VHDL Programming

CPU: Synthesis Description 327

When a rising edge is applied to input clk, the data on input ais stored

in the register.

Input enis used to control output q. When enis a ‘ 1 ’value, the register

state is driven to output q. When enis a ‘ 0 ’, output qis a high impedance

value and not driving. This functionality is implemented by entity trireg

shown in the following:

library IEEE;

use IEEE.std_logic_1164.all;

use work.cpu_lib.all;

entity trireg is

port( a : in bit16;

port( en : in std_logic;

port( clk : in std_logic;

port( q : out bit16);

end trireg;

architecture rtl of trireg is

signal val : bit16;

begin

triregdata: process

begin

wait until clk’event and clk = ‘ 1 ’;

val <= a;

end process;

trireg3st: process(en, val)

begin

if en = ‘ 1 ’ then

q <= val after 1 ns;

elsif en = ‘ 0 ’ then

q <= “ZZZZZZZZZZZZZZZZ” after 1 ns;

-- exemplar_translate_off

else

a

q

Trireg

en

clk

Figure 13-12
Trireg Symbol.