We could build this whole circuit from gates and wires, but we'll make it a lot simpler by using a register, and adder, and a clock. Here's what it will look like:

If you just finished the previous tutorial, then lets hide the simulator window so we have more room to work. To do so, select Hide from the Simulator menu.

Now lets get rid of the previous circuit. You have several options.

1. Pick the close option from the File menu, then create a new circuit as we did when we first started the previous circuit.
2. Get the cursor into the circuit editing area and type CTRL-A (select all), followed by CTRL-X (cut) or "delete".
3. Select all the elements in the old circuit by moving the cursor somewhere to the left/right top/bottom of the everything that is there, drag the mouse to draw a rectangle around everything, releasing it and cutting or deleting what was selected. After you select everything but before you delete everything, try dragging everything around (just so you know you can do it, even when you only select some of the elements in the circuit in this way).

Select the register element from the toolbar (near the middle, looks like a D flip-flop). Pick Neg Trig, give it a name, set the bits to 4 and click on OK. Place it in the middle somewhere. You now have a 4-bit register constructed from four negative-edge triggered D flip-flops.

Next select an adder element from the toolbar (the icon with the plus sign in the middle). Give it 4 bits, then place it to the right of the register.

Draw a wire from the Q output of the register to either the A or B input of the adder. This wire is actually a bundle of four wires - move the cursor over the wire and check the description in the cyan message area below the tool bar.

Create a constant element with value 1 and connect it to the other input of the adder. Create a constant element with value 0 and connect it to the Cin (carry in) input of the adder.

Connect the S output of the adder to the D input of the register with a multi-segment wire.

We now need to cycle the C input of the register to make the circuit count. The cycle time must be long enough get a value into and out of the register and through the adder. Check the propogation delays of the elements (50 for the register, 120 for the adder), so the cycle time must be at least 50+120 = 170. Its not a good idea to have the cycle time be exactly the propagation delay of the circuit (in fact the simulator actions will be undefined if you do), so we'll create a clock element with a cycle time larger than 170.

Select the clock element (just right of the adder on the toolbar), and set the cycle time to 175. You can leave the "One Time" set to 1 or change it to any value less than 175. The "One Time" is the lenght of time the clock output will be a 1. The remaining time the clock will be 0. Connect the clock to the C input of the register, drag the clock to the left a couple of inches, then put a probe on the wire connecting the clock and the register (call it "clock"!).

We could put a probe on hte wire connected to the D output of the register in order to trace the value in the register. However, a register (and a few other elements, like memory) can be watched, which will also produce a trace of the value in the register. Lets make the register watched by right clicking on it and selecting the Watch Element option. Notice that the element turns blue when the cursor is not on it. This indicates that it is being "watched".

We'll step the simulator for now since otherwise it will run for a long, long time (since the clock will keep cycling). Since the cycle time is long, it will be a pain to step one time unit at a time, so set the step time to something like 10 to make things go faster. Now start clicking the Step button. The signal traces will show the clock cycling and the output of the register increasing as you progress.

Click the Stop button when you've seen enough.

Now select the "stop" element from the toolbar (the one looks like a stop sign) and connect any of its four inputs to the Cout (carry out) output of the adder. The stop element will stop the simulator when its input becomes a one. Now click the Restart button to run the simulator. It will simulate until the counter becomes 15 and the carry out becomes 1, then stop automatically.

The signal trace ran off the screen, but it can be compressed to fit. Change the value in the "Scale Factor" box to 5 and click on Apply.

The values in the signal trace are in decimal (base 10). To see them in binary or hex, click on the appropriate button below the scale factor.

To re-arrange the stacking order of the traces, right click on the probe name of any signal and select the desired option.