Data Type Conversions

Andy202

I have been trying to interface with a software program which uses a message made up of a 20 word array of unsigned shorts;

unsigned short messageData[20];

I have been trying to use data from a home GPS system which includes Lat and Long and heading.; these have been converted from strings to floats. e.g. float lat, long, heading; My problem is that as I know the scaling factors and least Sig Bit (LSB) for the lat, long and heading in the messageData buffer; e.g. For lat and long LSB = 8.377E-08; Twos complement - 32 bits Max/ Min -180 to +180 For Heading LSB 0.00549306; Twos complement - 16 bits Max/ Min -180 to +180 Setting the heading:- unsigned short heading = (unsigned short)(newHeading/0.00549306); messageData[10] = heading; Getting the heading:-

bool negative;
unsigned short currentField = messageData[10];
if(currentField & 0x8000)
{
// Set the negative flag
negative = true;
}
else
negative = false;
float heading = (currentField & 0x7FFF) * g_nScalings[wordPos];
if(negative)
heading = -(180 - heading);

But this does not work as I expected; am I missing something? As for the 32 bit latitude, I was going to this:- Setting the Latitude:-

union
{
int lat_long;
unsigned short buff[2];
}convert;

unsigned short highWord, lowWord;
int newData = (int)(fEngValue/8.377E-08);

convert.lat_long = newData;
highWord = convert.buff[1];
lowWord = convert.buff[0];

I need to get and set data in the message that is of type unsigned short and use in my program as scaled floats. Any suggestions please.

CPallini

Could you please elaborate a bit. :confused: Could you post inputs, wrong and expected outputs? :)

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Mike the Red

If you're dealing with floats that have been converted to strings, just convert them back to floats. The unsigned short is the same as wchar_t - you should be able to use an explicit cast to 'convert' between them. _wtof() converts a wchar_t string to a double, which you can then cast to a float, if the value is withing the range of type float. _ecvt() converts a double (or float cast to double) to a wchar_t string, though you'll have to do some string manipulation to put the sign and/or decimal in the indicated place(s). Both functions are in <stdlib.h>. Hope this is useful, MZR

Luc Pattyn

1. could you give some actual data samples, where both the short representation and the real value are known? four or more pairs would be good. 2. for 32-bit data, for sure there only is one sign bit, so both halves need different treatment. :)

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Andy202

Sorry not made myself very clear. Forget about strings to floats thats OK. I need to cast from a float to a unsigned short using the information about the format about the unsigned short; e.g. top bit is sign, LSB is the scaling. Now if I run my code with a heading set to 3.45 this sets the unsigned short to 0xB999 which is not what I expected. Or looking at it the other way if the value of the unsigned short is 0x0001 then this would represent heading = (float)0x0001 * LSB. The messageData holds the data as unsigned shorts which need to be processed on Get and Set if that makes sense. For Lat and Long two elements are used, for heading just one.

Andy202

Just to recap and get things right in my mind; if a heading value is represented as an unsigned short (16 bits) with bit 1 = 0.000030517578125; so the range is -1 to + 1 and if we multiple by 180 we get -180 to +180 degrees - more useful Now for a test I set heading to 3.45 then the following occures. 3.45/0.000030517578125 = 1B999 in Hex; but as we can only store 16 bits in the unsigned short the debugger shows B999 as the value. Now when I come to decode that vaue via:- B999 * 0.000030517578125 = 1.44998169 which is wrong and if x 180 to get into degrees then get 260.99. Now for reading another value say, e.g. the value is 1234hex then we get:- 1234Hex * 0.000030517578125 = 0.14221191 and if we convert into degrees then get 0.14221191 * 180 = 25.598144 degrees. Hope this helps to explan what I am trying to do.

Luc Pattyn

I see you ignored all I asked and said. If I understood what you meant correctly, then: the LSB value is 360/0x10000 (approx 0.00549) assuming a monotonous binary representation (as opposed to e.g. sign+mantissa), we get:

0x7FFF = almost 180
0x0000 = 0
0x8000 = -180

3.45 --> 3.45*0x10000/360 = 628 = 0x0274 -3.45 --> -3.45*0x10000/360 = -628 = 0xFD8C :|

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Andy202

Sorry Luc, as you may have noticed I am getting more confused, but think getting there from your last post. One field in this array was height which ranged from 0 to 10,000, but the LSB was 0.5, and the sign not used; e.g. bit 15 was data. So if you had 0000 0000 0000 1000 which is 8 then that represented 4 meters. I got this working and then turned to see why the heading and lat/long were not! But with heading which did use the sign and had a range of -1 to +1 and a very small LSB. May next question is are they a formula to handle these types of conversions. Just for a Windows platforn with the VS environment. Still need to sort out the two unsigned short Lat and Long.