Test and Measurement Forum

USB serial decoding useless?

Post any questions you may have about our current range of oscilloscopes

USB serial decoding useless?

Postby admel » Tue Dec 15, 2015 2:30 pm

Using PicoScope 2206A I tried to analyze a USB v1.1 bus at 12 MHz with USB serial decode. All I can see is Start-Of-Frame (SOF) packets and occasionally a TOKEN IN and NACK packet, but the packets in which I am interested do not appear.

Does the 2206A take samples at random time intervals and skip large time intervals in between? How can one seriously decode a USB protocol?

In the ideal case I would expect that the scope samples continuously, decodes all samples to packets and displays only the filtered packets, is this possible with the 2206A?
If yes, which settings shall I use?
If no, is there a PicoScope that can do that? Which?

Thanks.
admel
Newbie
 
Posts: 0
Joined: Tue Dec 15, 2015 2:18 pm

Re: USB serial decoding useless?

Postby Bilal » Fri Dec 18, 2015 3:20 pm

Hi,

I have ran some tests with your device and unfortunately it turns out it does not possess sufficient buffer memory to capture USB 12 Mbps data rates but should be capable of 1.5 Mbps. I managed to decode USB 12 Mbps using a PicoScope 3204D which was perfectly capable of doing so. I haven't been able to test the 3203D as yet and will confirm this for you.
Bilal Patel
Technical Support Engineer
Pico Technology
Bilal
PICO STAFF
PICO STAFF
 
Posts: 33
Joined: Mon Oct 05, 2015 12:19 pm

Re: USB serial decoding useless?

Postby Bilal » Mon Dec 21, 2015 12:50 pm

Hi,

I have managed to test the 3203D which is the cheapest of the 3000 series devices and it is capable of decoding USB 12 Mbps.

If you require any further information, please feel free to contact us.
Bilal Patel
Technical Support Engineer
Pico Technology
Bilal
PICO STAFF
PICO STAFF
 
Posts: 33
Joined: Mon Oct 05, 2015 12:19 pm

Re: USB serial decoding useless?

Postby pietrzak » Tue Feb 23, 2016 12:37 pm

Hi,

I have similar issues. My picoscope model is 3404D MSO. I am wondering if my wiring is wrong. I tried both on A/B channels and digital inputs. With digital inputs I get nothing, with A/B channels I get something similar to the first post.

I stripped an USB cable and I plugged the picoscope channels to the white and green wires. I tried to plug the picoscope ground either to the shield or black wire. Which one is the best? The signal I observe when using channels A/B is not very square.

What sampling parameters should I use? What voltage thresholds should I use in the serial decoding parameters? I notived the voltage is around 3V, not 5V.

Thanks,
pietrzak
Newbie
 
Posts: 0
Joined: Tue Feb 23, 2016 10:16 am

Re: USB serial decoding useless?

Postby Bilal » Wed Feb 24, 2016 1:37 pm

Hello Pietrzak,

USB uses differential between the D+(Green) and D-(White) lines. You will need to use two probes:
Connect channel A to D+ with probe ground lead to Ground(Black) and
connect channel B to D- with probe ground lead to Black.
Both probes should be switched to the X10 mode and X10 selected under the 'channel options' in the software for each channel.

In order to pick up communication the host computer and device you are monitoring have to be actively communicating with each other for example if you want to monitor a USB pen drive, you will need to do it whilst it is transferring a file. You will not pick up anything from a wire connected to a computer with no device attached/communicating.

While the device and computer are communicating, you should press the Auto-setup button to set up the ranges of the device. The signal should be around 3.3 V and channel B will be an inverse of channel A. If you do not see 3.3 V on the data lines, you may need to probe at a different point.

Set the Trigger mode to 'Auto' and then set up the serial decoder with DataPlus: A and DataMinus: B. The thresholds will be automatically set up if the voltage levels of the D+ and D- are being displayed correctly.

You can change to a shorter collection time such as 100us/div to see the packets more clearly.

Hope this solves the problem.
Bilal Patel
Technical Support Engineer
Pico Technology
Bilal
PICO STAFF
PICO STAFF
 
Posts: 33
Joined: Mon Oct 05, 2015 12:19 pm


Return to USB PC Oscilloscopes

Who is online

Users browsing this forum: No registered users and 0 guests