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Emmo
Joined: 02 Feb 2017
Posts: 14
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| [solved] IEEE float and Microchip float confusion |
 Posted: Fri Jul 07, 2017 8:34 am |
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Hello everyone,
I've been a bit confused about float formats. I was working with a pic18f67j94 and ccs modbus library. I was reading out float values from a sensor and converted them to Microchip float:
| Code: |
unsigned int32 value = 0;
float Modbus_Values[MODBUS_VALUES_TO_READ] = {-256, -256, -256, -256, -256, -256, -256, -256};
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change_wordorder();
value = (((int32)modbus_rx.data[1]<<24) | ((int32)modbus_rx.data[2]<<16) | ((int32)modbus_rx.data[3]<<8) | ((int32)modbus_rx.data[4]));
Modbus_Values[TEMPERATURE_REQUEST] = f_IEEEtoPIC((int32)value); |
This was working very well. Now I've changed the controller to pic24fj512gb606 (Compiler v5.073) and I get strange values. I found out that pic24 is working inside with IEEE float format.
These are the 4 Bytes I get from modbus:
41 F3 FD 53
After shifting them to value, value is 30.498693 (IEEE float selected to display). This is still right. But after converting f_IEEEtoPIC((int32)value), Modbus_Values[TEMPERATURE_REQUEST] is -0,0 (displayed as Microchip float) or 4.20944358E9 (displayed as IEEE float). This is wrong :(
I also tried casting it like this:
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Modbus_Values[TEMPERATURE_REQUEST] = (float)value;
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But I got strange values, too and not 30.49. So how can I store an IEEE float, which is splitted into 4 Bytes, back into a IEEE float? What I'm doing wrong?
I spend now some hours to get it working, but now I'm out of ideas :-/
Thanks in advance!
Last edited by Emmo on Mon Jul 10, 2017 4:17 am; edited 1 time in total |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Fri Jul 07, 2017 1:06 pm |
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My advice is, get this floating point format conversion program:
http://www.piclist.com/techref/microchip/math/fpconvert.htm
In the Binaries section, click on the link for floatconv10.zip and download it.
Open the zip file and drag the floatconv.exe onto your Windows desktop.
Ignore the rest of it. Run the program, type in your byte values and
select the format you want on the left side, and click the button to
convert to float. Or vice versa. Investigate the problem. |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19539
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| Posted: Fri Jul 07, 2017 1:16 pm |
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All you need to do is use the same memory area for two types:
| Code: |
float fpval;
unsigned int32 intval;
#byte intval=fpval;
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Then with your number stored in 'fpval', you can use 'intval' where you previously used the result of the conversion.
Key point is that the conversion gives an int32, containing the four bytes in IEEE order. On the PIC30/24, the bytes are already in the right order, so all you need is an int32 variable that uses the same memory space as the float variable.
You can either do this as shown with #byte (which allows you to locate two variables to the same space), or using a union:
| Code: |
union {
unsigned int32 ival;
float fval;
} combine;
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Then if you write the float into combine.fval, combine.ival contains the int32.
Note that I'm explicitly using an unsigned int32. On the PIC18, the default integer is unsigned. On the PIC24, the default is signed. Hence it is better to be explicit. |
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Emmo
Joined: 02 Feb 2017
Posts: 14
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| Posted: Mon Jul 10, 2017 4:16 am |
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| Ttelmah wrote: | All you need to do is use the same memory area for two types:
| Code: |
float fpval;
unsigned int32 intval;
#byte intval=fpval;
|
Then with your number stored in 'fpval', you can use 'intval' where you previously used the result of the conversion.
Key point is that the conversion gives an int32, containing the four bytes in IEEE order. On the PIC30/24, the bytes are already in the right order, so all you need is an int32 variable that uses the same memory space as the float variable.
You can either do this as shown with #byte (which allows you to locate two variables to the same space), or using a union:
| Code: |
union {
unsigned int32 ival;
float fval;
} combine;
|
Then if you write the float into combine.fval, combine.ival contains the int32.
Note that I'm explicitly using an unsigned int32. On the PIC18, the default integer is unsigned. On the PIC24, the default is signed. Hence it is better to be explicit. |
Ohh yeah it is working! Thank you very much! You saved my week :-)
I just have to remove the ";" after #byte
| Code: |
float fpval;
unsigned int32 intval;
#byte intval=fpval
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Ttelmah
Joined: 11 Mar 2010
Posts: 19539
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| Posted: Tue Jul 11, 2017 8:56 am |
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My semicolon autopilot was on... 
Have been posting for a few days with a very small screen and slow 'broadband'. Makes it easy to not spot mistakes like this... |
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