My latest hobby is to learn VHDL and apply to the Mimas V2 FPGA board.  As with any language, reading about the language is all well and good, but attempting a real application is where the rubber hits the road.  I read through several tutorials and some introduction material to familiarize myself with some of the syntax.  I copied some code from example articles and observed how the code worked in simulation mode.  Then I finally decided to try and build a circuit without the code from a tutorial.  Keeping it somewhat simple, I chose to implement a shift register.  My goal was to create a 4-bit shift register that had one input that I could feed a “1” or a “0” per clock cycle.  I also wanted to see all 4 outputs to observe the data bits shifting down the line.  Here’s the circuit:

My first attempt was to just shift the outputs as though they are storage locations.  That caused an error: Cannot read from ‘out’ object output ; use ‘buffer’ or ‘inout’  I discovered that I needed some sort of storage inside my object (the flip-flops that represent my last state).  So I set up a “signal”:

`signal dflipflops: STD_LOGIC_VECTOR(3 downto 0):="0000";`

You can give your signal any name, I just called it dflipflops because that’s what popped into my head.  As you can see, the data in the signal can be pre-set to some value (in quotes because it’s a vector).

Next, I coded the reset.  I didn’t really need a reset for the simulation since I set the dflipflops to all zeros when the object is initialized.  However, if I decided to use this as a real circuit, I’d have to have a way to reset this at any time.  So I coded my reset as simple as possible:

```if (reset = '1') then
dflipflops <= "0000";
else
-- shift logic goes here
end if;```

Next, I hard-coded the logic of shifting bits (this is just the inner logic):

```if (clock='1' and clock'event) then
dflipflops(3) <= dflipflops(2);
dflipflops(2) <= dflipflops(1);
dflipflops(1) <= dflipflops(0);
dflipflops(0) <= datain;
end if;```

I did this because I wanted to see if the simulation worked, and it didn’t.  I ended up with unknown outputs:

Yup, forgot to translate my signal back out to the outputs:

```output(0) <= dflipflops(0);
output(1) <= dflipflops(1);
output(2) <= dflipflops(2);
output(3) <= dflipflops(3);```

That worked:

Next, I converted to “for” loops and here’s the final code:

```library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

-- 4-bit shift register
entity shiftregister is port (
datain,clock,reset: in STD_LOGIC;
output: out STD_LOGIC_VECTOR(3 DOWNTO 0)
);
end shiftregister;

architecture Behavioral of shiftregister is

signal dflipflops: STD_LOGIC_VECTOR(3 downto 0):="0000";

begin
process (datain,clock,reset)
begin
if (reset = '1') then
dflipflops <= "0000";
else
if (clock='1' and clock'event) then
for i in 2 downto 0 loop
dflipflops(i+1) <= dflipflops(i);
end loop;
dflipflops(0) <= datain;
end if;
end if;

for k in 3 downto 0 loop
output(k) <= dflipflops(k);
end loop;

end process;
end Behavioral;```

The test bench code is here:

```ENTITY shiftregistertest IS
END shiftregistertest;

ARCHITECTURE behavior OF shiftregistertest IS

-- Component Declaration for the Unit Under Test (UUT)

COMPONENT shiftregister
PORT(
datain : IN  std_logic;
clock : IN  std_logic;
reset : IN std_logic;
output : OUT  std_logic_vector(3 downto 0)
);
END COMPONENT;

--Inputs
signal datain : std_logic := '0';
signal clock : std_logic := '0';
signal reset : std_logic := '1';

--Outputs
signal output : std_logic_vector(3 downto 0);

-- Clock period definitions
constant clock_period : time := 10 ns;

BEGIN

-- Instantiate the Unit Under Test (UUT)
uut: shiftregister PORT MAP (
datain => datain,
clock => clock,
reset => reset,
output => output
);

-- Clock process definitions
clock_process :process
begin
clock <= '0';
wait for clock_period/2;
clock <= '1';
wait for clock_period/2;
end process;

-- Stimulus process
stim_proc: process
begin
-- hold reset state for 100 ns.
wait for 100 ns;

reset <= '0';

wait for clock_period*6;

-- test 1
datain <= '1';
wait for clock_period;

datain <= '0';
wait for clock_period*6;

-- test 2
datain <= '1';
wait for clock_period*2;

datain <= '0';
wait for clock_period*6;

wait;
end process;

END;```

For the test bench I first defaulted the reset to a “1” to force a reset at the beginning.  Then I set the reset back to “0” before testing data inputs.  The first test (test 1) feeds a “1” into the datain and then shifts it one clock cycle, then sets the datain back to “0”.  Then I shift 6 times to make the “1” shift all the way out of the shift register.  The next test (test 2), I set the datain to a “1” and shifted it in for two clock cycles, causing two “1”s to be inputted into the shift register.  Then I set datain back to “0” and shifted for 6 clock cycles to watch the two bits shift all the way through the shift register.  Here’s the simulation output:

The first test starts at 170ns and ends around 210ns.  The second test starts at 240ns and ends at 290ns.

One thing I noticed about the editor is that you must select the test source file before double-clicking on “Simulate Behavioral Model”:

Otherwise, you’ll get a result like this:

You also need to close the ISim window before you can run another simulation otherwise, you’ll get an error like:

ERROR:Simulator:904 – Unable to remove previous simulation file isim/shiftregistertest_isim_beh.exe.sim/shiftregistertest_isim_beh.exe. Please check if you have another instance of this simulation running on your system, terminate it and then recompile your design. System Error Message: boost::filesystem::remove: Access is denied: “isim\shiftregistertest_isim_beh.exe.sim\shiftregistertest_isim_beh.exe”ERROR:Simulator:861 – Failed to link the design

Once this error occurs, you’ll need to close the ISim window, then you will need to right-click on the “Simluate Behavioral Model” and select “rerun all”.  Double-clicking just gives this error:

INFO:ProjectMgmt – The selected process was not run because a prior process failed.

One other thing I find annoying about the editor is that there is no file name change capability.  I’ve attempted to change the name of a file and ended up with a mess.  There is a lot of smart linking that goes on between the project and the files that belong to it.  My quick fix is to create a new file with the new name and scrape the code from the old source and paste into the new source.  Then I delete the old file.  It’s dumb and dirty, but it’s also pretty quick.

Other than the few quirks that I’ve worked around, I am happy that the editor is similar to Visual Studio in commands and syntax highlighting.