Decoding a float value in a payload function

Application Development
24

This goes horribly wrong for any value that does not convert to exactly four characters. (If you found that code elsewhere, please refer the author to this answer…)

Instead, use something like explained above:

// Read sensor values and multiply by 100 to effectively keep 2 decimals
// Signed 16 bits integer, -32767 up to +32767
int16_t t = dht.readTemperature() * 100;
// Unsigned 16 bits integer, 0 up to 65535
uint16_t h = dht.readHumidity() * 100;

This can be encoded into just 4 bytes for the 2 values, using some “shifting” as explained in more detail in Decrypting messages for dummies:

byte buffer[4];
buffer[0] = t >> 8;
buffer[1] = t;
buffer[2] = h >> 8;
buffer[3] = h;
LMIC_setTxData2(1, buffer, sizeof(buffer), 0);

And decoded like also explained in more detail in the link above:

function Decoder(bytes, port) {
  var t = (bytes[0] & 0x80 ? 0xFFFF<<16 : 0) | bytes[0]<<8 | bytes[1];
  var h = bytes[2]<<8 | bytes[3];
  return {
    temperature: t / 100,
    humidity: h / 100
  }
}

You could probably even send humidity without any decimals, saving yet another byte:

// Unsigned 8 bits integer, 0 up to 255
uint8_t h = dht.readHumidity();

…with:

byte buffer[3];
buffer[0] = t >> 8;
buffer[1] = t;
buffer[2] = h;
LMIC_setTxData2(1, buffer, sizeof(buffer), 0);

…and:

function Decoder(bytes, port) {
  var t = (bytes[0] & 0x80 ? 0xFFFF<<16 : 0) | bytes[0]<<8 | bytes[1];
  var h = bytes[2];
  return {
    temperature: t / 100,
    humidity: h
  }
}
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