Embedded Systems Spring 2024
HW 4 Data Storage
Name: Mason Brady
Email: mrbrady1@fortlewis.edu

HW 4

Introduction: This homework is an introduction to Combinational Logic Blocks

Materials: gvim, vivado, basys 3

1) SR Latch

The SR Latch uses S to set and R to reset. The module and test bench are quite simple just detecting a change in either pin and setting the appropriate q and q_not. The details of the sub module, test bench, and simulation outputs can be seen in Figure 1 below.

Figure 1. SR_Latch sub module, testbench, and simulation showing correct outputs for the two primary input cases.

2)SR Flip Flop
The SR Flip Flop was a slight modification to the SR Latch so I coppied the logic of the SR Latch into a new sub module and rewrote the test bench to account for more cases. So far I prefer using for loops to generate test cases because I find I tend to miss key edge cases when I hard code them in so using the for loop lets me test every possible case (most of the time) with very little code. The sub module, test bench, and simulation can be seen below in Figure 2. If this was a rising edge trigger, the clk needs to be the first bit otherwise it will ignore most of the cases.

Figure 2. SR Flip Flop sub module, test bench, and simulation.

3)Edge Triggered D Flip Flop
The edge triggered D flip flop was also pretty similar to the previous case but instead of s, r we only have d and clr. Clr is the exact same as the reset pin on the last case and could use the same logic (minus the clk since we are already taking that into account). Otherwise, q is just set to d and qn is set to dn. The sub module, test bench, and simulation can be seen below in Figure 3.

Figure 3. D flip flop sub module, test bench, and simulation.
4)Edge Triggered JK Flip Flop

The JK flip flop is like a hybrid between the D and SR flip flops. It has three inputs allowing for basically a D flip flop with an inversing function (11). The testbench was setup nearly identically to the last one just had to put clr as the first bit of the count converter. This way the gate doesn't get reset on every other bit change, just after we have looped through everything else.

Figure 4. JK Flip Flop Sub module, test bench, and simulation.
5)T Flip Flop

The T flip flop was slightly more interesting using an xor gate for the logic. Other than that everything else is nearly identical. I had to use a placeholder bit in the test bench so that clr and t would stay the same for a full clock cycle. Another way to do this would be to add clk <= ~clk; #10; after the #10 in the tb.

Figure 5. T Flip Flop sub module, test bench, and simulation.
6)ROM

The Read Only Memory section of the homework was pretty cool. I started by just writing the submodule and test bench given to us. The readmemb operator and the definition of the memory register was new but the rest of this was pretty routine. The memory is defined as 256 8-bit registers which stores the memory from the storage file read in by $readmemb (read memory binary). The test bench was also pretty standard to how I have been setting up my test benches except for the $monitor which I still don't really understand. The % is clearly a variable string fill (the syntax and usage is very similar to what I have been doing with HTML strings for senior sem which was neat).

Figure 6. ROM submodule and test bench.

I generated a memory file by copying a bunch of 8 bit binary strings and then randomly changing bits. This creates a semiperiodic list fairly quickly. For room and my own sanity I'm only displaying the first 33 numbers in the memory and the first 120ns of the simulation. As you can see they line up with every other clock cycle being a 0 since it takes 2 cycles to read the memory.

Figure 7. ROM file and simulated read showing matching output to input.

I then did the same thing with 4 hex numbers or 16 bits. All that I had to change is the number of bits read in, the adress length, and the length of read in. For the next few demonstrations these are the only things to change between the files which made it quick to write. I am not going to talk about the rest of these too much because they are all almost identical except for changing the length of memory and the length of the stored info.

The readmem function also needs to be changed to read in hex or it will be confused. For this first example in Figure 8, I used the same memory file in the demonstration but I forogt to screen grab it before oberwriting it with the next example.


Figure 8. ROM Hexidecimal read in sub module, test bench, and simulation.

Then I set it up to read in more 2 digit hex numbers, as seen in Figure 9.




Figure 9. ROM submodule, test bench, memory file, and simulation output. The output clearly shows the same values as the input sepearated by the 00 bit of the first clock cycle.

And I modified the code one more time to do 3bit binary numbers.




Figure 10. ROM 3-bit binary sub module, test bench, memory file, and simulation.

Finally I had to use the IP ROM, I first set up my for loop to only increment by one but because of how the memory file is setup this resulted in pretty poor results as seen in Figure 11.

Figure 11. IP ROM test bench and simulation.

Because of this I changed the iteration step size to 50 and this gave results that showed it was actually reading the memory file provided.


Figure 12. IP ROM simulation with corrected step size.

Breadboard Debounce
I wired the breadboard debounce and the circuit was pretty simple, I just had an issue of not grounding my oscilliscope (Thanks Dr. J)


Figure 13. Breadboard debounce circuit output. The blue line is the debounce output and the yellow line is at points 1/2(identical output).
Discussion:
This weeks work felt very long and some of the tasks felt repetitive and redundant (Doing MUX several times and the large version of simple gates). Other than that the code was striaght forward, just seemed like it didn't stop. I also got very lazy with the writeups towards the end.