Specify Failure Rates Tool

Failure rates are used in testability analysis calculations and in run-time. In the testability analysis calculations, the failure rate of the component is used as a weighting factor in the fault isolation calculations. Fault isolation calculations which are weighted by component failure rate are more meaningful because the resulting testability analysis reflects the distribution of failure rates across the item. If parts with high failure rates can be isolated to the single part, it is more significant than parts with low failure rates that can be isolated to a single part. If many high failure rate parts are in a large ambiguity group, it is worse than if many low failure rate parts are in a large ambiguity group.

In run-time, failure rate data is used to identify the most likely fault among components when the diagnostic session results in an ambiguity group.

Failure rates are assigned at the part level and are allocated across the faults that make up the part. Failure rate numbers can be in any range and can be either relative or absolute values.

Inclusion of failure rates in the Diagnostic Profiler is optional.

The Failure Probability data is defaulted to a value of 1 for each part or design entity. The part's failure rate is equally distributed among the part's faults. EdgeIn fault locations are defaulted to a failure rate of 0, since it is assumed that inputs do not fail.

Using the Specify Failure Rates tool, you can:

1. enter failure rates at the component level as either relative or absolute values, or as MTBF.

2. change the distribution of the failure rate among the faults associated with a part.

3. assign failure rates by part type (e.g., set all resistors to a specific failure rate).

4. adjust failure rates or MTBF units to seconds, minutes, 1,000 hours, 1,000,000 hours, days or years.

To access the Specify Failure Rates tool, click on the Specify Tests and Repairs tab from the main Diagnostic Profiler screen. Click on Specify Failure Rates from the tool list.

The Specify Failure Rates tool provides, in tabular format, a listing of each part showing part type, part name and part failure rate. You can sort this list by either part name (alphabetic listing) or the part type. At the top of the table is a box where you can select the sort mechanism as wither Part Types or Part Names. Click on the sort mechanism you prefer to re-sort the list.

The default failure probability unit is failure rate in hours. You can change the failure rate display unit from the menu bar or from the failure rate column heading in the table. From the menu bar, select Views, or from the table, click on the failure rate column heading. A drop-down box will list the unit display options.

Click on Failure Rate or MTBF to select the failure probability measure. Click on Views or the column heading again to change the unit. Select the unit by clicking on that unit. The setting will stay the same each time you load the candidate until you change the setting.

To Enter a Failure Rate for a Specific Part

1. Click on the failure rate column associated with the part.

2. Enter the new number in the box.

To Change the Distribution of the Failure Rate among the Faults associated with a Part

1. Click on the part name. A pull-down arrow will appear in the box.

2. Click on the pull-down arrow, and a pull-down box will appear listing the faults or fault locations associated with the part. At the fault level, the failure rate is expressed as a percentage of the part's total failure rate.

To change the failure rate distribution, change the percentages.

3. When you are done, click on any other area on the screen outside the pull-down box. The pull-down box will disappear, leaving the parts listing.

If, based upon the modification you made, the sum of the fault failure percentages does not equal 100% for the selected fault, dialogue box will appear with a warning. From this box you can choose to have the tool normalize the numbers, adjust the unchanged faults, or go back to the fault-level pull-down box to make the changes yourself.

If you select normalize, the fault failure rate percentages will be adjusted so that they maintain the same relationships to each other, but their sum will be 100%. If you select Adjust Unchanged Faults, the percentages which you changed will remain, but the remaining failure rate will be redistributed equally among the unchanged faults so that the total will be 100%.

For example, a part with four faults, by default, has an even distribution of 25% of the failure rate assigned to each fault. Assume we want to make faults 1 and 2 each 10% of the total part failure rate. If we enter 10% for each of those, and subsequently select the Normalize pushbutton, the tool will automatically redistribute the failure rate to 14.29 for faults 1 and 2, and 35.71 for faults 3 and 4. If we select Adjust Unchanged Faults, the tool will automatically redistribute the failure rate as follows: 10% each for faults 1 and 2; 40% each for fault 3 and 5.

Failure Rate Wizard

In many cases, you may want to set failure rates based upon component types (e.g., set all resistors to a specific failure rate). To assign failure rates by part type press on the Failure Rate Wizard pushbutton at the bottom of the Specify Failure Rates screen or from the Tools selection on the menu bar.

The Failure Rate Wizard tool lets you select a part type, and then selectively assign failure rates to all parts of that part type.

1. Click on the pull-down arrow associated with the box labeled "Select a Part Type" to display a list of part types.

2. Click on the part type you wish to set a failure rate for.

3. Enter a failure rate number in the text box labeled "Define the Failure Rate."

4. Click on the "Next" pushbutton to proceed. A new screen will appear showing the selected part type, the defined failure rate, and the names of the parts which are of that part type.

5. Highlight those part names, from the Available Parts list, that you wish to set to the defined failure rate. You can hold down the Ctrl key on the keyboard to select multiple items from the list, or the Shift key to select a range of items (e.g. first through third).

6. Click on the right arrow pushbutton in the middle of the screen to move the selected part names to the Selected Parts box.

(To remove an item from the Selected Parts box, highlight the item and click on the left arrow pushbutton.)

7. To select another part type, press the Back pushbutton to go back to the last screen. You can continue in this fashion until you are completed.

8. Press Done when you are completed.