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The USB standard defines a suspend mode. When the host computer goes into sleep mode, it requests that all USB devices go into a low power suspend mode. Suspend mode is signaled to the devices by the absence of any USB activity.
V-USB does not implement suspend mode by itself. This is the task of the main application. However, it offers hooks to check for USB activity. Since the only USB activity seen may be the frame pulse on DATA–, this data line must be connected to an interrupt. The easiest way to do this is to use DATA– for USB interrupts (not DATA+ as usual). Then define USB_COUNT_SOF to 1 in usbconfig.h and watch the global variable usbSofCount in your main loop. If it stops incrementing, you should put the device into suspend mode and wait for activity on the USB.
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