Force USB device to run in Full Speed mode

Question

I have to test how a High speed USB device behaves when it runs at Full speed.

As High speed runs at 480 MHz and Full speed at 12 MHz I thought this would be easy with a "low pass filter" that does not allow the high frequencies to pass and so the host would automatically negociate a slower speed with the device.

I opened a USB cable and soldered a capacitor between Data+ and Data- like that:

But although I tried several values for the capacitor it did not work.

When I connect 3.3pF, 6.8pF or 12pF the device will work with High speed. How did I prove that? I connect a USB stick and transfer a big file. The transfer speed is 20 Megabyte/s which is too fast for Full speed.

When I increase the capacitor to 15pF or more I suddenly see the device working with Low speed (1.5MHz) or even failing completely (device enumeration failed)

I tried this on two different computers. The mainboard has USB2.0 hubs and USB3.0 hubs. No difference in bahaviour. I tried on Windows XP and Windows 7.

There is no way get Full speed. Either High speed or Low speed or nothing.

Can anybody explain me that? How do I force the device to run at Full speed?

UPDATE: 9 month after asking this question (and being more expert in USB stuff), I know today that USB does not have a fall-back mechanism. If a high speed device does not respond as expected the device enumeration will simply fail. The computer does not try to communicate with a high speed device in full speed after a high speed negociation has failed and neither does the device fall back to full speed mode. The only way to force full speed is to use a full speed hub (USB 1.1), which are very difficult to find nowadays.

Answer 1

The issue is that the FS mode is negotiated during attach/port reset event. Any HS device signals the connect event by pulling up D+ data line just as a FS device would do. The host, seeing the connect, initiates "USB_RESET" state, dragging both D+ and D- to ground with 45-Ohm driver. Then, if a device is HS, it pulls "Chirp-K" signal on D- wire. A HS device must always do this, you can't avoid this signal phase.

The host does recognize the Chirp-K, and after a special chirping sequence, both Host and Device recognize the HS mode. You can find more details HERE.

The problem in your case is that suppressing signal quality at HS mode doesn't remove the chirping sequencing. Since the chirping comes at much lower switching rate (~ 10MHz), you can't suppress it without harming the FS data signaling mode. That's why a simple impeding of HS data rate doesn't work.

There are three options to validate a HS device ability to run on FS mode:

1. Have a USB 1.1 hub, as suggested by duskwuff;
2. Get a USB host that supports only USB 1.1 (Arduino flavours, Cypress, whatever)
3. Get a normal (older) PC that is built around Intel EHCI controller, and disable the EHCI part of it in Device Manager. The remaining UHCI controller (or OHCI in AMD line) will provide the FS functionality.

CORRECTION: The chirp sequence J-K-J-K-... is specified to have pulses between 40us and 60us, see Section 7.1.7.5. of USB 2.0 specifications. All designs have it in the center, 50 us. Which makes the chirping frequency at 10 kHz, not 10 MHz, my bad. This frequency disparity makes it impossible to suppress chirps by filtering given the FS signaling rate is 6 MHz.

Answer 2

Attach the device to a USB 1.1 hub. They're still commercially available.

Answer 3

There are also USB isolators out there that will block the initial hand shake:

If the peripheral port happens to be high-speed-capable, then it sends a high-speed “chirp” pattern during enumeration. This would normally initiate negotiations for high-speed operation, but the ADuM4160 blocks the chirp signal and automatically forces the high-speed peripheral to operate at full speed.

You can find them here.

Also Adafruit sells breakout boards here based on ADuM4160 Full/Low Speed USB Digital Isolator (and ADuM5000 Isolated DC/DC Converter).