TUSB7340: PCIe differential signals - "P" and "N" swapping (polarity inversion)

Allen Fernandez

Intellectual 920 points

Part Number: TUSB7340

Can any of the "N' and "P" signals be swapped on the PCIe diff pairs (TX, RX, REFCLK) ? If so, which one(s) ?

over 3 years ago

0 Malik Barton57 over 3 years ago

TI__Guru 64326 points

Hi Allen,

This document explains the allowable differential pair swapping for the TUSB73x0 devices: http://www.ti.com/lit/ug/sllu149e/sllu149e.pdf See sections 5.4 SuperSpeed Differential Routing and 7.2 Transmit and Receive Links. TUSB7340 supports swap for the PCIE_REFCLKP and PCIE_REFCLKN

Please be aware that the TUSB7340 is NRND (not recommended for new designs) and support is limited to existing documentation and utilities.

0 Allen Fernandez over 3 years ago

Intellectual 920 points

Thank you Malik.

0 Malik Barton57 over 3 years ago in reply to Allen Fernandez

TI__Guru 64326 points

Hi Allen,

No problem.

0 Allen Fernandez over 3 years ago in reply to Malik Barton57

Intellectual 920 points

Follow-up question ... If I'm only using 3 of the 4 USB3.0 ports, is there anything that I need to do to disable the fourth unused port (e.g. control pin) ? Or will the IC automatically disable this unused port by detecting the absence of USB3.0 activity ?

0 Malik Barton57 over 3 years ago in reply to Allen Fernandez

TI__Guru 64326 points

Hi Allen,

You will need to set the PORTx_DIS bit(s), where x=[1:4], of the corresponding port in order to disable a downstream port of TUSB7340.

0 Allen Fernandez over 3 years ago in reply to Malik Barton57

Intellectual 920 points

Hi Malik,

I am enclosing a snap-shot of the TUSB7340 portion of my schematic. I will be interfacing to a Raspberry Pi 4 Compute Module.

Would you kindly take a look at this subcircuit and let me know if you see any schematic issues ?

Thanks,

~Allen

0 Malik Barton57 over 3 years ago in reply to Allen Fernandez

TI__Guru 64326 points

Hi Allen,

I will review and get back to you by EOD Thursday.

0 Allen Fernandez over 3 years ago in reply to Malik Barton57

Intellectual 920 points

Hi Malik,

I look forward to your review.

Looking at Table 104 (USB Control Register Description) in the TUSB7340 datasheet, I don't see any commands that will directly disable USB3.0 and enable USB2.0 only, but do you think there might be an indirect approach to forcing this configuration ? Is there a command to directly perform this function ?

There are 0.1uF series caps (C39 & C40) connected to the USB3.0 TX signals. Assuming I can't force USB2.0-only mode in software, will I need to remove these capacitors to force USB2.0-only mode ?

Thanks,

~Allen

0 Malik Barton57 over 3 years ago in reply to Allen Fernandez

TI__Guru 64326 points

Hi Allen,

Some notes on the schematic below.

For VDD and VDD33 power rails it is best to vary the decoupling caps used so different frequencies power supply noise are filtered apparently. Please include at least one or two 0.01uF decoupling caps.
FREQSEL should be pulled down by 4.7K resistor
Since a active reset is used please ensure that GRST follows the sequence listed in section 9.1.1 Power-Up Sequence of the datasheet.
Crystal load capacitance should be ~18 pF.

You are correct, to make the port USB 2 only you should disconnect SSTX and SSRX. You should place 0 ohm resistors on the SSTX path and leave the DNI as well as the SSRX AC coupling caps.

0 Allen Fernandez over 3 years ago in reply to Malik Barton57

Intellectual 920 points

Thank you Malik !

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TUSB7340: PCIe differential signals - "P" and "N" swapping (polarity inversion)