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Backgrounder
The bandwidth of the low-cost oscilloscopes that support basic digital troubleshooting has
doubled recently, reaching 200 MHz. Other valuable “high-end” measurement features,
including advanced triggering, FFT analysis, and color displays, have migrated into entry-level
instruments. Now designers have an affordable digital troubleshooting solution, such as the
TDS2000 Series, for their work on commodity embedded processors.
The balance of this application note will focus on the application of some digital troubleshooting
aids that are appearing in the emerging generation of low-cost DSOs.
MORE BANDWIDTH MEANS MORE VISIBILITY
A hidden “feature” of a processor manufactured a month ago, as opposed to the same device
manufactured a decade ago, is the increase in the speed of its signal edges (transitions). Based
on comparing the CMOS processes used 15 years ago, when many of these designs were
created, to 5 years ago, when the fastest 5V capable processes were developed, speeds
increased about three fold. Most new designs are either on these fastest 5V processes, or go to
lengths to reduce voltages in the core areas, using 5V only on the periphery. With this later
approach, even faster clock rates are possible. These faster speeds are a by-product of
shrinking feature sizes on the silicon chip.
Faster edge performance is generally a good thing, reducing skew problems, setup times, and
race conditions within the system. Faster propagation delays (driven largely by faster edge
rates in CMOS) also create issues. Margins in address decoding that often depend on delays
between address line logic and the bus control lines become more challenged when those
delays become uniformly less. Regardless, designers need to see and understand these
edges, as well as the narrower transients and other pulse characteristics that can arise in the
presence of high-speed transitions.
When selecting a DSO for digital design work including a 20 MHz embedded processors, one
might assume that an instrument with, say, 50 MHz or 100 MHz bandwidth is more than enough
for the job. Certainly that is true for answering basic troubleshooting questions such as “Is the
signal present? Is it correctly timed and synchronized?” But other details may be less obvious.
A DSO with high bandwidth provides far more insight into signal behavior than a lower-
bandwidth instrument. That is simply because the oscilloscope’s rise time becomes part of the
equation that determines the quality of the viewed signal, as described by this formula:
A pulse that appears to be “correct” when viewed at a lower bandwidth may have an amplitude
aberration in its leading edge, causing it to act like two pulses. Or a narrow transient on a bus
output might escape notice altogether, causing erratic operation on subsequent device inputs.
As the formula implies, a 200 MHz DSO, such as the TDS2000 Series, can capture details that
are invisible to a 100 MHz instrument.
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