Scan Sequence Question : Chaging Measure PO

[raviram80]

Scan Sequence Question : Changing Measure PO

Hi All,

I have following question

Scan operation has the following steps according to Mentor Graphics Document.

The operating procedure of the scan circuitry is as follows:
1. Enable the scan operation to allow shifting (to initialize scan cells).
2. After loading the scan cells, hold the scan clocks off and then apply stimulus to the
primary inputs.

3. Measure the outputs.

4. Pulse the clock to capture new values into scan cells.

5. Enable the scan operation to unload and measure the captured values while
simultaneously loading in new values via the shifting procedure

My Question is

What will happen if we move the Step 3 "Measure the Outputs" after the Capture pulse?

Please explain?

Thanks.

 

[jpvSoccer]

Hi,

I am assuming that you want to generate ATPG vectors using a custom load-unload sequence.

The standard steps are to sensitize and measure. The shift in and parallel load sensitize the circuit; at this time you can detect some faults on the regular outputs. The capture-clock captures the results of the sensitization for the rest of the chip. I could see where you might want to measure the outputs after the capture clock, but I don't think you want to not perform the initial parallel test (step 3).

By moving the "measure outputs" to after the capture clock you may adversely impact the ability to detect certain faults, or you may make the faults difficult to detect.

Why is it necessary to change the sequence?

You could measure the affects of changing the sequence by running ATPG with the standard sequence and then the custom sequence. Whichever sequence gives you the best coverage with the fewest vectors would be the way to go.

 

[ConradATPG]

Re: Scan Sequence Question : Changing Measure PO

You will make testing faults from a flip-flop to an output pin much more difficult.