Resistor on USB D+ and D- - Page 1
Forum section: electronics / beginners

author: chancs
author_rank: Contributor
timestamp: September 27, 2019, 05:13:04 am
content: I wish to use CP2102N as a USB to UART interface chip. And in the datasheet, they did not put any resistors in the USB data line. However, when I look at the Arduino UNO wifi ver2, they put 22Ohm resistors on the D+ and D- line. What is the purpose of these 2 resistors? To limit current in case of high voltage spike?Arduino UNO wifi ver2CP2102N Datasheet

author: chancs
author_rank: Contributor
timestamp: September 27, 2019, 05:51:04 am
content: "The Universal Serial Bus function controller in the CP2102N is a USB 2.0 compliant full-speed device with integrated transceiver andon-chip matching and pull-up resistors." Quote from the datasheet.They didn't explicitly say impedance matching. But is that what you meant?

author: Berni
author_rank: Super Contributor
timestamp: September 27, 2019, 06:13:43 am
content: These resistors also usually help save the chip from blowing up if 5V shorts to any of the data lines(Like a damaged connector or cable)

author: tszaboo
author_rank: Super Contributor
timestamp: September 27, 2019, 09:12:15 am
content: FYI, any Arduino isNOTa good source to learn best design practices. Or OSHW for that matter. They copy stuff from each other, without really understanding what is going on. For example it is possible that you need the resistors for FTDI, say FT232D and they just copy it and use it for the FT232H, which doesnt need it, and then it is copied by a chinese engineer, who replaces it with a CP2102N, because it is 5 cent cheaper than the FT232H... And so on.The difference is that everyone can tell apart a shoddy shed and a proper house, but it takes a bit of insight to spot bad electronics design.

author: chancs
author_rank: Contributor
timestamp: September 27, 2019, 02:29:09 pm
content: Quote from: NANDBlog on September 27, 2019, 09:12:15 amHow about dev board schematic provided by the vendor, like this one (https://www.silabs.com/documents/public/schematic-files/CP2102N-MiniEK-schematic.pdf). I suppose vendor knows what they are doing.

author: mbless
author_rank: Regular Contributor
timestamp: September 27, 2019, 02:45:05 pm
content: Quote from: chancs on September 27, 2019, 02:29:09 pmThat's fine. SiLabs is the chip manufacturer, so they're just applying their datasheet guidelines to the dev board...

author: rstofer
author_rank: Super Contributor
timestamp: September 27, 2019, 02:51:39 pm
content: Is it possible that the uC could be misprogrammed and those USB pins defined as outputs? Would that cause damage to any part of the USB chain?

author: iMo
author_rank: Super Contributor
timestamp: September 27, 2019, 06:19:29 pm
content: Adding those 22ohm resistor will not hurt even the chip includes some..

author: wraper
author_rank: Unknown
timestamp: September 27, 2019, 06:29:53 pm
content: Quote from: NANDBlog on September 27, 2019, 09:12:15 amArduino (original) is awful. They cannot even connect USB ESD protection device(s) properly. In addition to incorrect circuit they also use routing with long loops. For example Arduino UNO with ATmega8u2 is especially horrible in this regard. You are freaking supposed to tie it to GND is shortest possible way, not as in this particular case connect it though 4.7n (C1) cap which might actually be damaged itself by ESD thanks to this "protection".