Embedded Muse 45 Copyright 2000 TGG February 18, 2000

EDITOR: Jack Ganssle, jack@ganssle.com

CONTENTS:
- Editor’s Notes
- Delayed Sweep
- Upcoming Embedded Seminars
- Thought for the Week
- About The Embedded Muse

Editor’s Notes

This is a highly recommended event. Come and visit the show floor and attend the sessions. Clifford Stoll is repeating his keynote address: “Stalking the Wily Hacker”. You may have read his book, The Cuckoo’s Egg. I attended this talk in San Jose last September; it’s pretty hard to describe other than “wow”! The man is a zany, out-of-control, but fascinating speaker.

Hopefully I’ll see you there! See www.embedded.com for more info.

Delayed Sweep

What a loss! I find delayed sweep almost essential for working on fast digital signals. Here’s an overview:

Just as any decent scope has at least two vertical channels, most include two time bases as well. Seems odd, doesn't it? Double vertical channels intuitively makes sense, since each probe picks off a different sense point. Time, though, always flows in the same direction at the same rate, so a single axis is all that makes sense.

Novice scope users understand the operation of time base A: crank the time/division knob to the right and the signal on the screen expands in size. Rotate it to the left and the signal shrinks, but much more history (i.e., more microseconds of data) appears.

Time base B is a bit more mysterious. If enabled, it doesn't start until sometime after time base A begins. Try it on your scope: select "Both" (or "A intensified by B") and select a sweep rate faster than that used by A. You'll see a highlighted section of the trace whose width is determined by B's sweep rate, and whose starting position is a function of the delay time knob.

Switching from "Both" to "B" shows just the intensified part of the sweep: the part controlled by time base B. In effect, you've picked out and blown up a portion of the normal sweep. It's like a zoom control - and you can select the zoom factor using the sweep time, and the "pan position", or starting location, using the delay time adjustment.

Delayed sweep is always accompanied by a second trigger system. Any instrument with dual time bases will come with a second of these knobs to set the trigger point of the B channel.

The second trigger is important when working on digital signals that usually have unstable time relationships. Set the A trigger to start the sweep (as always), position the intensified part of the sweep to some point before the section you'd like to zoom on, and then adjust trigger B until the bright portion starts exactly on the event of interest.

This procedure guarantees that even though the second trigger event moves around with relationship to trigger A, you'll see a stable scope display after selecting the B time base. In effect you've qualified trigger B by trigger A, hopefully zeroing in on the area needing study

Suppose your microprocessor crashes immediately after RESET. Traditional troubleshooting techniques call for hooking up the logic analyzer and laboriously examining all of the data and address lines. Personally, I find this to be too much trouble. Worse yet, it tends to obscure "electrical" problems: the analyzer might translate marginal ones and zeroes into what look like legal digital levels. Logic analyzers are great for purely digital problems, but any problem at power-up can easily be related to signal levels.

Only a scope gives you a view of those crucial signal levels that can cause so much trouble. Trigger channel 1 on the RESET input and probe around with channel 2. Look at READ: every processor starts off with a read cycle to grab the first instruction or startup vector. You may find a puzzling phenomena: if the reset is provided by a source asynchronous to the processor's clock (as is the case with an RC circuit, a Vcc clamp, and even with many watchdog timers) READ will bounce around with respect to RESET. You'll never get a nice high resolution view of READ this way.

Triggering off READ will not help. You need to catch the FIRST read after reset (to look at the first instruction fetch), not any arbitrary read... and no doubt there will be millions of reads between resets.

The answer is delayed sweep. Put RESET into the scope's external trigger input and fiddle the knobs till you get a stable trigger. I like to put one scope channel on the external trigger while doing this initial setup to make sure the trigger is doing what I expect. Then, connect channel 1 to your processor's READ output and crank the time base till it appears over towards the right side of the display. Go to delayed (A intensified by B) mode, and rotate the B time base trigger adjustment until the bright part of the trace starts on the leading edge of the bouncing READ signal.

At this point time base A starts the sweep going on the asynchronous RESET, and time base B triggers the intensified part of the sweep when the first READ comes along. Flip the Horizontal Mode switch to B (to show only the intensified part of the sweep - that part after the B trigger), and a jitter-free READ will be on the left part of the screen. Cool, huh?

With the now stabilized scope display you can use channel 2 to look at the data lines, ROM chip selects, and other signals during the read cycle. It becomes a simple matter to see if the first instruction gets fetched correctly. A lot has to be perfect for this to happen. Very often a power-up problem comes from a bad data line, chip select or buffer problem, any of which is trivial to find with the scope triggered properly.

Delayed sweep was available way back in the vacuum tube scopes of Tektronix’s 540-series; it’s a shame to see it forsaken today.

Embedded Seminars in Boston and San Jose

The focus is uniquely on embedded systems. I'll talk about ways to link the hardware and software, to identify and stamp out bugs, to manage risk, and to meet impossible deadlines. If you’re interested reserve early as these seminars fill completely.

A lot of folks have asked me to bring this seminar to their company. Email me at mailto:jack@ganssle.com if you’re interested.

Thought for the Week

C'mon, give me a break;
else,
I'll continue;

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