USB isolation ATMEGA32U4 - Page 1
Forum section: electronics / beginners

author: wigman27
author_rank: Regular Contributor
timestamp: April 28, 2015, 01:56:39 am
content: Hi all,I'm currently working on rev c of mydummy loadand I am trying to learn a bit about the USB isolation.I am using the ATMEGA32U4 which has a USB communication interface already and separate pins. Does this mean that it is all isolated within the chip? So the d+ and d- lines are already isolated and all I need to isolate is the section that powers the rest of the board?For example I would run the 4 wires, USB power, d+, d- and USB ground directly into the micro then tap off the USB power and USB ground with an isolated DC/DC converter and that will be considered isolated.Is that correct or will I need to do more?ThanksLee

author: matseng
author_rank: Frequent Contributor
timestamp: April 28, 2015, 06:15:45 am
content: I'm fairly certain that UGND and GND are connected inside the chip, so you will have no isolation whatsoever.An alternative is to use a cheap MCP2221 USB<->UART/I2C chip powered by the USB+ and connect the rx/tx via optocouplers to your atmega. And then use the isolated dc/dc converter to power the mega and the rest of the stuff on the board.

author: Jeroen3
author_rank: Super Contributor
timestamp: April 28, 2015, 06:55:22 am
content: Why would you use optocouplers when you can get this stuff:Single chip USB isolator:http://www.analog.com/en/products/interface-isolation/isolation/standard-digital-isolators/adum3160.htmlDigital Isolator (for uart):http://www.silabs.com/Support%20Documents/TechnicalDocs/Si861x-2x.pdfThe USB peripheral has a separate power domain because USB requires a relative large amount of power for the line drivers.

author: matseng
author_rank: Frequent Contributor
timestamp: April 28, 2015, 07:11:25 am
content: Maybe because the Silabs stuff costs like 10x the price of two optos and doesn't add any extra value to the solution?

author: wigman27
author_rank: Regular Contributor
timestamp: April 28, 2015, 07:47:47 am
content: Thanks guys,Quote from: Jeroen3 on April 28, 2015, 06:55:22 amDoes that mean you think it has isolation? Does anybody have a chip off the board they can measure by chance?ThanksLee

author: Dago
author_rank: Frequent Contributor
timestamp: April 28, 2015, 08:19:39 am
content: Quote from: wigman27 on April 28, 2015, 07:47:47 amThe USB peripheral definitely does not have any kind of isolation. You need to isolate all lines coming from the USB.

author: electr_peter
author_rank: Unknown
timestamp: April 28, 2015, 08:22:44 am
content: No typical IC has an isolated USB by default because it would require isolated power supply.

author: wigman27
author_rank: Regular Contributor
timestamp: April 28, 2015, 08:36:27 am
content: OK, awesome! Thanks. Isolation it is.So, does anyone have any experience using theSingle chip USB isolatormentioned earlier with the ATMega32U4? Does it have its own USB drivers or will it still think it's an Arduino Leonardo and act purely as an isolator?I am thinking I will use either this or the opto isolator mentioned earlier and use with an isolated DC to DC converter as I will need about 200mA I am thinking.Thanks againLee

author: Jeroen3
author_rank: Super Contributor
timestamp: April 28, 2015, 08:54:25 am
content: You can buy those ADUM3160 in a USB Isolator. Which is literally a board with the following parts:- USB A Male.- ADUM3160 with a few power caps.- Separate dc/dc isolator block.- USB A Female.No digital stuff.Or you can isolate uart, with optocouplers or isolator ic, but you'll need an ftdi-like chip.Which does have its own advantages.Choice is up to you.