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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.

Thanks

Abd Alhaleem Bakkor
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    Regarding I2C, it normally doesn't require ESD diodes, because it's an onboard standard. Preferably it never even touches a connector. See: https://electronics.stackexchange.com/questions/461010/can-i-use-i2c-over-2m-cables – Tim Williams Oct 27 '23 at 01:39
  • @TimWilliams I2C is used as a basis for several other standards, where effectively I2C goes several meters between two consumer devices. – Justme Oct 27 '23 at 04:31
  • 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. – Justme Oct 27 '23 at 04:34
  • @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). – Tim Williams Oct 27 '23 at 05:34
  • @TimWilliams In HDMI, video link is negotiated up, and during watching a movie, there is constant monitoring of link and encryption errors, so any bit going wrong on I2C will cause the video to go away until link is re-negotiated up. I would not call that low priority from that perspective. It's not a life support or safety application, but you sure as a consumer don't want your gaming or movie session being disturbed by screen going blank every now and then. And true, passive cables may be 3-5 meters tops but that is not limited by I2C but video, and the active cables have tens of meters. – Justme Oct 27 '23 at 10:09
  • @Justme , I made an edit to the post, I added the MCU used and purpose of I2C and USB links. thanks – Abd Alhaleem Bakkor Oct 27 '23 at 15:14
  • @AbdAlhaleemBakkor Now which document says you must put 22 ohms in series with USB lines on a STM32L152CB? – Justme Oct 27 '23 at 15:32
  • @Justme web article, and what I remember that series will enhance the signal integrity. So, should I use them? – Abd Alhaleem Bakkor Oct 28 '23 at 19:19
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    @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. – Justme Oct 28 '23 at 21:10

1 Answers1

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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.

Ale..chenski
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  • 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. – Abd Alhaleem Bakkor Oct 27 '23 at 15:16