I have 2 questions:

  1. The USB differential pair D+/D- are impedance-controlled 90 Ohm transmission line..
    Why do I need to put a 22 Ohm series termination resistor in it? Isn't that a discontinued resistance in the transmission line that could technically cause reflections?

  2. should I implement TVS diodes on USB and I2C data lines?

Edit: I'm using STM32L152CB-A MCU, the I2C will be user to communicate with some ICs on the PCB itself, and the USB will be used to communicate with a computer which will act as USB host.. I'm using USB2.0.


  • 1
    \$\begingroup\$ Regarding I2C, it normally doesn't require ESD diodes, because it's an onboard standard. Preferably it never even touches a connector. See: electronics.stackexchange.com/questions/461010/… \$\endgroup\$ Oct 27, 2023 at 1:39
  • \$\begingroup\$ @TimWilliams I2C is used as a basis for several other standards, where effectively I2C goes several meters between two consumer devices. \$\endgroup\$
    – Justme
    Oct 27, 2023 at 4:31
  • \$\begingroup\$ To the original poster, we don't know which chip you mean so please mention that to have a specific answer for the chip you use. We also don't know what you are going to do with I2C so we can't answer if you need TVS diodes on I2C, please mention what you do with I2C. \$\endgroup\$
    – Justme
    Oct 27, 2023 at 4:34
  • \$\begingroup\$ @Justme Exactly: at best a few meters, in shielded cable. And even that's a low-priority application, AFAIK (if it fails, it can just keep retrying; or it's polled regardless, I don't know). \$\endgroup\$ Oct 27, 2023 at 5:34
  • 1
    \$\begingroup\$ @AbdAlhaleemBakkor Some chips may need them and some chips may not. It depends on the chip so if the manufacturer says to use or not then you either put them or don't, and adding them if not needed or leaving them off if they are needed are both bad. You can't apply a suggestion for one chip as suggestion to another chip. \$\endgroup\$
    – Justme
    Oct 28, 2023 at 21:10

1 Answer 1


Why do I need to put a 22 Ohm series termination resistor in it?

You need to use series resistors on D+/D- lines only if you use a cheap MCU operating only in full-speed (FS) mode. The 22-27 Ohm resistors are needed to match the driver impedance of non-sophisticated USB PHY drivers (typically 25 Ohms) to trace/cable impedance of 90/2 = 45 Ohm. This will help to have a nice FS eye diagram.

Regarding TVS diodes, see this answer.

  • \$\begingroup\$ it's STM32L152CB-A I'm using, The documentation doesn't say anything about the series termination resistors, but I'm yet to read the application note. \$\endgroup\$ Oct 27, 2023 at 15:16

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