Why Does the Logic Analyzer Report Different USB Current Requirements when Not Connected to the Logic Software?

All Saleae devices are powered from USB.

When first connected, before the Saleae Logic software has connected to the device, the device will stay in an uninitialized state. In that state, the interface descriptor will report a lower USB current requirement.

Once our software detects the device and causes it to re-enumerate, the interface descriptor will report a higher USB current, the actual worst case current the device could draw. We've rounded up these numbers and added extra margin.

This reported current requirement is only used by the operating system to determine if the device can operate through bus-powered USB hubs with other devices attached.

For instance, over USB 2.0, 500 mA is available from each port on the PC.

If a bus-powered USB hub is attached, then all devices connected to that hub and the hub itself can only draw up to 500 mA. The USB current reported by the interface descriptor is used to calculate the total current used by a USB hub and all downstream devices. If that total current exceeds the upstream port's limit, the operating system will inform the user that not enough power is available.

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