VHDL HelloWorld

First step is to write a "Hello World" program. This one just counts cycles of the input clock, and displays a binary count on the LED's. Sample code is included here. hello.vhd implements the counter and connects to the LED's. The ucf file defines the clock and LED connections on the board.These would need to be changed to use anything other than this specific Spartan 3AN development board.
It loads and the lights blink, so now we can do some real work.

-- $Id: hello.vhd,v 1.1 2008/05/01 15:18:18 jrothwei Exp $
-- Joseph Rothweiler, Sensicomm LLC Started 16Apr2008.
-- "Hello world" program. Count down the clock, and display a binary
-- count on the 8 LED's.
------------------------------------------------------------------------
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;      -- Common things.
use IEEE.STD_LOGIC_ARITH.ALL;     -- Makes the "div+1" instruction work.
use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity main is
 Port (
   CLK_50M : in  STD_LOGIC;  -- Input: 50 MHz clock.
   LED : out  STD_LOGIC_VECTOR (7 downto 0)
 );
end main;

architecture Behavioral of main is
 signal div: STD_LOGIC_VECTOR(29 downto 0);
begin
 process(CLK_50M) begin
   if rising_edge(CLK_50M) then
     div <= div+1;              -- Increment counter on every clock.
     LED <= div(29 downto 22);  -- Display the counter MSB's on the LED's.
     end if;   end process; end Behavioral;

# $Id: hello_3an.ucf,v 1.1 2008/05/01 15:23:01 jrothwei Exp $
# Joseph Rothweiler Sensicomm LLC started 01may2008.
# Settings for the Xilinx Spartan-3AN development board by Digilent.
# This file just contains the parts I'm using.

#################################################
# Settings specific to this Board.
CONFIG VCCAUX = "3.3" ;
CONFIG ENABLE_SUSPEND = "FILTERED" ;
CONFIG POST_CRC = "DISABLE" ;

#################################################
# The main onboard clock, with the recommended settings.

NET "CLK_50M"       LOC = "E12"  | IOSTANDARD = LVCMOS33 | PERIOD = 20.000 ;
OFFSET = IN  10.000 VALID 20.000 BEFORE "CLK_50M" ;
OFFSET = OUT 20.000 AFTER "CLK_50M" ;

#################################################
# The row of 8 LED's above the switches.

NET "LED<0>"        LOC = "R20"  | IOSTANDARD = LVCMOS33 | DRIVE = 8 | SLEW = SLOW ;
NET "LED<1>"        LOC = "T19"  | IOSTANDARD = LVCMOS33 | DRIVE = 8 | SLEW = SLOW ;
NET "LED<2>"        LOC = "U20"  | IOSTANDARD = LVCMOS33 | DRIVE = 8 | SLEW = SLOW ;
NET "LED<3>"        LOC = "U19"  | IOSTANDARD = LVCMOS33 | DRIVE = 8 | SLEW = SLOW ;
NET "LED<4>"        LOC = "V19"  | IOSTANDARD = LVCMOS33 | DRIVE = 8 | SLEW = SLOW ;
NET "LED<5>"        LOC = "V20"  | IOSTANDARD = LVCMOS33 | DRIVE = 8 | SLEW = SLOW ;
NET "LED<6>"        LOC = "Y22"  | IOSTANDARD = LVCMOS33 | DRIVE = 8 | SLEW = SLOW ;
NET "LED<7>"        LOC = "W21"  | IOSTANDARD = LVCMOS33 | DRIVE = 8 | SLEW = SLOW ;