The problem
I'm writing a function in a package which converts some values for a testbench. I want to check the if the output exceeds a maximum value, if it does I want to set it to that maximum value. What I tired was the following:
-- vec_in: 0...1023, returns -14...23.5 dB
function conv_dac602_scale (
vec_in : std_logic_vector)
return real is
variable val_in, dB : real := 0.0;
constant lower : real := -14.0;
constant upper : real := 23.5;
begin -- function conv_dac602_scale
val_in := real(to_integer(unsigned(vec_in)));
dB := (lower+(val_in*((upper-lower)/1024.0)));
return dB when dB <= upper else upper; -- this is the important line! (129)
end function conv_dac602_scale;
When I try to compile this I got the following errors:
** Error: myfile.vhd(129): near "when": expecting ';'
** Error: myfile.vhd(260): VHDL Compiler exiting
I then tried assigning it to a variable r first:
...
r := dB when dB <= upper else upper; -- this is the important line! (129)
return r;
end function conv_dac602_scale;
Which did not change the outcome. I know that I can use a simple if/else clause instead, but my question is why I can't use the when clause.
System
Modelsim SE 10.0b, VHDL 2008
The target <= signal when x is a so called concurrent statement, which is designed for easy creation of signal assignments outside a process. You can use it within an architecture, but not inside a process. If and else are designed for sequential statements within a process. In your case, you'll have to use if/else.
Edit: Seems this only holds true for Vhdl pre 2008. As fru1tbat pointet out, this is valid vhdl 2008 code and the problem is a not supported feature by the Modelsim compiler.