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Blog USB PD - CC decoding adventures part 2
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EMI-Reduction-Techniques
Engagement
  • Author Author: wolfgangfriedrich
  • Date Created: 19 Mar 2023 2:42 AM Date Created
  • Views 1591 views
  • Likes 11 likes
  • Comments 7 comments
  • power delivery
  • cc
  • usb-c
  • opamp
  • usb pd
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Recommended

USB PD - CC decoding adventures part 2

wolfgangfriedrich
wolfgangfriedrich
19 Mar 2023

As promised in my post about USB PD - CC decoding adventures, here is the conclusion of my journey to a usable tool to debug USB-PD messages on the CC interface. It also shows that picking the correct opamp characteristics is not only a good idea, but makes the circuit work.

I decided to build a board to pick up the CC1/CC2 signals and amplify them from a 1.2 V to ~3.3 V high level through a simple non-inverting op-amp with a gain of 2.5. The op-amp is a ST Micro TSV992 dual op-amp with 20 MHz GBW and 10 V/usec slew rate. I still made a mistake picking this particular part, as the datasheet clearly states it is stable for gain >4 only. I might have gotten lucky as I don't see any instability/oscillation in my setup. The part is a standard pinout for a dual op-amp, so the board is still useful and rev 1.1 will use a different op-amp.
Here is the schematic, board and pictures of the setup.

{gallery}CC Monitor

image

CC Monitor Schematic

Bare board front (who has good eyes?)

Bare board back

Populated board

image

Test setup


The test setup is straight forward with a ON Semi 60W PD eval board as source (not pictured), USB-C cable, the CC Monitor/ a TC66/C and my trusted CYPD3177 sink board. The sink is configured to request a fixed voltage contract of 5V / 1A. The CC Monitor has 2 dual pin headers to connect an oscilloscope and logic analyzer as CC decoder at the same time. It also has 2 linear regulators, because my standard part was out of stock everywhere and this gave me options. Of course, there are different pinouts for a simple linear regulator in SOT-89 package, magic smoke shall be expected. Both regulators are high voltage input up to 20V to be useful for the full USB-PD standard range and only one should be connected through a 0 Ohm resistor. As you can see, I like symmetry.

To show correct functionality, below is a scope capture of a CC transmission. Blue channel 2 is the CC signal as it appears on the USB-C interface with a voltage level of ~1.2V. Yellow channel 1 is the amplified signal that is used as the input to the logic analyzer. It replicates the nice slope at the rising and falling edges as a result of the sufficient slew rate and the overall 1s and 0s are also present with good symmetry, so the overall bandwidth is not exceeded as well.

image

And because I really like the Pulseview/Sigrok PD decoder here is another image of a decoded packet showing the sink request for a fixed 5V/ 1A contract.

image

Now I have a nice tool to debug my attempt to design a USB-C PD source. As usual, I will write in more detail when I have a working design.


Useful links:
If anybody is interested, the board can be ordered at OSHpark. Did I mention that I find the afterdark option really useful and sexy.

shabaz's USB PD recent blog posts:
USB Type-C and Power Delivery: Any Interest?
100W USB Type-C Power Delivery Source: Getting Started

 baldengineer's post about opamp characteristics

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  • wolfgangfriedrich
    wolfgangfriedrich over 2 years ago in reply to scottiebabe

    Thanks. I'll try to measure that and keep that in mind for the future.

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  • scottiebabe
    scottiebabe over 2 years ago

    We all learn by experimenting! I believe device discovery occurs with vbus unpowered so the op-amps input esd diode may load the cc lines before vbus is applied. Likely not enough to cause a problem, but something to consider in the future. Happy probing! 

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  • wolfgangfriedrich
    wolfgangfriedrich over 2 years ago in reply to shabaz

    Thanks, shabaz.

    The OSHpark option to make boards public is really neat. I was at the ordering end of this system as well. If it becomes hugely popular, I might set it up as a populated board for sale on Tindie. But I don't think so, the people doing low level USB-PD CC decoding is probably very narrow.

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  • shabaz
    shabaz over 2 years ago

    Hi Wolfgang,

    Great work! Thanks for setting it up as a project where the boards can be ordered easily. This will be something I will try to construct at some point too, to learn about the messaging with PD chips.

    This color option looks really good!

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  • wolfgangfriedrich
    wolfgangfriedrich over 2 years ago in reply to baldengineer

    Thanks James.

    Good grief, the D+/D- mix-up is definitely not intended. As you said, straight though, lucky me. Slight smile

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