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Survey: Special fixture requirements for Boundary Scan?

Question asked by adrianc

on Sep 15, 2011
Latest reply on Aug 1, 2012 by adrianc

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Hi all,
How's everybody doing?
I would like to poll everybody's experience in the testing of boundary scan using the 3070 generated boundary scan tests.

Do you need to have any special fixture requirements?
What are they? What issue does it solve? Is it effective?

A couple of examples that I know of includes:
1) needing to have a ground plane added to the probe plate to reduce ground bounce
2) needing to use a dual-stage fixture to reduce noise

What else ... ?
Tell me what you use or tried.

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jiguala Feb 28, 2012 2:46 AM
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Hi Adrian,

- If the board is not aligned with the IEEE1149.1 standard to ensure fail-safe operation: TMS=1, TDI=1, TRST*=1, TCK=0; coul be adapted in the ICT fixt.
- These TAP signals must be marked as a critical.
- Use a dual-stage fixture to prevent noise when there are differents BScan chains in the board.

Rgards,
Jose
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bsui Jul 23, 2012 8:55 PM
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Hi adrianc,

1. set critical nodes for TAP signals, sometimes, twisted wiring is preferred
2. add logical buffer plate in fixture if chain design have no buffer for logic translation
3. If board DFT is not good, we need to add some pull-up /down resistor for TCK,TMS,TDI,TDO
4. It is better to add debug connector which can route the TAP signal out from probes to board debug port,
    Sometimes, Boundary Scan doesn't work, we need to use external power to power up board and
    isolate fixture probes to verify whether it is the probe noisy problem or board/bsdl file problem.

Thanks,
Ben
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Jon_oc Jul 30, 2012 8:22 AM
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Hi Adrian,

couple of other points to mention -

if it is not possible to use a ground plane in the fixture , then mark TAP pins as CRITICAL, one other tip is during development is in the power supply definition section, to increase the current of the power supply to maximum that each supply can provide - and this means that more ground wires are added, which helps resolve ground bounce issues. This has no detrimental effect - but the setting will be dfiend same inthe tetsplan, so you need to ensure that power on sequence is ok in case the board current limit does need to be lowered in certain cases.

One of biggest causes of inteference is switch mode power supplies (SMPS), the noise fed back down the wires can make surrounding boundary scan tests unstable, so its always best if you dont hae to rely on these to generate teh pwoer on the board, but drive on the other side - but its not always possible.
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adrianc Aug 1, 2012 6:59 PM
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Thanks everyone for your comments, suggestions, recommendations!
I am sure more and more folks are using BScan or are seeing BScan on their boards. Your suggestions and recommendations will help everyone be more successful in their implementation!

Keep the suggestions flowing ....

Thanks ... adrian
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Survey: Special fixture requirements for Boundary Scan?

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