Forum: FPGA, VHDL & Verilog VHDL if construct assistance

von Rejoy Roy Mathews (Guest)

Rate this post
0 useful
not useful
Port w, x, y and z are defined as STD_LOGIC.

In my 'if' construct I use the below condition
if((not w) and (not x) and (not y) and (not z)) then
--Do something

instead of the actual proper syntax as below.

 if( w = '0' and x = '0' and y = '0' and z = '0') then
--Do something

The former implementation fails and returns the below error
type error near w ; current type std_logic; expected type boolean

I understand that VHDL is a strongly typed language and hence it returns 
this error.

However consider a second case i.e. if i had to do an assignment c <= 
not a (Where a and c are of type STD_LOGIC), the execution will proceed 
without an error.

I am looking for a proper reason to give my students as to why the 'if' 
construct fails but the assignment succeeds in VHDL.

Thank you.

von Lothar M. (lkmiller) (Moderator)

Rate this post
0 useful
not useful
The "not" operator for std_logic gives a std_logic in return.

A "=" operator gives a boolean in return.

A "and" operator expects boolean operands.

With that information the behavior becomes clear.

: Edited by Moderator
von Rejoy Roy Mathews (Guest)

Rate this post
0 useful
not useful
Thank you Lothar Miller for the clear and succinct explanation.

von Markus F. (mfro)

Rate this post
0 useful
not useful
VHDL 2008 relaxes the overly strict behaviour and allows std_logic in 
boolean context within if statements.

This can even be enforced with the '??'-Operator where this behaviour 
would lead to ambiguity.


Entering an e-mail address is optional. If you want to receive reply notifications by e-mail, please log in.

Rules — please read before posting

  • Post long source code as attachment, not in the text
  • Posting advertisements is forbidden.

Formatting options

  • [c]C code[/c]
  • [avrasm]AVR assembler code[/avrasm]
  • [vhdl]VHDL code[/vhdl]
  • [code]code in other languages, ASCII drawings[/code]
  • [math]formula (LaTeX syntax)[/math]

Bild automatisch verkleinern, falls nötig
Note: the original post is older than 6 months. Please don't ask any new questions in this thread, but start a new one.