for exploring how the sensor works.
public static void Main()
{
I2cConnectionSettings i2cConnectionSettings = new(1, 0x40);
// i2cDevice.Dispose in final program
I2cDevice i2cDevice = I2cDevice.Create(i2cConnectionSettings);
while (true)
{
byte[] readBuffer = new byte[3] { 0, 0, 0 };
// First temperature
i2cDevice.WriteByte(0xF3);
//Thread.Sleep(50); // no go -46.8
//Thread.Sleep(60);
Thread.Sleep(70);
//Thread.Sleep(90);
//Thread.Sleep(110);
i2cDevice.Read(readBuffer);
ushort temperatureRaw = (ushort)(readBuffer[0] << 8);
temperatureRaw += readBuffer[1];
//Debug.WriteLine($"Raw {temperatureRaw}");
double temperature = temperatureRaw * (175.72 / 65536.0) - 46.85;
// Then read the Humidity
i2cDevice.WriteByte(0xF5);
//Thread.Sleep(50);
//Thread.Sleep(60);
Thread.Sleep(70);
//Thread.Sleep(90);
//Thread.Sleep(110);
i2cDevice.Read(readBuffer);
ushort humidityRaw = (ushort)(readBuffer[0] << 8);
humidityRaw += readBuffer[1];
//Debug.WriteLine($"Raw {humidityRaw}");
double humidity = humidityRaw * (125.0 / 65536.0) - 6.0;
//Console.WriteLine($"{DateTime.UtcNow:HH:mm:ss} Temperature:{temperature:F1}°C");
//Console.WriteLine($"{DateTime.UtcNow:HH:mm:ss} Humidity:{humidity:F0}%");
Console.WriteLine($"{DateTime.UtcNow:HH:mm:ss} Temperature:{temperature:F1}°C Humidity:{humidity:F0}%");
Thread.Sleep(1000);
}
}
While tinkering with the sensor I found that having a short delay between initiating the temperature reading (TemperatureNoHold = 0xF3 was used so as not to hang up the I2C bus) and reading the value was important.
When I ran the application without a Thread.Sleep(70) the temperature and/or humidity the values were incorrect and sometimes quite random.
The .NET Core library didn’t validate the message payload Cyclic Redundancy Check (CRC) so I have added that in this version
void CheckCrc(byte[] bytes, byte bytesLen, byte checksum)
{
var crc = 0;
for (var i = 0; i < bytesLen; i++)
{
crc ^= bytes[i];
for (var bit = 8; bit > 0; --bit)
{
crc = ((crc & 0x80) == 0x80) ? ((crc << 1) ^ CrcPolynomial) : (crc << 1);
}
}
if (crc != checksum)
{
throw new Exception("CRC Error");
}
}
The CheckCrc is called in Temperature and Humidity methods.
public double Temperature()
{
byte[] readBuffer = new byte[3] { 0, 0, 0 };
if (_i2cDevice == null)
{
throw new ArgumentNullException(nameof(_i2cDevice));
}
_i2cDevice.WriteByte(TemperatureNoHold);
Thread.Sleep(ReadingWaitmSec);
_i2cDevice.Read(readBuffer);
CheckCrc(readBuffer, 2, readBuffer[2]);
ushort temperatureRaw = (ushort)(readBuffer[0] << 8);
temperatureRaw += readBuffer[1];
double temperature = temperatureRaw * (175.72 / 65536.0) - 46.85;
return temperature;
}
I’m going to soak test the library for a week to check that is working okay, then refactor the code so it can be added to the nanoFramework IoT.Device Library repository.
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