Seeing temperature values like 25.31, I wonder how the values are truly encoded. It is documented as:
Temperature measured in the device, expressed in degree Celsius. Encoded form using lo= - 44, hi= 85, nbits= 8, nresv= 0. It is given with a step of 0.5°C
But while decoding the step is calculated as 1 / (255 / 129) = 0.5058823529411764, yielding values such as 25.30588235294117. Here, JavaScript’s floating point numbers might be to blame partially. But even in exact math 1 / (255 / 129) will have an infinite number of digits, so I guess I would have used different low and high values, or use a different formula for the step.
One might be tempted to round the calculated step, but for the battery reading the calculated step yields 0.005533596837944666 and is documented as 5.5 mV. Or one might want to use the documented step, but even that will run into JavaScript rounding errors for (value - nresv / 2), if nresv is not zero:
function mt_value_decode2(value, lo, nresv, step_size) {
return (value - nresv / 2) * step_size + lo;
}
Also, with the given documentation there is just no way to tell if the above should really be interpreted as 25.3 or 25.5 (not even taking the sensor’s accuracy into account)…
Or: the above could also indicate a documentation error, if different values for the high and low values are used while encoding? If the device has any option to see the actual values, then I’d compare that to the output.
All said, while looking at that code, I happened to run into a wrong value for nresv in the temperature decoding, also present in @flodenh’s version. (And there might be more: validate against the documentation!) I fixed that in the version below, and also simplified the GPS handling, though I’ve no payloads to validate that. Just to be able to refer to this example in the future, I also changed the output of single-bit values into booleans, and made the error reporting more consistent:
/**
* Decoder for Abeeway Microtracker.
*
* 2019-02-09: initial release
* 2019-02-13: fixed "nresv" for temperature reading;
* using booleans for single-bit values;
* improved error reporting
* 2019-02-13: fixed message()
* 2020-06-16: simplified uint32()
* added warning about Data Storage Integration
*/
function Decoder(bytes, port) {
// nbits: number of bits used to encode
// lo: min value that can be encoded
// hi: max value that can be encoded
// nresv: number of reserved values, not used for the encoding
function step_size(lo, hi, nbits, nresv) {
return 1.0 / ((((1<<nbits) - 1) - nresv) / (hi - lo));
}
function mt_value_decode(value, lo, hi, nbits, nresv) {
return (value - nresv / 2) * step_size(lo, hi, nbits, nresv) + lo;
}
// Gets the zero-based unsigned numeric value of the given bit(s)
function bits(value, lsb, msb) {
var len = msb - lsb + 1;
var mask = (1<<len) - 1;
return value>>lsb & mask;
}
// Gets the boolean value of the given bit
function bit(value, bit) {
return (value & (1<<bit)) > 0;
}
// Gets a hexadecimal representation ensuring a leading zero for each byte
function hex(bytes, separator) {
return bytes.map(function (b) {
return ("0" + b.toString(16)).substr(-2);
}).join(separator || "");
}
// Decodes 4 bytes into a signed integer, MSB
function int32(bytes) {
// JavaScript bitwise operators always work with 32 bits signed integers
return bytes[0]<<24 | bytes[1]<<16 | bytes[2]<<8 | bytes[3];
}
// Decodes 4 bytes into an unsigned integer, MSB
function uint32(bytes) {
// Or, same result:
// return bytes[0] * 0x1000000 + bytes[1] * 0x10000 + bytes[2] * 0x100 + bytes[3];
// return bytes[0] * 0x1000000 + (bytes[1]<<16 | bytes[2]<<8 | bytes[3]);
// return bytes[0] * (1<<24) + (bytes[1]<<16 | bytes[2]<<8 | bytes[3]);
// return int32(bytes)>>>0;
// JavaScript bitwise operators always work with 32 bits signed integers;
// force conversion to unsigned 32 bits value using zero-fill right shift
return (bytes[0]<<24 | bytes[1]<<16 | bytes[2]<<8 | bytes[3])>>>0;
}
// Decodes 1 to 4 MAC addresses and their RSSI
function mac_rssi(bytes) {
var items = [];
for (var offset = 0; offset < bytes.length; offset += 7) {
items.push({
mac_address: hex(bytes.slice(offset, offset + 6), ":"),
// Sign-extend to 32 bits to support negative values; dBm
rssi: bytes[offset + 6]<<24>>24,
});
}
return items;
}
function message(code, descriptions) {
return {
code: code,
description: code < 0 || code >= descriptions.length ? "UNKNOWN" : descriptions[code]
};
}
var decoded = {};
var i;
var type = bytes[0];
// All message types, except for Frame pending messages, share the same header
if (type !== 0x00) {
// Note: the Data Storage Integration stores nested objects as text
decoded.status = {
mode: message(bits(bytes[1], 5, 7), ["Standby", "Motion tracking", "Permanent tracking",
"Motion start/end tracking", "Activity tracking", "OFF"]),
sos: bit(bytes[1], 4),
tracking: bit(bytes[1], 3),
moving: bit(bytes[1], 2),
periodic: bit(bytes[1], 1),
on_demand: bit(bytes[1], 0)
};
// Or, same result:
// // Unary plus-operator to cast string results of toFixed to a number:
// decoded.battery = +mt_value_decode(bytes[2], 2.8, 4.2, 8, 2).toFixed(2);
// decoded.temperature = +mt_value_decode(bytes[3], -44, 85, 8, 0).toFixed(2);
decoded.battery = Math.round(100 * mt_value_decode(bytes[2], 2.8, 4.2, 8, 2)) / 100;
decoded.temperature = Math.round(100 * mt_value_decode(bytes[3], -44, 85, 8, 0)) / 100;
decoded.ack = bits(bytes[4], 4, 7);
decoded.data = bits(bytes[4], 0, 3);
}
switch (type) {
case 0x00:
decoded.type = "FRAME PENDING";
decoded.token = bytes[1];
break;
case 0x03:
decoded.type = "POSITION";
switch (decoded.data) {
case 0:
decoded.position_type = "GPS fix";
decoded.age = mt_value_decode(bytes[5], 0, 2040, 8, 0);
// Signed 32 bits integers; LSB is always zero
decoded.latitude = (bytes[6]<<24 | bytes[7]<<16 | bytes[8]<<8) / 1e7;
decoded.longitude = (bytes[9]<<24 | bytes[10]<<16 | bytes[11]<<8) / 1e7;
// Estimated Horizontal Position Error
decoded.ehpe = mt_value_decode(bytes[12], 0, 1000, 8, 0);
break;
case 1:
decoded.position_type = "GPS timeout";
decoded.timeout_cause = message(bytes[5], ["User timeout cause"]);
for (i = 0; i < 4; i++) {
// Carrier over noise (dBm) for the i-th satellite seen
decoded["cn" + i] = mt_value_decode(bytes[6 + i], 0, 2040, 8, 0);
}
break;
case 2:
// Documented as obsolete
decoded.error = message(0, ["UNSUPPORTED POSITION TYPE " + decoded.data]);
break;
case 3:
decoded.position_type = "WIFI timeout";
for (i = 0; i < 6; i++) {
decoded["v_bat" + (i + 1)] = mt_value_decode(bytes[5 + i], 2.8, 4.2, 8, 2);
}
break;
case 4:
decoded.position_type = "WIFI failure";
for (i = 0; i < 6; i++) {
// Most of time a WIFI timeout occurs due to a low battery condition
decoded["v_bat" + (i + 1)] = mt_value_decode(bytes[5 + i], 2.8, 4.2, 8, 2);
}
decoded.error = message(bytes[11], ["WIFI connection failure", "Scan failure",
"Antenna unavailable", "WIFI not supported on this device"]);
break;
case 5:
case 6:
decoded.position_type = "LP-GPS data";
// Encrypted; not described in the documentation
decoded.error = message(0, ["UNSUPPORTED POSITION TYPE " + decoded.data]);
break;
case 7:
decoded.position_type = "BLE beacon scan";
decoded.age = mt_value_decode(bytes[5], 0, 2040, 8, 0);
// Remaining data: up to 4 beacons
decoded.beacons = mac_rssi(bytes.slice(6));
break;
case 8:
decoded.position_type = "BLE beacon failure";
decoded.error = message(bytes[5], ["BLE is not responding", "Internal error", "Shared antenna not available",
"Scan already on going", "No beacon detected", "Hardware incompatibility"]);
break;
// Test with: 0358D895090EC46E1FF44B9EB76466B3B87454AD500959CA1ED4AD525E67DA14A1AC
// or 032CD1890900C46E1FF44B9EC5C83A355A3898A6
case 9:
decoded.position_type = "WIFI BSSIDs";
decoded.age = mt_value_decode(bytes[5], 0, 2040, 8, 0);
// Remaining data: up to 4 WiFi BSSIDs
decoded.stations = mac_rssi(bytes.slice(6));
break;
default:
decoded.error = message(0, ["UNSUPPORTED POSITION TYPE " + decoded.data]);
}
break;
case 0x04:
decoded.type = "ENERGY STATUS";
break;
case 0x05:
decoded.type = "HEARTBEAT";
break;
case 0x07:
// Activity status message and configuration message share the same identifier
var tag = bytes[5];
switch (tag) {
case 1:
decoded.type = "ACTIVITY STATUS";
decoded.activity_counter = uint32(bytes.slice(6, 10));
break;
case 2:
decoded.type = "CONFIGURATION";
for (i = 0; i < 5; i++) {
var offset = 6 + 5 * i;
decoded["param" + i] = {
type: bytes[offset],
value: uint32(bytes.slice(offset + 1, offset + 5))
};
}
break;
default:
decoded.error = message(0, ["UNSUPPORTED POSITION TYPE " + decoded.data + "/" + tag]);
}
break;
case 0x09:
decoded.type = "SHUTDOWN";
break;
case 0xFF:
decoded.type = "DEBUG";
break;
default:
decoded.error = message(0, ["UNSUPPORTED MESSAGE TYPE " + type]);
}
// Just some redundant debug info
decoded.debug = {
payload: hex(bytes),
length: bytes.length,
port: port,
server_time: new Date().toISOString()
};
return decoded;
}
Still interested!
