Defining Logic Devices

In this exercise you will see how to create a new Logic Definition which can be used when categorising a Logic Device. It will build on the completed tutorial project; a copy of this project is installed as a zip file: CompletedTutorial4.zip.

The XJDemo Board hardware is not required for this exercise.

Background

When you categorise devices on the Categorise Devices screen you can assign them to be Test Devices, Logic Devices, Passive Devices, Unfitted Devices, Ignored Devices or Excluded Devices.

Devices with functionality that can be described by a truth table, such as logic gates and buffers, should be categorised as Logic Devices.

N.B. Devices such as flip-flops that hold state cannot be categorised as Logic Devices in XJTAG.

There are two main benefits to categorising a device as a Logic Device:

1. Connection Test - the connection test will fully exercise all of the elements of your Logic Devices that it can access.
2. Non-JTAG testing - where you have a Logic Device between your JTAG enabled device and the non-JTAG device you want to test, the underlying system will automatically manipulate the Logic Device to achieve the outcome you specify. For example if you have a buffer between your JTAG device and your RAM, the underlying system will automatically control the enable and direction control signals as needed in order to write to and read from the RAM.

There is a comprehensive library of common logic definitions installed as part of XJDeveloper. This exercise is designed to help you when you have to work with a device that is not in that library.

Creating new Logic Definitions

You are going to create Logic Definitions for two device types: first the dual NOR gate on the XJDemo board (U9) and then the RS485 transceivers (U4 and U7).

Dual NOR Gate

The logic symbol and pinout for U9 are shown below:

Normally you would choose the Use Built-in Truth Table option in the Add Truth Table dialog, which allows you to specify basic AND, NAND, OR and NOR gates of any width. For other types of gate the XJTAG Logic Library will often contain a suitable truth table. However, as an exercise, here you will create your own version of this truth table.

When you are creating a truth table you need to understand the structure of the device you want to categorise. In this case there are two instances of a simple NOR gate. The truth table only needs to define one of these gates as they operate independently from each other. You will then create two instances of the truth table in the Logic Definition to represent the two gates.

You have now created an empty truth table into which you can enter the logic behaviour for an individual NOR gate.

Each entry in the table must contain one of the following values:

• Inputs
• L - Low
• H - High
• X - Don't care - either low or high.
• Outputs
• Z - High Impedance
• 0 - Low
• 1 - High

For further information on entering and editing truth tables see Entering data into Logic Tables.

Now that the truth table is complete you can edit the Logic Definition to map the pins of the device into logic gates using the truth table you have just created.

Each Logic Definition can contain multiple footprints for a particular part number. In some cases these footprints will have the signals connected to the same pins, however the system does allow you to define different signal locations as well. You are now going to enter the 8 pin footprints, all of which have the signals connected to the same pins.

Now add the Power pins and Ground pins.

All of the information that you have entered so far will make it easier to find and use the Logic Definition. Now you are going to enter the pin information to link the Logic Definition to the truth table.

The Logic Definition is now complete, and can now be used for devices in the project. First we will remove the existing logic definition for U9:

Now the new definition can be added:

Your project is now back in its original functional state but is now using the logic definition for U9 that you have created.

RS485 Transceivers

You are now going to create a Logic Definition that can be used for each of the two RS485 transceivers on the XJDemo board, U4 and U7. The logic symbol and pinout for this device type are shown below:

Now we need to reassign the RS485 transceivers to use the logic definition we've just created.

You will notice that some errors have appeared in the project:

These errors are because XJTAG cannot find the Test Function it was using in the Test Device File for the two RS485 transceivers U4 and U7. This is because now that we have reassigned the transceivers the Test Device File is no longer associated with U4 or U7. With our new logic definition, XJTAG can automatically test U4 and U7 by verifying that each logic input state produces the correct output. This will be done by the Connection Test, and so there is no longer any need for the XJEase tests that were previously used - they can simply be removed from the test list.

The RS485 Tests are in red to show that there is a problem with these tests.

The errors have now disappeared, but you will notice that you now have two devices listed in the Uncategorised Devices panel at the bottom of XJDeveloper.

You should see that P2 and R36 are now being reported as Uncategorised. By providing our logic definition for the RS485 transceivers U4 and U7 the system now knows how to control the RS485 pins A (U7.6) and B (U7.7). With this information we can now perform testing on other devices on the nets connected to these pins such as P2 and R36. When you categorise a device as a Logic Device it may change what you have access to so always check the Categorise Devices screen for any uncategorised devices.

You now have an alternate way of testing the RS485 transceivers U4 and U7 using logic instead of an XJEase test.