Oscilloscope Digital Channel/MSO Tips
Digital channels! Also, watch pt. 2 of the video here:
• We got kicked out of the studio, so w...
We pre-recorded the first segment just in case we lost studio access, thank goodness we did! Unfortunately, we lost our stream when we switched over to my basement, so watch pt2 linked above to see the winners and the rest of the day's vid!
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I ran into a test problem the other day while working on a side project. I was working with some multi-segment displays that were acting up. When something goes wrong with my projects I almost always find that the error originated between the keyboard and the chair. So, of course, I powered up my scope and went poking around.
The problem, though, is that the 4-channels weren’t enough to get a full picture of my system.
And this happens a lot with embedded designs – there are a lot of peripherals and status lines that need to be checked. Individually that’s easy, but it gets tricky when you want to see them all at once.
That’s when an MSO becomes invaluable. Especially if you just want to see the collective status of different systems. So today we’re going to look at a few of my favorite tricks for using digital channels.
The first is to get good grounding. As always, the more grounding the better. But, grounding every pin isn’t always necessary. For low speed signals, the digital channel’s equivalent circuit looks like this
That’s not too bad, so you can usually get away with grounding every 8th channel or so. Back in the day that was one ground per “pod”
If you’re in an electrically noisy environment, though, I recommend grounding every 3rd channel.
When you get to high frequency systems, though, with rise times under 3ns or so, the equivalent circuit changes from this to this.
And, our impedance vs. frequency looks different.
At this point, you need to start grounding every lead.
If your layout reflects your test plan, this is actually pretty easy to do. Your digital probes come with some handy accessories, like this male/female adapter that let you hook up to an array of signals all at once, and these ground leads. If you put in a socket and route your grounds correctly the whole thing is a breeze. You can also use these leads to find a local ground point, especially if you’re using clips.
Once connected, you can play with your digital channel display. Turn channels on or off, scale them, and I like to assign them to a bus. This makes it easy to see both individual channel activity and get a hex value for that bus. You can then work out which hex values correspond to which device states. Checking a bus’s hex value is much easier than trying to verify each channel individually.
You can also use cursors to measure values, or trigger on specific channel activity or bus values.
If you’re using serial protocols you can also use digital channels to decode them, saving your analog channels for other debugging work.
It takes some work up front to setup, but this makes it really easy to go back and validate firmware changes or verify multiple boards.
Plastic clips – These accessories are super handy for designs where you planned ahead and have some grounded header pins.
These are nice when you just want to hook up to a nearby ground post, but it is still important to keep your ground leads as short as possible. These IC clips are also pretty handy.
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