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char conversion

 
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croc45
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char conversion
PostPosted: Sat Jul 14, 2007 1:18 pm     Reply with quote

Running into a bit of a snag that I believe I know why it is happening, but not sure how to fix it

I am reading data (temperature) from an I2C device (8bits of data), and the device returns it as celcius. I want to then convert it to fahrenheit using the following equation

int temp;
temp = I2C_data_read(); // function that returns the I2C data

temp = (((temp * 9) / 5) +32;

but when (temp *9) is higher than 255 my temp does not get displayed correctly on my LCD. I seems that even though I specify temp as an int, it still seems to be treated as a char and anything above 255 wraps around.

What am I doing wrong?
ckielstra



Joined: 18 Mar 2004
Posts: 3680
Location: The Netherlands

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PostPosted: Sat Jul 14, 2007 1:40 pm     Reply with quote

Quote:
What am I doing wrong?
You did not read the manual...

Code:
CCS Manual, Jan-2007, page 23
Basic Types

Type-Specifier
int1   Defines a 1 bit number   
int8   Defines an 8 bit number
int16  Defines a 16 bit number
int32  Defines a 32 bit number
char   Defines a 8 bit character
float  Defines a 32 bit floating point number
short  By default the same as int1
int    By default the same as int8         <<-------------
long   By default the same as int16
void   Indicates no specific type


In the C language the int datatype is defined to have the default size for the processor it is compiled for. In a Windows PC this is 32 bits, for the PIC it is 8 bits.

Because you can never assume the int datatype to have a certain size it is good programming practice to use other datatypes with a garuanteed size: int8, int16, etc.
croc45
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thanks ckielstra
PostPosted: Sat Jul 14, 2007 1:45 pm     Reply with quote

I thought it was something stupid that I was doing, those tend to be the hardest things to see ;-).

I'm new to the pics so I'm making assumptions that I should not, and your advise of reading the manual is well taken
ckielstra



Joined: 18 Mar 2004
Posts: 3680
Location: The Netherlands

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PostPosted: Sat Jul 14, 2007 2:41 pm     Reply with quote

Quote:
I thought it was something stupid that I was doing, those tend to be the hardest things to see ;-).
I totally agree. That said, there are no 'stupid' questions, better to ask soon before spending days searching. The problem is always to ask the 'right' question...
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