USB interface differential pairs - Page 1
Forum section: electronics / beginners

author: alireza7
author_rank: Regular Contributor
timestamp: March 08, 2018, 05:54:51 pm
content: hii 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 ohmwhat 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

author: ogden
author_rank: Super Contributor
timestamp: March 08, 2018, 06:07:39 pm
content: Quote from: alireza7 on March 08, 2018, 05:54:51 pmSeems you have a lot to learnhttps://www.eevblog.com/forum/projects/usb-differential-routing/https://en.wikipedia.org/wiki/Characteristic_impedancehttps://en.wikipedia.org/wiki/Microstriphttp://www.cypress.com/file/144296/download

author: alireza7
author_rank: Regular Contributor
timestamp: March 08, 2018, 06:20:23 pm
content: i some how understand the concept of impedance but i can't figure out what does " differential pairs with total grounding " mean

author: rstofer
author_rank: Super Contributor
timestamp: March 08, 2018, 06:25:39 pm
content: 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?

author: Wilksey
author_rank: Super Contributor
timestamp: March 08, 2018, 06:54:00 pm
content: 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!

author: ogden
author_rank: Super Contributor
timestamp: March 08, 2018, 07:02:45 pm
content: Quote from: alireza7 on March 08, 2018, 06:20:23 pmIn case you understand what differential pairs mean, "total grounding" sounds to me likesolesolid ground plane. So "differential pair with solid ground plane beneath".

author: alireza7
author_rank: Regular Contributor
timestamp: March 08, 2018, 07:11:20 pm
content: Quote from: rstofer on March 08, 2018, 06:25:39 pmthis module have capability of software debugging and firmware upgrade by USB interface.

author: paulca
author_rank: Super Contributor
timestamp: March 09, 2018, 07:33:34 am
content: Quote from: ogden on March 08, 2018, 07:02:45 pmAbove, below and either side as it says in the later image.