What is a Logic
Analyzer?
A
Logic Analyzer is basically a multi-channel version of a digital
oscilloscope which only displays
two logic levels - 0 and 1. Also logic analyzers usually
implement much more complex triggering capabilities than
oscilloscopes. An
oscilloscope can display digital signals like a logic analyzer, but is
better used for analog measurements such as rise-times, fall-times,
and peak amplitudes. You use an oscilloscope when you need to see
small voltage changes and when you need very accurate time interval
measurements. Logic
analyzers are ideal for when you need to see what effect signals have
on your digital logic, when you need complex triggering to find
specific events, and when you need to show lots of channels.
Until recently, logic analyzers were extremely expensive and
out of reach for individual purchasers.
That has changed dramatically with matchbox-sized USB-connected
adapters with small logic grabbers.
Whoa, Trigger!
A logic
analyzer continuously samples multiple input signals and stores the
values in a recycling FIFO buffer. When a preset trigger condition
occurs (rising/ falling edge or defined logic input pattern) writing
ceases to the buffer and the contents are displayed statically. Edge
triggering is can be set by the logical OR of one or more signal
edges. For instance, trigger when the write strobe falls or the read
strobe rises.
Pattern
triggering is set by the logical AND of various signals. For instance,
trigger when chip select is High, the write strobe is Low, AND the
other signals are in some selected state.
This is
the next stage beyond pattern triggering. The same events are defines
as in the previous answer - ORs of various edges, ANDs of various
levels. But these events are used to drive a state machine
within the logic analyzer. The state machine moves between states and
eventually arrives at the trigger state.
An
example of complex triggering may clarify this:
- start in state 0
- move from state 0 to state 1 when you see a certain
pattern
- move from state 1 to state 2 when the pattern goes away
- go back from state 2 to state 1 when an edge occurs
- trigger when you enter state 1 for the 500th time.
Notice
that complex triggering introduces one more feature - a counter. This
example illustrates all the essentials of a complex triggering
capability - a few states, a few pattern or edge recognition, and at
least one counter.
How many channels
do I need?
For some
applications, you can never have too many channels, and logic
analyzers are available with several hundred channels. Typically these
expensive systems are used to debug complex microprocessor buses and
it is a time consuming process to connect the myriad probes involved.
Simpler
tiny units like ANT-8 and ANT-16 are[LU2]
ideal when you want to see exactly what is happening on a few signals
- hooked up to a few
signals to give a trigger source, and to a few signals which are being
debugged. For instance, when debugging a first-in/first-out (FIFO)
memory with an 8-channel unit, you could look at full and empty
indicators, read and write strobes, and a few other signals for
reference. Additional signals in a 16-channel unit make it possible to
look at, say, an 8-bit data bus.
How fast should
sampling be?
You
might expect that the maximum sampling rate can never be too fast but
in practicality 10 times your clock speed should be fine. Even some of
the less expensive modules can sample at up to 500MHz, which gives a
timing resolution down to 2ns. Of course, the sample buffer fills up
quite quickly at that speed, so for slower signals the sampling speed
can be stepped down in stages to a minimum of 100Hz, which gives a
timing resolution of 10ms. Sampling beyond 500MHz needs sophisticated,
expensive probes.
How about
Isolation?
A
degree of isolation can be achieved by running adapter modules from an
isolated laptop computer, or by using an optical USB extender. But
otherwise the PC ground is your ground!
What’s available?
Watch
out when comparing models - the data buffer size can vary from 32kB to
4MB.
ANT-8
8-ch 500MS/s $222.00
www.saelig.com
ANT-16
16-ch 500MS/s $333.00
www.saelig.com
BitScope
8-ch 40MS/s with 100MHz scope
$345.00 www.bitscope.com
LogicPort
34-ch 500MS/s $379.00 www.circuitspecialists.com
USBee
8-ch 24MS/s $445.00 www.USBee.com
Digiview
18-ch 100MS/s $499.00 www.tech-tools.com
LA-2124
24-ch 100MS/s $800.00 www.link-instruments.com
Cleverscope
8ch 100MS/S with 100MHz
scope $999.00 www.saelig.com
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