In this lab, a synchronous sequential circuit is design to recognize a sequence of 11*0 where 1* represents any number of logic 1s, e.g., 0 logic 1, 1 logic 1, 2 logic 1s, .. The sequential circuit has one input I and one output Z, and Z outputs logic 1 when the machine identifies a sequence of 11*0. A sample input sequence and output response is given below.

I = 0 1 1 0 0 1 0 1 0 0 1

Z= 0 0 0 1 0 0 1 0 1 0 0

Task 1

The sequential circuit must be a Moore machine and can use 3 states: S0, S1, and S2. You must design this machine using D-FFs. You will see lots of dont cares to help you reduce the circuit size since two FFs will be used and S3 is NOT required (and thus can be used for dont cares). The workstation has a clock generator with tunable frequency. You must use an appropriate clock rate, e.g., one clock cycle per second. Note that if the clock is too fast, you cannot input the sequence on time. However, if the clock is too slow, you lose your patience, since you have to wait for so long to give the next input bit. Input I can be implemented using a switch, and output Z can be implemented by an LED. Use whatever circuit minimization methods, e.g., K-map, to reduce the circuit size such that your wiring efforts can be minimized. Go through pages 236 to 239 of the textbook to learn the design procedure.

Note that you can also use push-button to generate the clock signal such that it can be fully controlled during the debugging phase. The clock generator keeps pumping clock signal to the sequential machine which may make debugging extremely difficult. You are recommended to use a push-button (instead of clock generator) to generate the clock signals. The breadboard orientation contains an introduction to breadboard workstation where operations for push-buttons can be found. The paragraph is duplicated below.

Two Debounced Pushbuttons

Two pushbuttons are provided on the CADET workstation each of which can be configured to provide a momentary Gnd-5V-Gnd (LO HI-LO) pulse. Since mechanical switches naturally bounce when they are switched, a simple electronic circuit can be added to the switch (thereby making the switch debounced) that delays the output of the switch by several milliseconds to ensure the switch is solidly closed or open. By default the switch is in the normally closed (NC) which ties the NC outputs to ground. When depressed, the NO are tied to ground. The most important thing is to add a 300 Ohm resistor from Vdd to a NC (NO) pin as shown in Fig. 1(a) (Fig. 1(b)) to generate a 0-1-0 (1-0-1) pulse when the button is pressed. Clock implemented by

(b)

Figure 1. Wiring diagrams for pushbutton switches. Normally HI output when button is depressed is shown in (a), OUT output when button is depressed is shown in (b).

Notes: (1) Never try to use a switch to generate clock signals since they can be very noisy and can make your sequential circuit non-deterministic. (2) Make sure that you have PR = CLR = 1 after CLR =0 for state initialization where PR is an active low preset and CLR is an active low clear.