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USB cdc virtual com port and USB otg

harinath
Associate III

‎2015-09-10 7:51 AM

Posted on September 10, 2015 at 16:51

Designing STMF405 Board. Bit confused about USB functionality. 

We need multi role USB functionality (from one usb connector). 

The current usb circuit is as shown below.

0690X00000605ChQAI.png

The board should communicate with computer usb (Virtual com port) for data transfer.

Also, when the board connected to computer, it should appear simply as a usb mass storage device (like usb flash drive).

Do I need USB OTG ? As far as I know, it supports multi-role for usb.

I got confused when I saw the schematics of STM3240G-EVAL

http://www.st.com/st-web-ui/static/active/en/resource/technical/document/user_manual/DM00036746.pdf

It has two USB OTG connectors.

What is external PHY and USB OTG HS ? 

What is USB OTG FS ?

If the STM3240G-EVAL has what I'm looking for, what part of the usb I can adapt ?

Because lack of expertise, we have populated the MicroSD card socket for the board ( FAT file system). But for final product, we do not want MicroSD card.

Thank you

#usb-otg #cdc
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1 REPLY 1
waclawek.jan
Super User

‎2015-09-11 12:19 PM

Posted on September 11, 2015 at 21:19

> The board should communicate with computer usb (Virtual com port) for data transfer.

> Also, when the board connected to computer, it should appear simply as a usb mass storage device (like usb flash drive).

>

>Do I need USB OTG ? As far as I know, it supports multi-role for usb.

No. Multi-role means that it can be host or device - not simultaneously, but at different times. For example, some digital cameras, they have only one USB connector;  when connected to printer, they act as a host and control the printer; when connected to PC, they act as a device (e.g. mass storage).

The expression you are looking for is ''composite device'', see e.g. https://en.wikipedia.org/wiki/USB#System_design second paragraph. It's only a device, you don't need OTG nor dual-role.

The STM32F4xx High-Speed OTG USB device is PHY-less (more precisely, its PHY is Full-Speed only), so if you want to use it as High-Speed, you need an extra chip, the High-Speed PHY. That's what is on the EVAL board; but the chip used there is hopelessly obsolete so.

JW

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