7 Replies Latest reply on Aug 24, 2014 12:01 PM by anko_284866

Connecting a pin to both an uncommitted op amp and an analog mux (psoc5lp)

anko_284866 Aug 24, 2014 12:01 PM

I'm trying to drive a calibration voltage from a VDAC through an op-amp (for higher current drive) and monitor the voltage at the same time (I'm driving a variable load and want to manually compensate it).

I can't figure out how to get PSoC Creator to let me do this. From a routing perspective it's trivial, I can hand-route it, but I'm wondering if there's a way to do it "normally" and just not seeing it.

The pin I'm using (bottom left) is the direct output from one of the uncommitted op amps. When I try to generate this design, Creator complains that the op-amp output is only designed to drive one pin. If I try to use an AMUX (with the intention of connecting both "inputs" to the "output", Creator complains that you can't connect the common of a mux to the input of another. If I rotate the AMUX I get similar errors.

I really don't want to use a "throwaway" pin to feed the signal back. Is there a Creator-sanctioned way to do this?

threeway.png 21.7 K

1. Re: Connecting a pin to both an uncommitted op amp and an analog mux (psoc5lp)

helic_263931 Aug 21, 2014 5:30 PM (in response to anko_284866)

Did you try to use the SARADC alone (not the sequencing SAR)? It might be (without looking at the data sheets) that the SAR mux want to connect its inputs to pins directly, and that Creator doesn't like that.

Did you specify P3[3] as OpAmp out manually, or got that assigned by Creator?
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2. Re: Connecting a pin to both an uncommitted op amp and an analog mux (psoc5lp)

anko_284866 Aug 21, 2014 9:20 PM (in response to anko_284866)

Yes, using another ADC solves it, but I am looking for a more generic solution. A future application may require both ADCs and thus not work.

It looks like it can be done by forcing the analog routing to close an additional switch. Is there an "official" way to do this? I tried net ties and net joins without any sucess, and I'd really like to avoid hardcoding it in software.
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3. Re: Connecting a pin to both an uncommitted op amp and an analog mux (psoc5lp)

JoMe_264151 Aug 22, 2014 2:37 AM (in response to anko_284866)

The mux that is used with the sequebcing SAR is the Analog Hardware Mux and it does not allow for any additional internal connections (see datasheet), so Hli suggestion is completely right: you cannot solve that except connecting external pins.

Bob
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4. Re: Connecting a pin to both an uncommitted op amp and an analog mux (psoc5lp)

helic_263931 Aug 22, 2014 4:15 AM (in response to anko_284866)

Alternatively, you can use a normal SAR ADC together with the non-hardware analog mux. It means you lose the automatic sequencing, but should be able to connect the feedback path.
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5. Re: Connecting a pin to both an uncommitted op amp and an analog mux (psoc5lp)

dakn_263916 Aug 22, 2014 4:34 AM (in response to anko_284866)

Consider filing a CASE on the original problem, to see if

any of the net components can force a route.







To create a technical case at Cypress -



www.cypress.com

“Support”

“Technical Support”

“Create a Case”



You have to be registered on Cypress web site first.



Regards, Dana.
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6. Re: Connecting a pin to both an uncommitted op amp and an analog mux (psoc5lp)

anko_284866 Aug 23, 2014 3:23 PM (in response to anko_284866)

Thank you all for your ideas and thoughts on this.



For now I've gone the same route that hli suggested which was just to use the second SAR without a MUX.



There are still a lot of "hidden" restrictions when using the various higher level constructs which I suppose is par for the course (trading implemenation speed with flexibility). I'm not completely sure *why* a hardware mux restricts things in this way, since it appears that the actual routing fabric would allow it. This isn't a criticism so much as just a recognition of the learning curve these parts present.
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7. Re: Connecting a pin to both an uncommitted op amp and an analog mux (psoc5lp)

JoMe_264151 Aug 23, 2014 4:21 PM (in response to anko_284866)

When you switch to "Analog" view in the .cydwr file you can see how the Analog Hardware Mux is realized and how it affects further routing. The analog routing capabilities are a bit reduced compared to the PSoC5.

Bob
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8. Re: Connecting a pin to both an uncommitted op amp and an analog mux (psoc5lp)

anko_284866 Aug 24, 2014 12:01 PM (in response to anko_284866)

That's actually my point; I can see clearly that I should be able to close a switch and realize what I want without trouble. This is why I asked how to try to show it to Creator. :-)
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