Author Topic: USB interface differential pairs  (Read 485 times)

0 Members and 1 Guest are viewing this topic.

Online alireza7

  • Regular Contributor
  • *
  • Posts: 87
  • Country: us
USB interface differential pairs
« on: March 09, 2018, 04:54:51 am »
hi
i want to design USB interface between a GPS/GPRS Module (Quectel EC21) and a microcontroller.

in Hardware Design manual of Quectel EC21 page35 it say :
It is important to route the USB signal traces as differential pairs with total grounding. The impedance of USB differential trace is 90 ohm

what does exactly it mean?

differential pairs with total grounding ???????????

The impedance of USB differential trace is 90 ohm????????????


Hardware Design manual of Quectel EC21 :
https://www.codico.com/shop/media/datasheets/Quectel_EC21_Hardware_Design_V1.0.pdf
 

Online ogden

  • Super Contributor
  • ***
  • Posts: 1307
  • Country: lv
Re: USB interface differential pairs
« Reply #1 on: March 09, 2018, 05:07:39 am »
It is important to route the USB signal traces as differential pairs with total grounding. The impedance of USB differential trace is 90 ohm

what does exactly it mean?

differential pairs with total grounding ???????????

The impedance of USB differential trace is 90 ohm????????????

Seems you have a lot to learn :)

http://www.eevblog.com/forum/projects/usb-differential-routing/

https://en.wikipedia.org/wiki/Characteristic_impedance

https://en.wikipedia.org/wiki/Microstrip

http://www.cypress.com/file/144296/download

 

Online alireza7

  • Regular Contributor
  • *
  • Posts: 87
  • Country: us
Re: USB interface differential pairs
« Reply #2 on: March 09, 2018, 05:20:23 am »
i some how understand the concept of impedance but i can't figure out what does " differential pairs with total grounding " mean :o
 

Online rstofer

  • Super Contributor
  • ***
  • Posts: 5056
  • Country: us
Re: USB interface differential pairs
« Reply #3 on: March 09, 2018, 05:25:39 am »
What the performance penalty for using the UART interface?  Implementing USB in a non-trivial task even beyond physical things like traces.  Do you have to write a driver?  Is the microconntroller USB gadget capable of being a host?  Or is it just a device like UART or HID or mass storage?

 

Offline Wilksey

  • Super Contributor
  • ***
  • Posts: 1105
Re: USB interface differential pairs
« Reply #4 on: March 09, 2018, 05:54:00 am »
I only bother with ESD protection if it is going to an external connector, R1 + R2, I typically use 22R in these positions.
I wouldn't bother with the test points which removes the warning for the "stubs".
I typically route diff and impedance controlled pairs / traces on the top layer, with the 2nd layer (inner plane) a solid ground, make sure the traces do not cross any other signal if you can help it, if you must, make sure it not a emissive source like a clock source etc.  I wouldn't usually bother surrounding the pair with ground, just make sure I had good plane referenced.

To calculate, you can use a calculator with a known stackup, or you can ask for the track and gap from your usual PCB supplier for 90R differential, they will be able to tell you for their stackup what the values are.

If it is just for testing, then don't worry too much, just make sure the trace lengths are roughly the same length (for timing), for production i'd do it properly, if you plan on taking it for EMC testing then definitely do it properly!
 

Online ogden

  • Super Contributor
  • ***
  • Posts: 1307
  • Country: lv
Re: USB interface differential pairs
« Reply #5 on: March 09, 2018, 06:02:45 am »
i some how understand the concept of impedance but i can't figure out what does " differential pairs with total grounding " mean :o

In case you understand what differential pairs mean, "total grounding" sounds to me like sole solid ground plane. So "differential pair with solid ground plane beneath".
« Last Edit: March 09, 2018, 08:39:55 am by ogden »
 

Online alireza7

  • Regular Contributor
  • *
  • Posts: 87
  • Country: us
Re: USB interface differential pairs
« Reply #6 on: March 09, 2018, 06:11:20 am »
What the performance penalty for using the UART interface?  Implementing USB in a non-trivial task even beyond physical things like traces.  Do you have to write a driver?  Is the microconntroller USB gadget capable of being a host?  Or is it just a device like UART or HID or mass storage?

this module have capability of software debugging and firmware upgrade by USB interface.
 

Offline paulca

  • Super Contributor
  • ***
  • Posts: 1714
  • Country: gb
Re: USB interface differential pairs
« Reply #7 on: March 09, 2018, 06:33:34 pm »
i some how understand the concept of impedance but i can't figure out what does " differential pairs with total grounding " mean :o

In case you understand what differential pairs mean, "total grounding" sounds to me like sole solid ground plane. So "differential pair with solid ground plane beneath".

Above, below and either side as it says in the later image.
"What could possibly go wrong?"
Current Open Projects:  3 Channel Audio mixer with DAC, BT, pre-amps and h/phone amp, WS281x LED controller Version 2 5V/5A w/Atmega328, FY6600 Power supply, 5A DC Load (Still!)
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf