[SI-LIST] measuring i/o power without disrupting signal quality
- From: "Kim Flint" <kflint@xxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Mon, 29 Jul 2002 15:34:03 -0700
Hi-
I'm developing a bringup board for a large ASIC project. One of the
things I need this board to do is provide a way to measure power
consumed in the I/O while running in system. We would then use this
information to check analysis of thermal and power supply requirements.
I'm concerned about how I might do this measurement without degrading
the signal quality and otherwise causing problems for the main goal of
ASIC validation. How do other people approach this problem?
The ASIC is a 788 ball BGA with two 64 bit 166MHz DDR/sstl2 memory
interfaces, one 16 bit 166MHz ddr/sstl2 memory interface, one 32bit in /
32bit out 100MHz cmos source sync interface, and one 100MHz 64bit cmos
common clock cpu interface. In other words, a whole lot of i/o running
at reasonably high speed, with potentially sensitive timing. The core of
the ASIC is 1.8V, and fortunately all of the i/o is separate on 2.5V. It
is a .18u chip, so rise times are likely in the neighborhood of a few
hundred ps.
Measuring the 1.8V core power is pretty easy, since I can isolate that
as it's own island with it's own regulator, and run it through a sense
resistor.
With the I/O however, I am concerned about isolating the ASIC i/o 2.5V
power from the i/o power of the devices it is communicating with. I
don't care much about layer count here, so I can easily add enough
ground planes so that the signals are only adjacent to ground planes all
the way. Is that sufficient for handling return currents? I think
current on 2.5 still has to return somehow, so I wonder if I risk
coupling or SSO related problems as a result of forcing that current
through some roundabout path to where I can measure it. Or would
switching current that does travel around be of sufficiently low
frequency to not cause me any trouble?
The second doubt I have is whether I will be able to get an honest
measurement of I/O power at all. Measuring current of a locally isolated
2.5V supply would help me with my power supply spec, but does that tell
me everything about how much power is actually dissipated in my i/o, and
thereby affecting thermal solutions? Here is my understanding of it,
perhaps some of you could check whether I've got it right or not?=20
With a cmos i/o driving 50ohm transmission line and another cmos
receiver, there will be some current driven from 2.5V to ground directly
through the transistors as the i/o switches. Measuring current on 2.5V
and multiplying by 2.5 would accurately represent this component. If the
i/o is switching high, current also drives through the high-side
transistor from local 2.5V into the line, charging the line to a high
level. Since the voltage across this transistor is changing, multiplying
that current component by 2.5V wouldn't accurately represent that power
dissipation. When the i/o switches low, current drives from the line
through the low side transistor to ground. This wouldn't be represented
in the local 2.5V current measurement at all. Measuring 2.5V current
doesn't appear to capture the whole picture here.
With the sstl2 ddr i/o, we have a termination resistor to 1.25V at the
far end. In this case when the i/o drives the line high, there should be
some initial power dissipation in the I/O as the voltage goes high. But
once the voltage is high, there is still current through the transistor
but the actual power is dissipating in the resistor, minus some in the
transistor due to the saturation voltage across it. Measuring local 2.5V
supply current in this case would not easily tell me how much was
actually in my i/o. Similar to cmos, switching low drives current from
the line and the termination supply, but not through the local 2.5V
power supply so that power dissipation is not represented in a
measurement there.
hmm, so I seem to be convincing myself that measuring current on my 2.5V
supply is not going to give me a correct measurement of power
dissipation in my i/o. Do you agree? Is there some other way for this
power to be measured? I'm curious how others address this, or if you do
what I've done in the past and convince everybody that it is too
problematic to be worth the trouble?
any opinions or suggestions are very much appreciated, thanks!
kim
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