XC8 novice question

Novice question using MPLABx + XC8

If i define my_variable using;

#define my_variable 0b10000000

And then I use it in a function that expects a 'char';

void my_function(char i);

I get a compiler warning;

warning: (752) conversion to shorter data type

By changing the function prototype to expect an 'int' the warning goes away so I think the defined my_variable is being assigned as an 'int'

So my question is how or can I structure my define so the defined my_variable is assigned as a 'char'..

I cannot see anything useful in the mplabx documentation.

Thanks.

(also posted to comp.arch.embedded)