Cayenne Low Power Payload (LPP) Encoder

I originally started building my own Cayenne Low Power Protocol(LPP) encoder because I could only find one other Github repository with a C# implementation. There hadn’t been any updates for a while and I wasn’t confident that I could make the code work on my nanoFramework and TinyCLR devices.

I started with the sample Mbed C code and did a largely mechanical conversion to C#. I then revisited some of the mathematics where floating point values were converted to an integer.

The original C++ code (understandably) had some language specific approaches which didn’t map well into C#

uint8_t CayenneLPP::addTemperature(uint8_t channel, float celsius) {
    if ((cursor + LPP_TEMPERATURE_SIZE) > maxsize) {
        return 0;
    }
    int16_t val = celsius * 10;
    buffer[cursor++] = channel; 
    buffer[cursor++] = LPP_TEMPERATURE; 
    buffer[cursor++] = val >> 8; 
    buffer[cursor++] = val; 

    return cursor;
}

I then translated this code to C#

public void TemperatureAdd(byte channel, float celsius)
{
   if ((index + TemperatureSize) > buffer.Length)
   {
      throw new ApplicationException("TemperatureAdd insufficent buffer capacity");
   }

   short val = (short)(celsius * 10);

   buffer[index++] = channel;
   buffer[index++] = (byte)DataType.Temperature;
   buffer[index++] = (byte)(val >> 8);
   buffer[index++] = (byte)val;
}

One of my sensors was sending values with more decimal places than LPP supported and I noticed the value was not getting rounded e.g. 2.99 ->2.9 not 3.0 etc. So I revised my implementation to use Math.Round (which is supported by the nanoFramework and TinyCLR).

public void DigitalInputAdd(byte channel, bool value)
{
   #region Guard conditions
   if ((channel < Constants.ChannelMinimum) || (channel > Constants.ChannelMaximum))
   {
      throw new ArgumentException($"channel must be between {Constants.ChannelMinimum} and {Constants.ChannelMaximum}", "channel");
   }

   if ((index + Constants.DigitalInputSize) > buffer.Length)
   {
      throw new ApplicationException($"Datatype DigitalInput insufficent buffer capacity, {buffer.Length - index} bytes available");
   }
   #endregion

   buffer[index++] = channel;
   buffer[index++] = (byte)Enumerations.DataType.DigitalInput;

   // I know this is fugly but it works on all platforms
   if (value)
   {
      buffer[index++] = 1;
   }
   else
   {
     buffer[index++] = 0;
   }
 }

I then extracted out the channel and buffer size validation but I’m not certain this makes the code anymore readable/understandable

public void DigitalInputAdd(byte channel, bool value)
{
   IsChannelNumberValid(channel);
   IsBufferSizeSufficient(Enumerations.DataType.DigitalInput);

   buffer[index++] = channel;
   buffer[index++] = (byte)Enumerations.DataType.DigitalInput;

   // I know this is fugly but it works on all platforms
   if (value)
   {
      buffer[index++] = 1;
   }
   else
   {
      buffer[index++] = 0;
   }
}

The code runs on netCore, nanoFramework, and TinyCLRV2 just needs a few more unit tests and it will be ready for production. I started with an LPP encoder which I needed for one of my applications. I’m also working an approach for a decoder which will run on all my target platforms with minimal modification or compile time directives.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.