Basic Drawing and Timing Analysis (Tutorial 1)

This tutorial demonstrates the basic timing diagram editor features of WaveFormer Pro, Timing Diagrammer Pro, VeriLogger Pro and TestBencher Pro. It teaches you how to draw timing diagrams using delays, setups, clocks and part libraries and how to use timing diagrams to help detect timing errors in digital designs. It also covers the waveform editing features, measurement and quick access buttons.

You will draw the timing diagram for the circuit shown in Figure 1. This circuit divides the clock frequency in half. Both the flip-flop and the inverter have propagation times that delay the arrival of the Dinput signal. If the Dinput is delayed too long it will violate the data-to-clock setup time. This increases the risk of the flip-flop failing to clock in the data and may lead to the flip-flop entering a metastable state.

Circuit Parameters:

clk	20MHz	(50ns period)
DFFtp	5-18ns	D flip-flop (74ALS74): Clock to Q propagation time
Dsetup	15ns minimum	D flip-flop (74ALS74): D to rising edge Clock setup time
INVtp	3-11ns	Inverter (74ALS04): propagation time

Figure 2: Completed timing diagram (captured with copy-to-clipboard function).

Figure 2 is the completed timing diagram. The first thing you may notice is the gray signal transitions caused by the min/max values of the component delays. The gray areas of the signal transitions are uncertainty regions, which indicate that the signal may transition any time during that period. This is a little disconcerting especially if you have been using a low-end simulator that cannot compute both min and max at the same time. This representation shows the entire range of possible circuit performance. With WaveFormer Pro, there won't be any surprises during production when you get components at extreme ends of their tolerance range.

1) Set the Base Time Unit

At the beginning of each project, you will set the base time unit. The base time unit is the smallest representable amount of time that WaveFormer Pro can display. The base time unit determines the range of times that can be represented in your timing diagram. All time values are internally stored in terms of the base time unit.

In the circuit in Fig.1, the propagation times for the gates are in units of nanoseconds and the clock has a period of 20ns. Generally it is a good idea to set the base time unit for your project one unit below the units you are working in for best rounding performance during division operations (clock frequencies are inverted and stored internally as clock periods). Therefore we will set the base time units to picoseconds. To set the base time unit:

Select the Options > Base Time Unit menu option. This displays the Base Time Unit dialog box with radio buttons that set the base time unit. The other options control how any existing parameters or signals are changed when the base time unit is changed and have no effect on an empty timing diagram. See the on-line help if you want to know more about these options.
Click on the ps radio button to make picoseconds the base time unit (if it is not already selected).
Press the OK button to close the dialog.

2) Set the Display Time Unit

Next you need to set the display time unit. The display time unit sets the units for times which you enter and for times which are displayed. Set the display time unit to the units you most commonly use in the design. To set the display time unit:

Select the Options > Display Unit menu option. This will display a submenu of display time units. The checked time is the current display time unit (Default is ns = nanoseconds).
Click on ns, to make nanoseconds the display time unit if it is not already checked.

3) Add the Clock

First we will create the clock. The clock is named clk, has a period of 50ns (20MHz), and starts with a low segment.

To add a clock:

Move the mouse cursor over the Add Clock button (DO NOT CLICK), located in the top left hand corner of the Diagram window.
Notice that the status bar at the very bottom of the window reads "Left click to add a clock signal, right click to set clock name prefix" . This status bar changes depending on the mouse location and the mode that you are in. Move the mouse around and watch the status bar change. The status bar is very useful when you want to know what buttons do or when you need to know what to do next in the middle of a program operation.
Press the Add Clock button .
The Edit Clock Parameters dialog box will appear.
Enter the name clk in the Name: edit box.
Enter 50 in the Period: box. Make sure the MHz/ns radio button is selected. Note that the frequency will change to match the new period value, when you move the selection to another control.
Check the invert check box. Clocks are normally displayed high at time zero, so "invert" makes the clock start low at time zero.
Press the OK button to close the dialog box.

Note: For more information on clocks, master clocks, clocks with formulas, and clock grids read Chapter 2: Clocks in the on-line help. If you made a mistake designing the clock, then double left click on the clock segment to reopen the Edit Clock Parameters dialog box. Double left clicking on a clock edge opens up the Edge Properties dialog box which displays the edge time. You may also reach the Edit Clock Parameters dialog box by double clicking on the clock name and choosing the clock properties button in the Signal Properties dialog.

4) Add Signals

Next, add two signals and name them "Qoutput" and "Dinput".

Left click twice on the Add Signal button to add two signals. The signals will be have default names like SIG0 and SIG1.
Double left click on the SIG0 signal name to open the Signal Properties dialog.
Enter Qoutput into the Name: edit box. (DO NOT CLOSE THE DIALOG)
Press the Next button or ALT-N to move to the next signal on the list. SIG1 is now displayed in the Name: edit box.
Enter Dinput into the Name: edit box and press the OK button to close the dialog.

If you accidentally close the Signal Properties dialog, double click on the the signal name to open the dialog again. The Boolean Equation and Simulation features of the Signal Properties dialog are covered in the Interactive HDL Simulation tutorial. The Signal Properties dialog is modeless, so you can leave the dialog open while you perform actions on the timing diagram.

5) Drawing Signal Waveforms

Next, we will draw some random waveforms to become familiar with the drawing environment.

Notice the buttons with the waveforms drawn on them . These are the State Buttons. The active button is colored red and indicates the type of signal state that will be drawn next. In this case, the HIGH signal state is active .
Move the mouse cursor to inside the Diagram window at the same level as the signal name Qoutput, and at about 40ns.
Left click to draw a waveform segment from 0ns to the cursor. Notice that a HIGH signal was created.
A different state button is now activated. The State Buttons automatically toggle between the two most recently activated states. The small red T above the signal name denotes the toggle state, for instance, . (If you have a 3 button mouse, click the middle mouse button to toggle between the two most recently activated state buttons.)
Move the cursor to about 80ns on the same signal and left click. Now a LOW segment is drawn from the end of the HIGH signal to the location of the cursor.
Left click on the TRI button to activate the tristate State Button and draw another waveform segment.
Draw more segments, using all the states except the HEX button. The HEX state button is used in defining multi-bit signals and signals which have a user defined VHDL type. This button is covered in later tutorials. For the time, experiment with the graphical states.

Your drawing should be a mess, or at least look nothing like figure 2.

6) Edit signal waveforms

There are four main editing techniques used to modify existing signals (Note: these techniques will not work on clocks). The most commonly used technique is the dragging of signal transitions to adjust their location. The other three techniques all act on signal segments, the waveforms between any two consecutive signal transitions. The segment waveform can be changed, deleted, or a new segment can be inserted within another segment. Use each of the following techniques:

Move a signal transition: Left click and hold down the mouse button on a signal transition and drag it to the desired location. A green bar will appear that follows the mouse cursor. Release the mouse button when the green bar is at the location where you wish to place the transition.
Change the state of a segment: A segment is the waveform between two consecutive signal transitions. Left click on the segment to select it (a selected segment has a highlighted box drawn around it ). Then left click on the State Button of the new state desired.

If you try to select a narrow segment and one of the transitions gets selected, widen the segment by clicking the Zoom In button , located on the right hand corner of the button bar.
Delete a segment: Select a segment (see above) and then press the delete key on the keyboard.
Insert a segment: Inside a large segment, click and drag to the right or left then release. A new segment will be added in the middle of the original segment. For this operation to work the original segment must be wide enough to be selected.

These techniques will not work on clocks. This is because clocks have fixed edges and segments. To edit a clock, double click on a segment of the clock waveform in the Diagram window. This causes the Edit Clock Parameters dialog box to appear. All clock parameters can be changed in this dialog box. If you cannot double click on a segment without selecting a transition, zoom in until the segment is large enough.

For more information, read WaveFormer's on-line help "Chapter 1: Signals and Waveforms".

7) Adjust your diagram so it resembles the figure below

Now use the above techniques to edit the signals so they have roughly the same transitions as the signals in the figure below. This is not the normal way to create a timing diagram, but it will teach you how to use the editing features of WaveFormer Pro. Make sure you try all the editing techniques.

Tile the Parameter and Diagram windows so that you will be able to see the interaction between the two windows. The Report window is not used in this tutorial, so you can minimize it if your screen is small.

Select one of the Window > Tile menu options in the timing window.

Adjust the zoom level of the drawing so that only 3 whole clock periods are shown on the screen.

Left mouse click the Zoom In or Zoom Out buttons, which are located on the right hand corner of the button bar, to show less or more of the waveforms.

8) Moving and Reordering Signals

All signals are moved by dragging and dropping the signal's name. When several signals are highlighted and moved as a group, they will reorder themselves according to the order in which they are selected. This ability to quickly reorder signals by the order of selection will help you deal with the large numbers of member signals of buses.

Moving a single signal:

Select the signal clk by left clicking on the name. The selected signal is highlighted.
Put the mouse cursor near the very bottom of the selected signal. When the mouse cursor changes from a normal arrow to an up/down arrow, click and hold the left mouse button down. A green bar will appear.
Drag the green bar until it is in between Qoutput and Dinput.
Drop the green bar by releasing the left mouse button. Notice that the timing diagram has redrawn itself.
Try dropping clk at the very top and at the very bottom of the diagram. Leave clk at the bottom of the diagram.

Moving and reordering multiple signals:

Select Qoutput, then select Dinput by left clicking on the signal names in that order.
Move the signals to the bottom of the diagram. Notice that Dinput is above Qoutput because that is the order in which they were selected.
Select Dinput and then select Qoutput.
Move the signals to the top of the diagram. Notice that Dinput is above Qoutput, because the signals were selected in that order. This is a quick way to reorder a large group of signals.
Return the signals to their original order, (clk, Qoutput, Dinput).

9) The right mouse button (mode buttons):

In the next sections we will add delays, setups, and comments to the timing diagram. These objects are added using the right mouse button. The function of the right mouse button is determined by the second group of buttons on the button bar marked DELAY, HOLD, SETUP, TEXT, SAMPLE and MARKER . The red or active mode button indicates the current functionality of the right mouse button. To activate a different mode button, left click on it.

10) Add the D flip-flop propagation delay

Add the delay that represents the propagation time from the positive edge of the clock to the Qoutput of the D flip-flop. To add the delay do the following:

Activate the Delay mode .
Left mouse click on the first rising edge of the clock.
Right mouse click on the first falling edge of the Qoutput signal.

This will draw the D flip-flop delay, and creates a blank delay in the Parameter window.

When delays are added, they are blank and do not enforce any timing restraints. Notice that the delay is drawn with gray colored lines, this indicates that the delay is not forcing either the min or max edges of the Qoutput signal. Now edit the delay's parameters.

Double left click on the parameter name D0 in the Parameter window to open the Parameter Properties dialog. Adjust the position of the Parameter Properties dialog so that you can see the parameter in the Diagram window and at least part of the parameter in the Parameter window. For simplicity, we will refer to the dialog as Parameter Properties, even though the name at the top may say Delay Properties or Setup Properties.
Type 5 into the min edit box and press the TAB key to move control to the max edit box (leave max blank for now). This enters 5 display time units, or 5ns for this project.

Two things happened when you pressed the TAB key:

First, the falling edge of the Qoutput signal adjusted itself so that it was 5ns from the clock edge. Measure this for yourself using the time readouts above the signal name window. Left click on the first transition and then move the cursor to the second transition. Notice that the blue readout shows approximately 5ns, depending on the zoom level and the base time unit. The delay can also be made to display the exact distance by choosing the distance choice of the Display label section of the Parameter Properties dialog (make sure to return D0 to "Global Default" when you are done experimenting).
Second, the delay changed from a gray color to a blue color. Delays are color coded to indicate which delays are forcing the min and max edges of a transition. This type of critical path display is necessary in diagrams where multiple delays drive a single signal transition. The colors are: Gray = none, Blue = Min only, Green = Max only, Black = both min and max. After this tutorial you may want to experiment with the multdely.tim file to see the effects of multiple delays on a single transition and critical path color coding.

Next, finish editing the rest of the parameter. The parameter is named "DFFtp", has a max time of 18ns, and a comment of "Ck to Q propagation time". Use the Parameter Properties dialog that is still open to add the following data:

Left mouse click in the Name edit box and type DFFtp into it.
Press TAB twice so that the max edit box is selected.
Type 18. This means 18 display time units, or in this project 18ns.
TAB once so that the comment cell is selected.
Enter Ck to Q propagation time and press the Enter key.

Notice that the DFFtp delay is black which indicates that it is forcing both edges of Qoutput. Also notice the falling edge of Qoutput now has a gray uncertainty region. Use the time measure readouts to verify that the edges of the region are 5ns and 18ns from the clock edge (13ns of uncertainty). Double click on the edge to see the exact edge values.

The Parameter Properties dialog is modeless (other operations can be performed while the dialog is open) and interactive (any changes in the dialog fields are reflected in the diagram after you move out of that field). This tutorial has you open and close it several times so that you learn all the different ways to open the dialog. Also, the tutorial attempts to conserve screen area for laptop users. However, in a normal design you will probably want to keep this dialog open much of the time.

Tip: When the Parameter Properties dialog is open you can edit a different parameter by double clicking in the Diagram or Parameter window on the parameter you want to change. If you double click in the diagram window, that instance of the parameter will be edited (the Change All Instances checkbox will NOT be checked). If you double click in the Parameter window, ALL instances of the parameter will be edited, (the Change All Instances checkbox will be checked).

11) Add the Inverter propagation delay

Add the delay that represents the propagation time of the inverter from its input Q to its output D. To add the delay do the following:

Activate the DELAY mode by clicking on the delay button.
Left click on the first falling edge of the Qoutput signal (the same edge that ends the "DFFtp" delay).
Right click on the first rising edge of the Dinput signal.

This will draw the inverter delay, and create a blank delay in the Parameter window. Now let's edit the parameters from the inside of the Diagram window instead of going to the Parameter window.

Double left click on the new delay in the Diagram window and enter the following values in the dialog box that appears:
- Name is INVtp.
- Min time is 3 ns.
- Max time is 11 ns.
- Comment is Inverter (Q to D) delay.
- Click on the OK button to close the dialog.

Notice the large uncertainty region for the Dinput transition. Click on the first rising edge of Dinput, then use the blue delta readout to verify that the uncertainty region lasts for 21ns (13ns from DFFtp + 8ns from INVtp = 21ns). Next, click on the first edge of clk and measure to the end of the uncertainty region of Dinput. If both the inverter and the D flip-flop are slow, Dinput may not transition until 29ns after the clock edge.

12) Add the setup for the Dinput to clock

Next add the setup for Dinput to clock transition.

Activate the Setup mode .
Left click on the first rising edge of the Dinput signal (the same edge that ends the "INVtp" delay).
Right click on the second rising edge of the clock.

This will draw the setup parameter. Notice that the arrows of the setup are pointing to the control signal. This means that you added the setup correctly.

Like delays, setups are also created with empty min/max values. They must have a min value before they start to monitor the data signal's position. Now we will edit the setup

Double left click on the setup name in the Diagram window. This will open the Parameter Properties dialog box.
Enter Dsetup into the Name edit box.
Enter 15 into the min edit box.
Enter Check for metastable condition into the comment edit box.
Click the OK button to close the dialog.

Notice that the margin column in the Parameter window says that there is a 6ns safety region before the setup is violated. Verify this by clicking on the second rising edge of the clock and placing the cursor on top of the maximum edge of the Dinput signal. The blue time readout should say -21ns (setup time 15ns - measured 21ns = -6ns margin).

Next, we will demonstrate what happens when a setup is violated. Increase the inverter's delay so that the maximum delay is 18ns instead of 11ns:

Double left click on INVtp in the Diagram window.
Type 18 into the max edit box and TAB to another control.

Notice that the setup has turned red in both the Diagram and Parameter windows. Change the inverter delay back to 11ns and click OK to close the dialog.

13) Add a free parameter

So far we have always directly edited a parameter's values. This is inefficient and error prone if the circuit is large. It would be better to define one variable that held the value and make everything that needed that value reference this variable. Then if the value needs to be changed, you only have to edit one variable.

Free parameters act as variables that can be referenced by other parameters. They are called "free" because these parameters are not attached to any signal transitions in the Diagram window. Let's add a free parameter to hold the propagation times for the inverter.

To add a free parameter:

Click the Add Free Parameter button in the Parameter Window. A blank free parameter is added to the Parameter window.
Double click on the free parameter to open the Parameter Properties dialog box and enter (tpFreeInv, 3ns, 11ns, and 74ALS04 inverter delay) for the (name, min, max, and comment cells) of the free parameter.
Use the Previous and Next buttons in the Parameter Properties dialog to locate the INVtp parameter.
Type tpFreeInv into the min and max cells of INVtp. Changes to the timing values of the free parameter will now affect INVtp.

Note: Free parameters can be saved to special library files which can later be merged into other projects. You can also reference free parameters without including them into your project file by placing libraries in your library search path (Libraries > Library Preferences menu option). For more information on free parameters and libraries read the on-line help Chapter 10: Libraries or perform the Parameter Libraries tutorial.

14) Using formulas and constants

Parameters can contain mathematical formulas as well as numeric time values. Legal operations are: multiplication(*), division(/), addition(+), and subtraction(-). For example, the inverter in this circuit could represent 3 cascaded inverters used to generate a minimum delay of 9ns. To represent this in your timing diagram:

Enter the following equation into INVtp's min edit box:

3 * tpFreeInv

Free parameter names can also be used with an attributed parameter name such as tpFreeInv.min and tpFreeInv.max. This gives you the flexibility to specify formulas any way you need. If no attribute is added then a min or max is assumed depending on whether the formula is in the min or max column.

15) Summary

Congratulations! You have completed the Basic Drawing and Timing Analysis tutorial. In this tutorial we have covered three main topics. The first is how to start a project. Next we covered signals, which includes clocks, signals, and drawing the waveforms of the signals. And finally we covered parameters.

1) Starting a project

Always set the Base Time Unit one unit below the Display Time Unit to avoid rounding errors. Default values are: Base Units= ps and Display Units = ns.

2) Drawing a timing diagram with Signals and Clocks

Use the Add Clock and Add Signal buttons to add clocks and signals to the diagram. Double left click on the signal name to edit the signal name.
Left click to draw a waveform with the state of the selected State Button.

Editing waveforms

Drag and drop signal transitions.
To change the graphical state of a segment, select it then press a State Button to indicate the new graphical state.
To Delete a segment, select it then press the delete key on the keyboard.
To Insert a segment, left click and drag to the right.

3) Timing Analysis with Parameters

Add delay, setup, and hold parameters by (1) activating the mode button of that name, (2) left clicking on the first signal transition, and (3) right clicking on the second signal transition.
Edit parameters by double left clicking on the parameter in either the Diagram or the Parameter window.
Free parameters are variables that other parameters reference. Use the Add Free Parameter button in Parameter window to add a free parameter.
To use a free parameter, type the name of the free parameter in the min or max column. Free parameters can also use the dot min/max property to specify a specific value. For example, Inverter.min retrieves just the minimum value of the parameter called Inverter.