Timing tutorial

Okay you went through the full adder tutorial. Great! You have a fully functional full adder. But when you try to simluate it using timing, you find out that there is no delay! You must modify your circuit to incorporate IPAD, OPAD, IBUF, and OBUF. This tutorial will guide you on the steps to add those devices into your circuits, so that you can simulate the timing delays for your projects.


Our original half adder.

Here we can see our half adder. It has terminals, and we want to replace those terminals with input pads and output pads, and add input buffers and output buffers to our circuit. So go ahead and delete the terminals and the first thing you will want to do is add the input and output buffers. Place the input buffers where there was an input terminal (IBUF and OBUF in your device box).


Half adder with IBUF and OBUF

We aren't done yet though. We need to add the Input pads and output pads, which more or less replace the input and output terminals we had previously. Bring up the device toolbox and add IPAD and OPAD to your circuit, before and after the input and output buffers, respectively.


Half adder with IPAD, OPAD

You are almost done! But not quite. If you try and simulate this circuit you will notice that you can't add anything. That is because the IPADs and OPADs have no names! So to add the names to them (such as A, B for the inputs and SUM, CARRY for the outputs), double click on the wire between the IPADs and the IBUFs and similarily the wires between the OPADs and the OBUFs and name them, just like you did for the terminals.


Our finished half adder

Now you are done! You can go ahead and simulate it. Select "timing" and voila, you will notice that you have a nice delay! For lab 1, this is what we want you to notice (as well as see in your lab reports!).


Timing diagram with delay

Notice that there is indeed a delay when we add binary 0 + 1. In fact in this case, there is a 2ns delay. To get that pretty green marker, go to waveform -> measurements -> measurements on and mark the two points to see how long the delay acutally is. You will likely have to zoom up a bit in order to magnify and cleary see the delay.