Why are SSRX- and SSRX+ backwards on CX3 ?

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HuEl_264296
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Is there a good reason why SSRX+ and SSRX- seem to be swapped relative to SSTX+ and SSTX- ?

If they were the right way round, it would be very convenient to route to a USB connector.  Was this by design, by accident, or have I made a terrible mistake ?

USB_Swapped.png

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YangyangC_06
Employee
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Thanks for the link, but this doesn't really answer the question. It just says that for USB 3.0 you can connect then backwards and it doesn't matter, but you can't for USB 2.0.

But still, why?   This is not the only example of odd pin placement on a Cypress part. On the PSoC parts, the XRES pin is on the other side of the chip from the programming pins, making it akward to connect to the programming connector.

Hugo

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Hi Hugo,

The KBA explains that SSRX+ and SSR- ,SSTX+ and SSTX- could be swapped to make it easier when you design PCB. USB 3.0 define this feature so that user could do it in this way. That's the reason for USB 3.0 diff pairs.

I am sorry but I think I could not understand what you are asking. Could you please check the screenshot below?

BaiduShurufa_2019-2-24_14-52-56.png

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Hi,

Here is the image again with labels.

USB_Swapped_Labelled.png

The following are the USB pins of the CX3 device, as described in the datasheet:

USB_Pinout.png

As you can clearly see, the USB connector has its pins in a sensible order:

  • RX+
  • RX-
  • TX+
  • TX-
  • DM+
  • DM-

If the CX3 had its pins in the same order, then wiring from the USB connector to the CX3 device would be trivial.

However the CX3 device has a different order:

  • A3:   RX-
  • A4:   RX+
  • A5:   TX+
  • A6:   TX-
  • A9:   DM+
  • A10: DM-

Note that RX- and RX+ are swapped compared to the order in the USB connector.

However, what I think I understand from the article you linked to (KBA218753), is that the polarity of the RX and TX pins of the CX3 is basically meaningless, because their polarity will be automatically negotiated. So I can actually connect them like this, and everything will be fine. Is that correct?

USB_Not_Swappedpng.png

Many thanks

Hugo

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Hi Hugo,

Yes, you're right.

The pin SSRX- on USB receptacle side ,for example, could be connected to SSRX- or SSRX+ on CX3 side(same rule for SSTX+/-). It is supported by USB spec and you don't need to pay other attention.

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