Sabin's Computer Exercise 2
A decoder is a device which takes in binary inputs and returns the information into unique, separate outputs. In CE 2, we will program a simple decoder using both the structural and behavioral architectural designs.
The truth table shows the functionality of a decoder; inputs are taken in and the decoder returns unique outputs.
The first decoder that was made was using structural architecture. This means that the decoder consisted of various components that were put together using VHDL. Three VHDL modules were created: inverter, and3, Decoder_Structural. As it sounds, the inverter takes an input and outputs the inverse of the input. and3 takes in three inputs and outputs the AND of the three. Finally, Decoder_Structural is used to combine the other two modules in such a way to program a functional decoder.
Under the architecture part of the Decoder_Structural module, the inverter and the and3 modules were declared so that these components may be used to program the decoder. As shown below, inverter and and3 have the appropriate number and types of inputs and outputs. The intermediary signals, I0_NOT and I1_NOT are declared here as well.
COMPONENT inverter
PORT(
I : IN std_logic;
O : OUT std_logic);
END COMPONENT;
COMPONENT and3
PORT(
I0 : IN std_logic;
I1 : IN std_logic;
I2 : IN std_logic;
O : OUT std_logic);
END COMPONENT;
SIGNAL I0_NOT, I1_NOT : std_logic;Following the schematic, the decoder module was programmed to use the appropriate components and signals for the decoder. Below is an example of the connection for the Y3_and3 component:
Y3_and3: and3 PORT MAP(
I0 => I0,
I1 => I1,
I2 => EN,
O => Y3);After completing the connections for all of the components in the circuit, the next step was to test the module using a test bench. NOTE: it was interesting that after creating a gate, a vhdl representing that gate was automatically created under the Decoder_Structural module.
For this test bench, all of code pertaining to clocks were commented out. Along with the inputs (I0, I1, and EN) and outputs (Y0, Y1, Y2, and Y3), a counter signal (count) was also created to be used in the stimulus process. count was initialized to a value of 000, and would go through all eight of the input combinations. Thus, in the UUT (Unit Under Test), count(0) was set to I0, count(1) as I1, and count(3) as EN. The outputs were kept the same.
A for-loop was used to test the eight input combinations. Starting with 000, the simulation would read through the three inputs and print out the corresponding output values. Immediately after printing the results in the console, the simulation displayed the waveform of the inputs and outputs. After every iteration, count was incremented by 001. This was made possible by using the unsigned.all library.
These results may be compared with the truth table above or the waveform below.
The three inputs, I0, I1, and EN are represented by the count signal, and therefore are not shown to actually take in the input values for the simulation.
The decoder was made once more using behavioral programming techniques. A new module was created which implemented the behavioral architecture. This was relatively simple to do by following the provided circuit schematic. I coded in the connections part of the module as shown below.
I0_NOT <= not I0;
I1_NOT <= not I1;
Y0 <= I0_NOT and I1_NOT and EN;
Y1 <= I0 and I1_NOT and EN;
Y2 <= I0_NOT and I1 and EN;
Y3 <= I0 and I1 and EN;Afterwards, I created a test bench for the behavioral decoder. I used the same code from the test bench used by the structural decoder. As expected, the results were the same. Below are images of the behavioral test bench showing that the results are indeed the same.
SAME AS ABOVE!!! NO WAY!
To review, a decoder is a device that takes in inputs, converts the inputs and returns unique output values. The third input, EN, stands for enabler. The enabler is used to limit, or distinguish the unique outputs. If EN is 0, then the entire output will have to be 0 as well because all of the AND gates are connected to the enabler. By connecting the enabler with each of the other inputs and utilizing the two NOT gates, we have a way to return three unique outputs.
Decoders are used in numerous ways. If certain information is encoded, the decoder can take the information and return the original information before it was encoded. For example, this process is used by cable TV companies who only want to allow pay-per-view customers to have access to a movie or TV show. The streaming of the movie/show is encoded by the company and the viewer's decoder will return the decoded/viewable movie/show.