Bill said:
The point is, a typical integrated circuit like a memory chip has thousands
of transistors inside, and there is no way you can test them, as they are
all interconnected within the IC. This is NOT the case for testing a
single discrete device, like a diode (has 2 leads) or a transistor (has 3
leads), or a resistor, capacitor, etc.
And even with that simple CMOS inverter, you couldn't check it with a
multimeter (except to see if there were shorted pins, a pretty limited
check). The only way to check it out would be by applying a logic LO and a
logic HI input voltage to the input, and monitoring the output voltage in a
test circuit.
Or better yet, by applying a pulse input waveform while simultaneously
monitoring the output waveform on an oscilloscope (a dynamic check)
I don't see a point to probing LSI devices *at all* with a multimeter.
But there will be situations where you're probing other elements
on the PCB, and the wiring just happens to be connected to a large
IC as well. By using the low power ohms scale, you might avoid interference
from the large IC. If it is "transparent" and not affecting your measurement,
then you may be able to get a better reading on something, say, next
to that chip.
Again, in the lab, I didn't spend a lot of time probing boards with
ohmmeters. I would typically use a multimeter for checking VCC voltage
feeding some chip, to see if the chip is powered and that the voltage
is within spec (+/-5%). Much more of my time would be spent with a
four channel storage scope, or a logic analyzer. The logic analyzer
on some occasions would even be tied to the storage scope,
so I could take an analog voltage snapshot of something happening
when a certain digital event was captured by the logic analyzer.
One of our analyzers, even had a storage scope built in, for those
kinds of mixed captures.
There isn't much point in adding a stimulus to a modern PCB, because
the LSI chips are more than capable of generating useful patterns
you can verify. I worked with a guy programming the LSI chips
(FPGAs) - he'd cook up a new design file, and later in the day,
I would verify the patterns coming out of the chip, then leave
him some notes on things that needed fixing. That would be
a typical scenario now. If the FPGA designer is good with
testbenches, in fact his design might work on the first try.
(You can simulate everything on the computer, leaving nothing
to chance.)
Years ago, we had an ancient kit in the lab, perhaps made by HP.
It consisted of a logic pulser (would sink up to 0.5 amps to ground
for a period down in the nanoseconds). The second item in the kit
was a logic probe. The intention was, to test simple logic gates,
like a 2-in NAND. The logic probe would touch the output pin,
while the pulser probe would drive an input low (and drive it
so strongly, as to overrule the previous chip in the chain).
Since the pulser 0.5 amp pulse was so short, there was no
damage to the circuit it was strong-arming. So that is a toy
we'd have used in the "jelly bean era". I think I played with
it once, just to see "history in action". But for modern LSI
components, that isn't useful any more. It is more a matter
of observing the system with instruments. Frequently a test
stimulus comes from some software program being run on
the hardware. Like writing a register in a chip somewhere,
to create a sequence on some I/O pin. If your logic analyzer
is set up to catch the write to the register, you can then trace
in time, the response coming from the LSI.
Paul
