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dowhatnow
Joined: Sep 05, 2011
Posts: 6
Location: CA
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 Posted: Mon Sep 05, 2011 5:38 pm Post subject:
Help troubleshoot my multiplexer issues! |
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I have a multiplexer that seems to trigger multiple outputs at once for some reason.
There are 4 inputs coming from a counter. I was poking around with a logic probe and found that when I was touching one of the inputs of the multiplexer, the problem went away!
I thought maybe it might need a pull down resistor...nope. Same results.
I thought maybe power was giving me a problem...nope, caps across ground and power near every chip. Same results.
I actually connected the probe without the leads connect to the circuit, and repeated the touch, and it still fixed the problem while touching. Weird. Then I just used a metal screw driver and touched it, and it fixed it!?
I thought maybe it was a physical joint problem or something, so I tried to just push down on it a bit with a non conductive tool. This did not fix the problem.
This is making my brain hurt. How can I resolve this?? |
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dowhatnow
Joined: Sep 05, 2011
Posts: 6
Location: CA
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| Posted: Mon Sep 05, 2011 6:32 pm Post subject:
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Keep in mind, this doesn't look like a full pulse on multiple outputs, but very quick pulses on the wrong outputs.
For example, when output number 7 is high, output number 4 has a very quick pulse.
Also, this seems to only happen with every 4th output (outputs 3,7,11,15).
Am I just dealing with a fried chip here, or can someone confirm this behavior with a 4067? |
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dowhatnow
Joined: Sep 05, 2011
Posts: 6
Location: CA
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| Posted: Mon Sep 05, 2011 7:11 pm Post subject:
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| Also, I am wired to and from a panel...propagation delay may be the culprit here. I need to look into this a bit more. |
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brock
Joined: May 26, 2011
Posts: 75
Location: Canada
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| Posted: Mon Sep 05, 2011 8:00 pm Post subject:
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| dowhatnow wrote: | | Also, I am wired to and from a panel...propagation delay may be the culprit here. I need to look into this a bit more. |
If you're using a ripple counter for the select inputs you have an inherent delay issue because the bits change sequentially A-B-C-D on a clock transition.
If you have X7 selected where DCBA = 0111 and you are switching to X8 where DCBA = 1000 you will have a series of transitions that go 0111, 0110, 0100, 0000, 1000. You can have three unwanted outputs on X6 ,X4 and X0. They sometimes go away when you put the logic probe on the input because the capacitance of the probe is slewing the edge of the fast select signal that's causing the glitches.
If this is causing a real circuit problem rather than just the observation that you can see some glitches you probably need to rethink your design. |
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dowhatnow
Joined: Sep 05, 2011
Posts: 6
Location: CA
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| Posted: Mon Sep 05, 2011 8:45 pm Post subject:
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Wow, I was not aware of this. Thank you for the insight!
I guess typically this wouldn't be a problem, but it definitively is for my design  |
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corex
Joined: Mar 02, 2010
Posts: 100
Location: Las Vegas
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| Posted: Tue Sep 06, 2011 12:06 pm Post subject:
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| Do you have a schematic drawn up? |
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dowhatnow
Joined: Sep 05, 2011
Posts: 6
Location: CA
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| Posted: Tue Sep 06, 2011 1:36 pm Post subject:
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No, not really. This is a huge project and I haven't had a chance to document it all yet.
I already have a new design for this section, so I should be good now.
Thanks guys! |
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