![]() ![]() Have a USB 1.1 hub, as suggested by duskwuff.There are three options to validate a HS device ability to run on FS mode: ![]() That's why a simple impeding of HS data rate doesn't work. Since the chirping comes at much lower switching rate (~ 10MHz), you can't suppress it without harming the FS data signaling mode. The problem in your case is that suppressing signal quality at HS mode doesn't remove the chirping sequencing. The host does recognize the Chirp-K, and after a special chirping sequence, both Host ans Device recognize the HS mode. A HS device must always do this, you can't avoid this signal phase. Then, if a device is HS, it pulls "Chirp-K" signal on D- wire. The host, seeing the connect, initiates "USB_RESET" state, dragging both D+ and D- to ground with 45-Ohm driver. Any HS device signals the connect event by pulling up D+ data line just as a FS device would do. The issue is that the FS mode is negotiated during attach/port reset event. The only way to force full speed is to use a full speed hub (USB 1.1), which are very difficult to find nowadays. 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. If a high speed device does not respond as expected the device enumeration will simply fail. How do I force the device to run at Full speed?ĩ month after asking this question (and being more expert in USB stuff), I know today that USB does not have a fall-back mechanism. Either High speed or Low speed or nothing. The mainboard has USB2.0 hubs and USB3.0 hubs. ![]() 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) The transfer speed is 20 Megabyte/s which is too fast for Full speed. How did I prove that? I connect a USB stick and transfer a big file. When I connect 3.3pF, 6.8pF or 12pF the device will work with High speed. I opened a USB cable and soldered a capacitor between Data+ and Data- like that:īut although I tried several values for the capacitor it did not work. I have to test how a High speed USB device behaves when it runs at Full speed.Īs 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. ![]()
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February 2023
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