• AVR Freaks

Helpful ReplyHot!Questions about the VBUS voltage comparator

Page: 12 > Showing page 1 of 2
Author
jtron
Starting Member
  • Total Posts : 36
  • Reward points : 0
  • Status: offline
2020/07/31 07:51:15 (permalink)
0

Questions about the VBUS voltage comparator

My dsPIC33EP256MU806 is functioning as a self-powered USB device. I'm using the PIC's built-in VBUS voltage comparator to detect whether or not a host is connected. The documentation talks about the VBUS voltage comparator thresholds (the "session valid" and "session end level" voltages) but I can't find specifications for these voltages anywhere. Also, I can't find a spec for the VBUS input current. I need to know all of these specs in order to create a solid design that will reliably detect host connect/disconnect events. Specifically, I need answers to the following questions:
  1. What is the "session valid" voltage (minimum VBUS voltage for guaranteed "host connected" detection)?
  2. What is the "session end level" voltage (maximum VBUS voltage for guaranteed "host disconnected" detection)?
  3. What is the maximum VBUS input current?
Update
I need to know the VBUS specs because in my design the PIC is galvanically isolated from the USB cable, and consequently my isolator must "reconstruct" the cable 5V and apply it to PIC's VBUS pin. To do this reliably, my isolation circuitry must take into account the VBUS threshold voltages and maximum input current.
post edited by jtron - 2020/07/31 09:58:46
#1
du00000001
Just Some Member
  • Total Posts : 3946
  • Reward points : 0
  • Joined: 2016/05/03 13:52:42
  • Location: Germany
  • Status: offline
Re: Questions about the VBUS voltage comparator 2020/07/31 08:07:43 (permalink)
5 (1)
Seems it's all in the standards.
Secondary source:  https://www.beyondlogic.org/usbnutshell/usb2.shtml

PEBKAC / EBKAC / POBCAK / PICNIC (eventually see en.wikipedia.org)
#2
jtron
Starting Member
  • Total Posts : 36
  • Reward points : 0
  • Status: offline
Re: Questions about the VBUS voltage comparator 2020/07/31 08:15:48 (permalink)
0
du00000001
Seems it's all in the standards.
Secondary source:  https://www.beyondlogic.org/usbnutshell/usb2.shtml



That doesn't seem to answer any of my questions. It doesn't define the PIC comparator voltage thresholds nor does it mention the PIC's VBUS input current.
#3
Antipodean
Super Member
  • Total Posts : 1908
  • Reward points : 0
  • Joined: 2008/12/09 10:19:08
  • Location: Didcot, United Kingdom
  • Status: offline
Re: Questions about the VBUS voltage comparator 2020/07/31 12:16:33 (permalink)
5 (2)
jtron
du00000001
Seems it's all in the standards.
Secondary source:  https://www.beyondlogic.org/usbnutshell/usb2.shtml



That doesn't seem to answer any of my questions. It doesn't define the PIC comparator voltage thresholds nor does it mention the PIC's VBUS input current.




Then go back and figure out what "unit load" means.
 

Do not use my alias in your message body when replying, your message will disappear ...

Alan
#4
Jerry Messina
Super Member
  • Total Posts : 550
  • Reward points : 0
  • Joined: 2003/11/07 12:35:12
  • Status: offline
Re: Questions about the VBUS voltage comparator 2020/07/31 12:29:53 (permalink)
5 (1)
The "session valid" and "session end level" comparator levels are part of the USB OTG specification
 
#5
jtron
Starting Member
  • Total Posts : 36
  • Reward points : 0
  • Status: offline
Re: Questions about the VBUS voltage comparator 2020/07/31 13:43:02 (permalink)
0
Jerry Messina
The "session valid" and "session end level" comparator levels are part of the USB OTG specification

 
Thanks, Jerry. I found various definitions in the OTG spec for "session valid" and "session end" voltage thresholds, but I'm not sure which ones are relevant to a dsPIC33 operating purely in device (vs. OTG) mode. It seems that "session end" is probably 0.8 V, and "session valid" is either 0.8, 2.0, 2.1 or 4.0 V. Do you know which of these applies to my situation (dsPIC33 in device mode)? Also, do you have any idea where I might find a spec for VBUS input current?
#6
Jim Nickerson
User 452
  • Total Posts : 6736
  • Reward points : 0
  • Joined: 2003/11/07 12:35:10
  • Location: San Diego, CA
  • Status: offline
Re: Questions about the VBUS voltage comparator 2020/08/03 09:27:10 (permalink)
0
... there seems to be a missing post.
 
#7
jtron
Starting Member
  • Total Posts : 36
  • Reward points : 0
  • Status: offline
Re: Questions about the VBUS voltage comparator 2020/08/03 09:39:03 (permalink)
4 (1)
Hi Jim. Yes, I've tried multiple times to respond to an earlier post, but for some reason none of my replies appear. Oddly, the forum software tallies my replies even though it won't show them. The original post says:
 
Then go back and figure out what "unit load" means.
 
To which I replied (or attempted to reply, anyway):
 
It's not clear to me how USB "unit load" is related to the PIC's voltage comparator thresholds. Can you please elaborate?
 
I have no idea why the forum software keeps deleting my replies and edits. I hope this one gets through.
#8
Jim Nickerson
User 452
  • Total Posts : 6736
  • Reward points : 0
  • Joined: 2003/11/07 12:35:10
  • Location: San Diego, CA
  • Status: offline
Re: Questions about the VBUS voltage comparator 2020/08/03 10:02:36 (permalink)
3 (2)
I suspect this thread by Ian may help in your failure to post.  https://www.microchip.com/forums/FindPost/777101
When a post has been blocked the thread is not able to be marked as read and always shows up as a new post.
Also a missing post at the end of a page may cause "blinking" ( reloading of the post over and over ).
#9
jtron
Starting Member
  • Total Posts : 36
  • Reward points : 0
  • Status: offline
Re: Questions about the VBUS voltage comparator 2020/08/03 10:13:40 (permalink)
5 (1)
Thanks for the tip, Jim. Unfortunately I don't see anything there that explains my post rejections -- my posts had no URLs or markup of any kind, though I did try quoting another editor's post. Perhaps it has something to do with that editors "signature" line, which says "Do not use my alias in your message body when replying, your message will disappear ..."
#10
Jim Nickerson
User 452
  • Total Posts : 6736
  • Reward points : 0
  • Joined: 2003/11/07 12:35:10
  • Location: San Diego, CA
  • Status: offline
Re: Questions about the VBUS voltage comparator 2020/08/03 10:22:59 (permalink)
5 (2)
LoL: LoLLoL: LoLLoL: LoL perhaps.
#11
jtron
Starting Member
  • Total Posts : 36
  • Reward points : 0
  • Status: offline
Re: Questions about the VBUS voltage comparator 2020/08/03 10:30:38 (permalink)
3 (2)
So how can I reply to an editor without using their alias? And why does the forum software drop my posts when I address an editor via their alias? I need to do this because my reply doesn't immediately follow the related post, and I can't think of another way to associate my reply with the related post. Grrrr!
#12
Jim Nickerson
User 452
  • Total Posts : 6736
  • Reward points : 0
  • Joined: 2003/11/07 12:35:10
  • Location: San Diego, CA
  • Status: offline
Re: Questions about the VBUS voltage comparator 2020/08/03 10:32:14 (permalink)
5 (1)
Maybe you could edit his alias by inserting spaces between each character ?
#13
du00000001
Just Some Member
  • Total Posts : 3946
  • Reward points : 0
  • Joined: 2016/05/03 13:52:42
  • Location: Germany
  • Status: offline
Re: Questions about the VBUS voltage comparator 2020/08/03 10:33:45 (permalink) ☄ Helpfulby Jim Nickerson 2020/08/03 10:34:54
5 (3)
jtron
Thanks for the tip, Jim. Unfortunately I don't see anything there that explains my post rejections -- my posts had no URLs or markup of any kind, though I did try quoting another editor's post. Perhaps it has something to do with that editors "signature" line, which says "Do not use my alias in your message body when replying, your message will disappear ..."

 
Let me guess: you tried to quote   A n t i p o d e a n 's  message, although his footnote clearly tells that this is not possible (See my "garbling" of his user name. A dash like in Anti-podean does the same trick.)
If you ignore his footnote, your posts are accepted but display is suppressed. Repeat this multiple times and the thread display starts behaving more than "weird".

PEBKAC / EBKAC / POBCAK / PICNIC (eventually see en.wikipedia.org)
#14
Jim Nickerson
User 452
  • Total Posts : 6736
  • Reward points : 0
  • Joined: 2003/11/07 12:35:10
  • Location: San Diego, CA
  • Status: offline
Re: Questions about the VBUS voltage comparator 2020/08/03 10:35:46 (permalink)
5 (1)
As evidenced by the blinking page now in this thread Smile: Smile
#15
du00000001
Just Some Member
  • Total Posts : 3946
  • Reward points : 0
  • Joined: 2016/05/03 13:52:42
  • Location: Germany
  • Status: offline
Re: Questions about the VBUS voltage comparator 2020/08/03 10:37:40 (permalink)
0
We've got 15 message displaying, but 21 message posted (both numbers prior THIS post).

PEBKAC / EBKAC / POBCAK / PICNIC (eventually see en.wikipedia.org)
#16
jtron
Starting Member
  • Total Posts : 36
  • Reward points : 0
  • Status: offline
Re: Questions about the VBUS voltage comparator 2020/08/03 10:45:05 (permalink)
3 (2)
Well it's probably moot at this point because I'm fairly confident that the editor has no idea how to answer my questions. What's really frustrating (aside from forum behavior) is that no one (not even at Microchip, apparently) seems able to answer questions about a few essential specifications that are missing from the documentation.
post edited by jtron - 2020/08/03 10:47:12
#17
du00000001
Just Some Member
  • Total Posts : 3946
  • Reward points : 0
  • Joined: 2016/05/03 13:52:42
  • Location: Germany
  • Status: offline
Re: Questions about the VBUS voltage comparator 2020/08/03 11:52:03 (permalink)
5 (1)
jtron
Well it's probably moot at this point because I'm fairly confident that the editor has no idea how to answer my questions. What's really frustrating (aside from forum behavior) is that no one (not even at Microchip, apparently) seems able to answer questions about a few essential specifications that are missing from the documentation.



The datasheet is simply referencing the standard, so what beyond that are they expected to be obliged ?
(OK - they could do more. But considering how their business is humming: why "waste" capacity on some very-small size customers ?
(Some years ago I've brought a PIC16Fxxxx design to fruition that's now falling off the conveyor at maybe 10.000.000 (in words: ten millions) units/year. Created with some of the latest PIC16s, this would be a gross worth of no more than 1.500.000 USD. Still a small customer...)

PEBKAC / EBKAC / POBCAK / PICNIC (eventually see en.wikipedia.org)
#18
Antipodean
Super Member
  • Total Posts : 1908
  • Reward points : 0
  • Joined: 2008/12/09 10:19:08
  • Location: Didcot, United Kingdom
  • Status: offline
Re: Questions about the VBUS voltage comparator 2020/08/03 12:40:36 (permalink)
4 (3)
jtron
Thanks for the tip, Jim. Unfortunately I don't see anything there that explains my post rejections -- my posts had no URLs or markup of any kind, though I did try quoting another editor's post. Perhaps it has something to do with that editors "signature" line, which says "Do not use my alias in your message body when replying, your message will disappear ..."



Yep, the reason my posts disappear is given in that thread. Careful reading required.
 
Coming back to your original question, whatever you do for your isolator, to meet the USB spec it HAS to supply a minimum of 100mA, i.e. one unit load. So the current input of the Vbus comparator is insignificant.
 
If you don't arrange for such a supply current then there will be insufficient current to supply the D+/D- line drivers (which I agree should take considerably less than 100mA, but I would expect anywhere up to 20mA). This is irrespective of the chip being self powered.

Do not use my alias in your message body when replying, your message will disappear ...

Alan
#19
Antipodean
Super Member
  • Total Posts : 1908
  • Reward points : 0
  • Joined: 2008/12/09 10:19:08
  • Location: Didcot, United Kingdom
  • Status: offline
Re: Questions about the VBUS voltage comparator 2020/08/03 12:45:20 (permalink)
5 (1)
jtron
Well it's probably moot at this point because I'm fairly confident that the editor has no idea how to answer my questions. What's really frustrating (aside from forum behavior) is that no one (not even at Microchip, apparently) seems able to answer questions about a few essential specifications that are missing from the documentation.


I'll answer this one to stop the next page from flashing ... well, it doesn't, it needs someone else to post as well, then it will stop flashing.


It comes down to reading the USB standard correctly. I haven't tried using OTG, only as a device, in which case the thresholds you state are not applicable. As you are also using it as a device they won't be applicable to you either.
 
post edited by Antipodean - 2020/08/03 12:46:33

Do not use my alias in your message body when replying, your message will disappear ...

Alan
#20
Page: 12 > Showing page 1 of 2
Jump to:
© 2020 APG vNext Commercial Version 4.5