The sequential network represented in the next figure is a asynchronous counter. Click on the figure to open in the d-DcS a trace of the schematic, and then complete it as shown. Note that the !Q output of each flip-flop is back-connected to the D input of the same one, and observe also how the Ck inputs are connected.
Compile a table, as the following one, that will describe the numerical sequence generated on Q2Q1Q0 (Q2 = MSB, Q0 = LSB). Fill the last two columns with the decimal representation of the number: one as unsigned number, the other as signed number (two-complement coded):
Now, complete the new schematic assigning as outputs the !Q of the flip-flops. Using the timing simulation , verify the network behaviour.
How does the counting sequence change?
Simulate the circuit by animation : this could be useful to understand the behavior of the network. In this mode of simulation, it seems to be perfectly operating: but a timing simulation will proof the limits of the asynchronous implementation of counters (observe the "spurious" outputs):
Use the Home cursor to position the diagram view on the point of interest. Enlarge the timing scale to point out the behavior of the outputs in the critical transitions. In particular, analyse the transition between the activation of LED 3 and LED 4. Try to explain what happens around this transition.