VHDL Programming

22 Chapter Two

AB

Delay = 20 ns

A

B

0 10 20 30 40

Figure 2-5
Transport Delay
Buffer Waveforms.

propagation.

At time 10 nanoseconds, the buffer model is executed and schedules an

event for the output to go to a 1 value at 30 nanoseconds. At time 20

nanoseconds, the buffer model is re-invoked and predicts a new value for

the output at time 40 nanoseconds. With the transport delay algorithm,

the events are put in order. The event for time 40 nanoseconds is put in

the list of events after the event for time 30 nanoseconds. The spike is not

swallowed but is propagated intact after the delay time of the device.

Inertial Delay Model

The following model shows how to write an inertial delay model. It is

the same as any other model we have been looking at. The default delay

type is inertial; therefore, it is not necessary to specify the delay type to

be inertial:

LIBRARY IEEE;

USE IEEE.std_logic_1164.ALL;

ENTITY buf IS

PORT ( a : IN std_logic;

PORT ( b : OUT std_logic);

END buf;