4 Hex bytes To Float - exponent and mantissa [SOLVED]

[Gabriel]

Hi all,

I have tried this:

[PCM programmer]

Download this zip to your Windows desktop.
http://www.piclist.com/images/floatconv10.zip
Open it, and drag floatconv.exe onto your desktop.
Run it. You can select the float format. CCS uses
"Microchip 32-bit" format for the PCM and PCH compilers.
You can enter the 4 bytes, then click the Convert to Float
button, and it will show you the decimal floating point number.

[Gabriel]

Hi PCM, thank you for your quick reply.

In retrospect i dont think I asked the right question so i will further explain.

I am reading a Modbus device from my PIC, the device has the data i need as a 4 hex bytes that add up to a float which is the device's sensor reading.

I want to display the sensor value to terminal as a float number that any human can understand.

Just to be clear This is not a modbus question.... its a how to convert data to float question.

is there any function to do this? how do i build the float from those bytes in code?

Edit:
Just for reference my modbus device seems to be using IEEE754 according to your app and the example provided on the original post.

Thanks.
G.
_________________
CCS PCM 5.078 & CCS PCH 5.093

[Gabriel]

Hi PCM...

I'm looking at the code of that little app... seems like i can get the conversion code from the .H files...

I'll let you know if run into any problems.

Thanks!
_________________
CCS PCM 5.078 & CCS PCH 5.093

[Gabriel]

For those interested in this below is my "working" code.

Works fine for numbers ending in .0 or .5 but its always a bit off for other decimals for reasons i dont yet understand.

[Jerson]

I would think the right way to approach this is to make a union to which you assign the individual bytes. Then read the union as a float.

union {
float f;
int8 bytes[4];
} Float_bytes;

Only, you will need to identify which byte is MSB and which is LSB and do the stuffing accordingly.

[Ttelmah]

Jerson's answer is right if this was a CCS float.
There is though a caveat. The data being sent is in IEEE format, not CCS. With the compiler there is a library 'ieefloat.c'. This contains the routines to convert to/from an int32 containing the bytes, to a float (and vice versa).
So:

[Gabriel]

Well this worked like a charm! Thanks Mr. T.

Now I'm left with an itch cause i don't know why my attempt was failing with the decimal precision.

Thanks
G.
_________________
CCS PCM 5.078 & CCS PCH 5.093

[PCM programmer]

[Gabriel]

Huh... i thought i was shifting INTO "Mantissa" and thus the shifting byte size did not matter.

Thanks for clearing that up!
No wonder i was losing all decimal presicion!

Thank you.
G.
_________________
CCS PCM 5.078 & CCS PCH 5.093

[Ttelmah]

No.
That is a general C thing. All arithmetic sizes are based on the size of the sources not the destination. The result is put into the destination, but not till after the maths is done.

So two int8 values (say 130 and 150 in 'a' and 'b'). Destination int16 (say 'c'). Add them together as:

c=a+b;

or multiply etc., and the maths will overflow.

However if one source is larger this size will be used for the maths. So:

c=a+150L; //here the '150' is declared as a 'long'

or:

c=(int16)a+b; //convert 'a' to int16 before the arithmetic.

Will both use int16 arithmetic.

On some chips (for example the Intel processors particularly when using the hardware maths processor), the arithmetic will automatically be handled using a larger type, but this is specific behaviour to these chips, not how C defines this to be done....

[RF_Developer]