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.
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:
- Unpowered Analog Tests
- CALL PWRUP(); /* power up normally */
- Test Onboard Power
- Powered Analog
- Powered Hybrids
- CALL PWRDN();
- CALL PWRUP_ALT(); /* back-power */
- CALL PWRDN();
In some cases you may want to do the Powered Hybrids, Opamps and
Powered Analog under PWRUP_ALT() as well.