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I'm trying to implement an audio DAC using PCM2707 as a receiver and, looking over a couple schematics of previous implementations - at least one being verified as correct, I noticed D+ being connected to VDD through a 1.5k resistor. Considering I trust the source of the schematics, I take this to be correct.

But why is implemented that way? I'd really want to understand how it works... and Google has not helped me so far.

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It is part of how USB negotiates the speed at which the host and device plan to communicate, as well as allowing for presence detect of a device being plugged in. Basically, the host has a 15K pull-down on both D+/D-, and the device has a 1.5K pull-up on one of the 2 lines to indicate full or low-speed. You can read more at the following link: USB in a NutShell.

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