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My First VHDL

18 September 2006: Not quite true

Ok, I should be honest: This is not my first vhdl design but it is the first test that I am creating from the bottom up ...
The goal of this 'design' is not to create some real functional piece of hardware but to explore VHDL programming for an FPGA and the Quartus tool from Altera.

You will find some weird constructions in the code but that's just because part of the exercise is to explore real problems that will appear in larger designs.

download the complete design

The design: it looks so simple ...

This is it, the circuit shown is what will be created - it actually is the output of the RTL schematics viewer of Quartus.
A simple edge triggered latch with an output that is depending on both latched and non-latched input data.

The output C will only be high from the time when input A changes to the next rising clock edge as can be seen from the timing diagram.

The clock signal CLK is driven with a 100 MHz signal. Output B follows A at every clock edge and C is only high from the time that A changes until the next rising clock edge.

All this is created with a a few lines of VHDL code:

    USE IEEE.std_logic_1164.ALL;
    entity all_mine is                    (1)
          clk : IN std_logic;
          a : IN std_logic;
          b : OUT std_logic;
          c : OUT std_logic
    end all_mine;
    architecture structure of all_mine is (2)
    signal a_latched : std_logic;
       b <= a_latched;                    (3)
       c <= a xor a_latched;
       latch_a : process (clk)            (4)
          if rising_edge(clk) then
             a_latched <= a;
          end if;
       end process;
    end structure; 

  1. Defined the entity all_mine by specifying a port containing all input and output signals.
  2. This is where the implementation of the components starts.
  3. The assignment of the output signals. Remember that this all happens in parallel.
  4. This defines the process for the latch. All statements within the process will be executed in sequential order.
    Assigning a value to a_latched will only occur only occurs after a rising edge on the clock.


Compiling the code: the tedious job of specifying all constraints

In order to compile the code you have to create a new project and add a file with the component to that project. There is a nice tutorial delivered with Quartus guiding you in doing so.

During compilation some warnings will be shown to tell you that you have not specified a load capacitance for the output pins and that the CLK line is not assigned a clock setting. In order to get proper results from the timing analyses, these values should be given the appropriate values. If the output load is not specified the default load will be used - it's best to specify this value for each pin in order to prevent these warnings.

Specifying the output pin load and clock settings is done in the Assignment Editor (from the Assignments menu):

The clock settings are specified using the Timing Analysis Settings (from the Assignments menu). Click on the 'Individual Clocks' button to specify clock frequency and duty cycle. I specified 66 MHz and 50% duty cycle.

This resolved all warning during compilation.
A simple job for this design, but it can be a tough thing to do for a large design and it may even fail. You will learn to accept that there are some warnings that seem too hard to fix ...


Simulating: not so simple after all ...

In order to simulate the design the waveforms for the input signals have to be specified. This is done using the waveform editor as shown in the PDF tutorial from Altera. You only need to specify the A and CLK signals since the other signals are output.

Now simulation can be started. In the Settings Editor (from the Assignments menu) there is a separate page with the Simulator Settings. Here you can select if you want a functional simulation or a full timing simulation. From the two simulation waveforms given below you will see that timing and pin loads have a real impact on the output.

Functional Simulation


Timing Simulation

Compare these results with the different pages under the Timing Analyzer section in the compilation report. Look at the tco (clock to output times) and tpd (point to point delay) pages. Play around with this for some time and change the output load, clock frequency and try to add a second latch (in front of the current circuit).
With this simple design and using a fast clock it's easy to test out other weird things. Try yo gain experience with this simple design before creating more complex ones.

More to come

Gaisler Research has a great IP library available under the GPL license, also has some nice IP (not all IP on the site conforms to the same standards though) and this nice board from Gleichmann has some nice features that you will not quickly find in this price range.

Enough material to fill a complete website ...