Uwharrie Test Solutions

LLC

TSN_logo.JPG (12298 bytes)

 

Problems with On-Board Power

You thought you did everything right. You added lots of ground probes to your fixture. You added twisted pair to all of your critical signals. You have all the necessary disables and inhibits. All of your oscillators are shut-off. You're using the prescribed logic levels. All the analog tests are working. The board powers up perfectly. Then, all of your digital tests are intermittent. It's not uncommon for an experienced test engineer to spend days staring at this problem with no resolve.

It can't possibly be the on-board power supplies. They come up perfectly. You look at them with a scope, everything looks clean and stable.

Time to think outside the box. Murphy's law says your on-board power will only misbehave when your not looking. Connect your oscilloscope probe to an on-board power rail. Try the 3.3V first. Now put your failing test in to BURST mode. Did I tell you to put on your safety glasses first? Look at your signal now. The Evil Doer has come out of his cave! Quick, hit the space bar before you let the smoke out of your UUT!

On-board power may look real good while the board is powered and idle. The effect of back-drive can cause an undue power surge that your onboard regulator(s) will try to compensate. Active signals from the tester can wreak havoc on switching signals and feed-back dividers. Sometimes large ASICs have the capability to shut-down their own core voltage. When you attempt to do a boundary-scan test on them, they do just that.

Here are some on-board regulator types and how to deal with them.

  • Common DC to DC converter with no feedback

Voltage goes in, voltage comes out. These should never hurt you.

  • Linear regulator with feedback connected to a resistive divider

Sometimes the fix is as simple as moving the probe wire connected to the feedback pin to a place where it does not pick up cross-talk from other signals. Sometimes you have to remove the probe from the fixture, however that will illuminate your ability to test the resistors in the feed-back network. You can sometimes add a series resistor in the wire near the probe, then you can still do a marginal test of the feed-back resistors. The higher the impedance of the feedback network, the more likely you will have trouble.

  • Switcher Regulators

These are the worst nightmare for ICT. First look for control signals coming from other parts of the board that may me interrupting this circuit. Look for a way to cut these off. It may be necessary to remove the fixture probe from feed-backs and gate drivers. Often the only solution is to shut it down and back-power it.

  • Recipe for Back-Power

Fixture

First of all, have your GR228X or TestStation equipped with enough Alliance Supplies to back-power your on-board voltages.

In your fixture, place at least 3 extra probes on every net that you anticipate you may need to back-power. And it doesn't hurt to add a power island to your fixture and wire it up to a spare supply.

Program

Modify power SEQUENCE2: and SEQUENCE3: these need to shut-down and disconnect every power supply your tester has. Not just the ones you are currently using. If you don't remember to do this you may end up damaging your tester.

Create SEQUENCE4: from SEQUENCE1:. Except that this sequence should turn on all of your back-power, and you probably should leave your main input power at 0V.

Create a new subroutine called PWRUP2() or PWRUP_ALT(). Make this from your PWRUP() subroutine. This subroutine needs to SET PS SEQUENCE = 4; not 1. You will also need to change the MEAS DCV statements to connect and measure the supplies that your are back-powering.

In most cases you should leave (turned off), the supply connected to the main input power. If this is not possible, you have a couple of alternatives. Find a way to shut off the on-board supply, or simply back-power it at 100MV greater than its output voltage. For a 3.3V supply, back-power it to 3.4V. The on-board supply is satisfied and it goes to sleep and your back-power takes over.

Your board test-flow should look something like this:

Shorts
Unpowered Analog Tests
CALL PWRUP(); /* power up normally */
  Test Onboard Power
  Powered Analog
  Opamps
  Powered Hybrids
CALL PWRDN();
CALL PWRUP_ALT(); /* back-power */
  Digital
  ISP
CALL PWRDN();
  done

In some cases you may want to do the Powered Hybrids, Opamps and Powered Analog under PWRUP_ALT() as well.

 

Return to Main Page