CODEWITHSUNDEEP
javascriptsphero-apisphero
Michael Oryl
Michael Oryl

Reputation: 21662

Interpreting Sphero 16-bit values (such as in Collisions)

I'm using Cylon.js and Node.js to work with a Sphero, and I'm not sure how I am supposed to reconstruct the 16 bit values from the data returned from a collision event.

The docs imply that they should be signed 16-bit integers, and that the numbers come in (MSB, LSB) byte order, but I get oddball values when I attempt to convert two bytes into what should be negative power ratings (such as impacts from behind or from the left).

The Javascript code I am using to convert a two-btye Sphero value is this:

  function convertToSignedInt(msb, lsb) {
    var negative = msb > 128;
    if (negative) {
      msb -= 128;
    }
    var value = msb*256 + lsb;
    if (negative) {
      value = 0 - value;
    }
    return value;
  }

Given raw collision data like the following:

{"SOP1":255,"SOP2":254,"ID_CODE":7,"DLEN":17,"DATA":[18,31,255,104,0,0,1,0,67,0,6,0,0,9,1,95],"CHK":156}

I then attempt to convert the DATA property to a more readable object using this:

  function convertCollisionData(data) {
    var obj = {};
    obj.xPower = convertToSignedInt(data[0], data[1]);
    obj.yPower = convertToSignedInt(data[2], data[3]);
    obj.zPower = convertToSignedInt(data[4], data[5]);
    obj.impactAxis = data[6];
    obj.xImpact = convertToSignedInt(data[7], data[8]);
    obj.yImpact = convertToSignedInt(data[9], data[10]);
    obj.speed = data[11];

    return obj;
  }

To come up with this:

{"xPower":4639,"yPower":-32616,"zPower":0
    ,"impactAxis":1,"xImpact":67,"yImpact":6,"speed":0}

It's obvious that the yPower is completely wrong, since this was a right-side (positive X) collision. Since the xImpact and yImpact values are positive, they appear to be converting correctly.

So the short version of the question is, what is the proper way to convert the two bytes to a signed integer?

Upvotes: 0

Views: 139

Answers (0)

Related Questions