Using XTAG-3 via USB isolator

Technical questions regarding the XTC tools and programming with XMOS.
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CousinItt
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Using XTAG-3 via USB isolator

Post by CousinItt »

Hi all,

I'm trying to program an xcore-200 explorerkit via the xtag-3 adapter using a USB isolator. This is in preparation for a related project that I hope will be able to use the same isolator.

Windows sees the xtag-3 and will install its driver when the isolator is plugged into a new USB port, but xrun -l says that no devices are available. I've tried various USB ports, and various cables to and from the isolator and the problem persists.

The set-up works fine if the isolator isn't there. The PC is running Windows 7 64-bit, but there are no USB3 ports.

Does the xtag-3 have to use high speed (480 Mbps) USB comms? The isolator is limited to full speed (12 Mbps), so would that explain why it's not working?

Please let me know as it'll save me a lot of pain. Thanks.


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mon2
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Post by mon2 »

Hello

Image

Not an audio developer but believe your issue is the requirement for HIGH SPEED support for this USB device. The USB enumeration occurs at the low speed. The issue must be surfacing in the need for HIGH SPEED mode. However be sure that ample current is being supplied on the secondary side of the isolator.

A proper isolator design will feature an isolated DC-DC power source. Does the Vbus on the secondary (isolator) side remain @ 5 volts?

We are aware of the LOW / FULL speed USB isolator from Analog and also the Murata USB Isolator module. Have you tested the LOW and HIGH speed settings of these parts? Believe that the Analog isolator requires to be logic selected for the respective speed.

Consider to isolate your circuits at a different stage of your design. For example, why not isolate at the interface pins on the XTAG3 and the XMOS processor? As long as the pins are unidirectional, you should be fine to use a slew of different digital isolators. We prefer Silabs but there are others. Do review the speeds of such parts and you will need to confirm if the jitter of such devices is acceptable for your project. There are assorted posts about using such low / full speed isolators for audio with XMOS processors. That is, while this XTAG-3 tool may not function with the same USB isolator, the isolator concept may still be ok for the end product using USB if the HIGH speed support is not required.

If you must have HIGH speed USB isolator in your design, then there is:

http://www.prweb.com/releases/2015/10/prweb12995871.htm

Lewis Boore
V.P., WW Sales
Silanna Semiconductor, Inc.
4795 Eastgate Mall | Suite 100 | San Diego, CA 92121
(O) +1 858.373.0452
(C) +1 619.227.7953
(skype) lewboore
lboore@iosemi.com
lboore@silanna.com
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CousinItt
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Post by CousinItt »

Hi Lewis, thanks for the quick reply. The question is whether the xtag is used in high speed mode. I imagine there are USB debugging tools that would tell me but I thought it would be quicker to ask.

On the target side of my setup, the xtag is powered via a link to the 5V on the explorer kit. This then feeds the supply on the secondary side of the USB isolator via the USB cable. The primary side is powered via the PC.

I'm using the Analog ADUM4160. It's set up for full speed (12 Mbps) operation. I haven't tried low speed operation with these devices.

The reason I inserted the isolation at the USB interface is that it's the simplest and cleanest way to deal with signal and power isolation. The xtag can be powered via pin 1 of the jtag connector (not connected on the explorer kit) and should be able to power the secondary side of the USB interface so there's then no need for an additional supply converter just for the isolator. I also want to use the xscope interface on the xtag so that adds a few signals with (I expect) tight timing constraints.

Thanks for the tip on your isolators. I'll take a look.
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CousinItt
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Post by CousinItt »

Well, no proof, but pretty strong circumstantial evidence that the xtag uses high speed mode.

1) My full speed isolator works fine with other devices.

2) For a test scenario of connecting the debugger, and running xrun -l, the traffic shown by USBpcap/wireshark is consistent initially between the isolated and non-isolated cases, but then data traffic is missing from the isolated case. (The isolator filters out the chirps for setting up high speed mode).

3) The XS1-U8A used in the xtag is fully USB 2.0 conformant, so there's no reason why it shouldn't use high speed mode.

I think the way forward for me is to move the isolation barrier to the JTAG side. If there are any problems I can use the --jtag-speed setting to slow it down. If I need to dispense with xscope in the final application, so be it.
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