August
1998, Issue 97
BitScope
A Mixed-Signal Capture Engine
by
Norman Jackson
Has
your office become so cluttered that you can’t
find your oscilloscope or logic analyzer? No problem,
Norman will help you build a low-cost, mixed-signal
capture engine that connects to your computer via
the serial port.
Some
time ago, I had a bad experience with a bus—a logic
bus. It had six rampaging DSP cards and a SCSI controller
all trying to ride at the same time.
About
once an hour, there was a sickening crash. After going
through the usual stages of blaming the software, I
relented, admitted possible culpability, and borrowed
a mixed-mode DSO.
This
machine has a digital sampling oscilloscope and a logic
analyzer effectively joined at the hip. They share a
common trigger module that enables the user to identify
a complex event and record the state of the target hardware
before and after the trigger—in both the analog
and digital domains.
In
the case of my erratic bus logic, the culprit turned
out to be a delinquent GAL with a ground bounce problem.
The offending chip had its duties reassigned and the
documentation police were alerted. Engineer triumphs
over bug.
By
employing a high-tech piece of test equipment, I could
trigger on a complex digital event and correlate this
event to an oscilloscope trace that showed what was
really happening in the analog domain. I was saved in
the nick of time, but despite having formed a deep attachment
to the trusty ’scope, I had to give it back.
Following
this adventure, I started musing about how to roll my
own version of that useful electronic gadget. After
some mental tinkering and with the added incentive of
Design98, I was soon sketching electronic stuff on the
grid pad. BitScope began to emerge (see Photo 1).
Photo 1—BitScope was prototyped on a two-layer
PCB. Notice that the components are arranged
to separate analog and digital sections of the
circuit.
|
