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-- $Id: simbididly.vhd 793 2016-07-23 19:38:55Z mueller $
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--
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-- Copyright 2016- by Walter F.J. Mueller <W.F.J.Mueller@gsi.de>
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--
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-- This program is free software; you may redistribute and/or modify it under
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-- the terms of the GNU General Public License as published by the Free
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-- Software Foundation, either version 2, or at your option any later version.
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--
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-- This program is distributed in the hope that it will be useful, but
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-- WITHOUT ANY WARRANTY, without even the implied warranty of MERCHANTABILITY
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-- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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-- for complete details.
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--
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------------------------------------------------------------------------------
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-- Module Name: simbididly - sim
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-- Description: Bi-directional bus delay for test benches
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--
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-- Dependencies: -
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-- Test bench: tb_simbididly
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-- Target Devices: generic
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-- Tool versions: xst 14.7; viv 2016.2; ghdl 0.33
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-- Revision History:
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-- Date Rev Version Comment
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-- 2016-07-23 793 1.0.1 ensure non-zero DELAY
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-- 2016-07-17 789 1.0 Initial version (use separate driver regs now)
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-- 2016-07-16 787 0.1 First draft
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------------------------------------------------------------------------------
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library ieee;
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use ieee.std_logic_1164.all;
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use ieee.numeric_std.all;
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use work.slvtypes.all;
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entity simbididly is -- test bench bi-directional bus delay
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generic (
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DELAY : Delay_length; -- transport delay between A and B (>0ns!)
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DWIDTH : positive := 16); -- data port width
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port (
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A : inout slv(DWIDTH-1 downto 0); -- port A
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B : inout slv(DWIDTH-1 downto 0) -- port B
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);
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end entity simbididly;
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architecture sim of simbididly is
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type state_type is (
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s_idle, -- s_idle: both ports high-z
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s_a2b, -- s_a2b: A drives, B listens
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s_b2a -- s_b2a: B drives, A listens
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);
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constant all_z : slv(DWIDTH-1 downto 0) := (others=>'Z');
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signal R_STATE : state_type := s_idle;
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signal R_A : slv(DWIDTH-1 downto 0) := (others=>'Z');
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signal R_B : slv(DWIDTH-1 downto 0) := (others=>'Z');
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begin
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process
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variable istate : state_type := s_idle;
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begin
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-- the delay model can enter into a delta cycle oszillation mode
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-- when DELAY is 0 ns. So ensure the delay is non-zero
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assert DELAY > 0 ns report "DELAY > 0 ns" severity failure;
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while true loop
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-- if idle check whether A or B port starts to drive bus
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-- Note: both signal R_STATE and variable istate is updated
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-- istate is needed to control the driver section below in the
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-- same delta cycle based on the most recent state state
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istate := R_STATE;
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if now > 0 ns then -- to avoid startup problems
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if R_STATE = s_idle then
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if A /= all_z then
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R_STATE <= s_a2b;
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istate := s_a2b;
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elsif B /= all_z then
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R_STATE <= s_b2a;
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istate := s_b2a;
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end if;
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end if;
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end if;
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case istate is
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when s_a2b =>
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R_B <= transport A after DELAY;
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if A = all_z then R_STATE <= s_idle after DELAY; end if;
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when s_b2a =>
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R_A <= transport B after DELAY;
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if B = all_z then R_STATE <= s_idle after DELAY; end if;
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when others => null;
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end case;
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-- Note: the driver clash check is done by comparing an internal signal
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-- with the external signal. If they differ this indicates a clash.
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-- Just checking for 'x' gives false alarms when the bus is driven
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-- with 'x', which can for example come from a memory model before
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-- valid data is available.
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if now > 0 ns then -- to avoid startup problems
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case istate is
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when s_a2b =>
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assert B = R_B report "driver clash B port" severity error;
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when s_b2a =>
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assert A = R_A report "driver clash A port" severity error;
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when others => null;
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end case;
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end if;
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wait on A,B;
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end loop;
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end process;
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A <= R_A;
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B <= R_B;
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end sim;
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