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Anik , 06-19-2024, 11:14 AM

I can not make any bank after this(photo attached) while I go for routing differential pair routing on kicad 8.0. When I click for make any bank and turn over then it stop routing.

Anik , 06-19-2024, 02:26 PM

Is it possible to cross D+ and D-?

Mini , 06-19-2024, 06:14 PM

I'm sure you can cross them with such a short distance especially if you have ground plane in between. But instead of crossing them like that I have used 0 ohms resistors instead and avoided vias if possible. If you are doing it USB 2.0 or below then USB speeds are quite low and very robust. I have even seen USB working without having differential pair at all.

Anik , 06-19-2024, 07:02 PM

Thank you so much for this information. And yes it is 4 layer. There are a solid ground plane on layer 2

QDrives , 06-19-2024, 07:06 PM

1) Why 4 (6) Vias? You just need one to change layers and the additional via creates even more impedance discontinuity.
2) Unlike @Mini , I would not use 0 ohm resistors for high speed signals. However, You often need to place 22 ohm series termination resistors in USB so you could use those.

Anik , 06-19-2024, 07:15 PM

let me show something

Anik , 06-19-2024, 07:16 PM

Maybe something like this?

Anik , 06-19-2024, 07:17 PM

R0402 resistor was not working for this. So that I have changed it to R0603.
And this is 0ohm resistor near to the usb device.

Anik , 06-19-2024, 07:17 PM

Will this work?

Mini , 06-19-2024, 07:25 PM

Yes if you need series resistors with USB, many MCU-s have them built in though. Depending for which USB it is, I can tell USB 2.0 and below are extremely robust.

QDrives , 06-19-2024, 07:40 PM

I have yet to see a USB device that has the termination resistors internal. There are good reasons to have them external, ie. surges.

Mini , 06-20-2024, 10:54 AM

Look STM32 microcontrollers. Only very old ones needed external ones. Same goes about 1.5k pull up resistor. I have used many times STM32L4 series and they actually tell you in the datasheet not to use external ones. I went extra to datasheet to find that line for you.

Mini , 06-20-2024, 10:54 AM

Mini , 06-20-2024, 10:55 AM

Also you are free to google about it. But I would agree about having external ones would be better perhaps.

Mini , 06-20-2024, 10:55 AM

This is from STM32L476RG datasheet.

QDrives , 06-20-2024, 03:58 PM

Zdrv is between 28 and 44 ohm. So for a 50 ohm impedance, you would still need resistors.
The datasheet merely states "*No external termination series resistors **are required**...*", that is something else than "*they actually tell you in the datasheet **not to use** external ones*". It states that it will work without them.

Mini , 06-20-2024, 04:10 PM

My english not the best so my word choice might not be best. You are nitpicking now. Yes you are correct if we start to nitpicking words. You don't need external resistors with STM32. You can go read their forum or search their development boards. Since you stated you haven't seen any MCU like that I made an example for you.

QDrives , 06-20-2024, 04:23 PM

A common quote: "a datasheet must be assumed incorrect until proven otherwise". I do not care if you call me nitpicking, the output impedance should be better. I pointed out why you should still add the resistors.

Mini , 06-20-2024, 04:41 PM

That's why I said you are free to google, read their forums, look at their development boards etc. You can definitely put a place for external resistors. But there is no need for these STM32. They are only USB FS most of them which means 12 Mbit/s max. Only few have full USB 2.0 speed. But you can't argue that you haven't seen any MCU without external resistors.

QDrives , 06-21-2024, 07:32 PM

"*But instead of crossing them like that I have used 0 ohms resistors instead and avoided vias if possible.*" -- Why add 0 ohm resistor? It is worse than two vias.
USB differential impedance should be between 72 and 120 ohm (https://www.usb.org/sites/default/files/USB_3_0_e-Compliance_methodology_0p5_whitepaper.pdf)
With STM32 being 28 to 44 ohm (single ended). This comes to 56 to 88 ohm.
So STM32 is low in impedance. I do not care what the forum states, as well as the datasheet comment. You should add some resistance as it is not compliant.

Mini , 06-21-2024, 08:42 PM

You can go ahead and nitpick about your impedance. Whatever. You are free to do whatever you want. I don't use external resistors and many development boards from the manufacturer either. But you can't argue about you haven't seen any MCU without external resistors. The whole purpose to include resistors inside is that you don't have to use external ones. Otherwise why bother adding them at all. Yeah I get that it's not that precise, but it's not a problem. We are not designing here USB 3.0 or whatever speeds. And I personally add 0 ohm resistors because I might switch MCU like I have many older MCU-s which do need external resistors. So it makes them simply compatible. Pinout is same on most of STM32 MCU-s.

QDrives , 06-22-2024, 03:08 PM

You cannot state that you are using "USB" if you do not comply with the requirements. And yes, ST should know that too, but I guess the marketing department has more say then the engineers.

Mini , 06-22-2024, 04:47 PM

I guess you can go ahead and compare eye diagrams or whatever STM32 direct USB and with resistors. I can't be bothered with that. It works and thats how they meant to do it. Why they chose this resistance like they have chosen I have no idea. But it works and you I'm pretty sure it's hard to cause any problems. In fact you can even route USB full speed even without using any impedance control. And I have seen it work. Not sure how reliable though. But this STM32 built in is quite reliable, no problems so far.

QDrives , 06-23-2024, 04:28 PM

You can place a USB logo on your product too, but unless you are a member of USB organization, that is not allowed. Same goes for the impedance.