A loop problem which does not do what is expected

Hi, I rewrote the above code, but it still does not work.

in the project package, I define the following function: function Get_Index (x: std_logic_vector) return integer is variable index : integer; begin index := 0; for j in x'low to x'high loop if x(index) = '1' then index := j; -- if j-th bit is asserted, it returns j+1, to distinguish 0 as no bit set return index; end if; end loop; return 255; -- no bit is asserted end Get_Index;

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Error_p : process(RESET, CLK) variable v_Index : integer; procedure INIT is begin Error_Oi

Hi Hans and Rick,

Please help me! I don' work for any company and nobody can help me except the web.

Weng

Hi Weng,

Fortunately I do work for a company and free time is very scares. This newsgroup is probably also not the best place to ask VHDL questions, stack exchange or the many vendor mailing lists might be better.

I just had a quick look at your code and in your first example no latching occurs as you never assert Error_O_v so your loop is never entered:

Error_O_v := '0'; for j in 0 to G_TOP_LEVEL loop if Error_O_v = '0' then -- always 0 if Error_O_m(j) = '1' then Error_O_v := '1';

In your second example you never clear Error_O but instead you clear Error_Oi. So I suspect you get an 'X' for Error_O?

Simulation should answer all your questions, just keep your code as simple as possible.

Good luck, Hans

Hi Hans, Thank you for your help.

Error_O_v := '0'; -- its value is initialized here for j in 0 to G_TOP_LEVEL loop ...if Error_O_v = '0' then -- always 0? it is true on first entry ......if Error_O_m(j) = '1' then -- an error happens if true ..........Error_O_v := '1'; -- it is refreshed here if an error is detected, after that above if-condition = false, and all remaining testing skips

You are right, but it is an error that is corrected after my posting, Error_Oi is introduced to be the internal value of output port Error_O for VHDL-2002: Error_O

.. snip

Hi Weng,

index will remain 0 if x(0) is '0', perhaps you want "if x(j)='1'" ?

I would suggest you single step through your design as this will show you the issue. Just open the file in the Modelsim editor, set a breakpoint on the index:=0 line hit run -all then single step through the design.

Good luck,

Regards, Hans.

Hi Hans, Under your guidance, the problem is resolved. Here is my latest version of Get_Index:

function Get_Index (x: std_logic_vector) return integer is begin ......for j in x'low to x'high loop ............if x(j) = '1' then -- it is the error you help me to find, originally if x(index) = '1' then ..................return j; ............end if; ......end loop; ......return 255; -- no bit is asserted end Get_Index;

This error gave me a deep lesson: I must check again and again to make sure no design error exists!

Thank you very much!!!

Weng

I can't tell if your Get_Index procedure works. You initialize index to 0 which is the value used to index X the first time through. But the value returned is X'low if that first X(index) is '1'.

The comment, -- if j-th bit is asserted, it returns j+1, to distinguish 0 as no bit set" seems to be saying the value returned should be either j+1 or 255 if no '1' bits are found.

index := 0; if x(index) = '1' then index := j; return index; end if; end loop; return 255;

This looks wrong to me. I would try....

index := 255; if x(j) = '1' then index := j+1; return index; end if; end loop; return index;

Hi Rick, I have found the error indicated by Hans, and before your posting, I post my final result:

function Get_Index (x: std_logic_vector) return integer is begin ......for j in x'low to x'high loop ............if x(j) = '1' then -- it is the error you help me to find, originally if x(index) = '1' then ..................return j; ............end if; ......end loop; ......return 255; -- no bit is asserted end Get_Index;

My code is simpler than yours.

Thank you.

Weng

Yes, your code is more simple, but does not do what you specified.

Your comment, "-- if j-th bit is asserted, it returns j+1, to distinguish 0 as no bit set" clearly asks for the value of J+1 to be returned if index J is asserted. Your code does not do that, it returns the value J or 255 if no bit is set.

So is your specification wrong, or is the code wrong?

It is not at all infrequent that failures in code is directly related to failures in the specification. That's why it is important to start with a well designed specification and test the code to the specification at the unit level.

At this point I have no idea if your code is wrong or if your specification is wrong.