VHDL Programming

Predefined Attributes 163

If the optional time expression for attribute ’DELAYEDis not specified,

0 ns is assumed. A signal delayed by 0 ns is delayed by one delta. (Delta

delay is discussed in Chapter 2.)

Another application for the ’DELAYEDattribute is to perform a hold-check.

Earlier in this chapter, we discussed what setup and hold times were and

how to implement the setup check using ’LAST_EVENT. Implementing the

hold-check requires the use of a delayed version of the clksignal. The

example shown earlier has been modified to include the hold-check function

as shown here:

LIBRARY IEEE;

USE IEEE.std_logic_1164.ALL;

ENTITY dff IS

GENERIC ( setup_time, hold_time : TIME );

PORT( d, clk : IN std_logic;

PORT( q : OUT std_logic);

BEGIN

setup_check : PROCESS ( clk )

BEGIN

IF ( clk = ’ 1 ’ ) and ( clk’EVENT ) THEN

ASSERT ( d’LAST_EVENT >= setup_time )

REPORT “setup violation”

SEVERITY ERROR;

END IF;

END PROCESS setup_check;

hold_check : PROCESS (clk’DELAYED(hold_time))

BEGIN

IF ( clk’DELAYED(hold_time) = ’ 1 ’ ) and

( clk’DELAYED(hold_time)’EVENT ) THEN

ASSERT ( d’LAST_EVENT = 0 ns ) OR ( d’LAST_EVENT >

hold_time )

REPORT “hold violation”

SEVERITY ERROR;

END IF;

END PROCESS hold_check;

END dff;

ARCHITECTURE dff_behave OF dff IS

BEGIN

dff_process : PROCESS ( clk )

BEGIN

IF ( clk = ’ 1 ’ ) AND ( clk’EVENT ) THEN

q <= d;

END IF;

END PROCESS dff_process;

END dff_behave;