Freescale Semiconductor TWR-MCF52259-Ethenet - Manual (Page 109)

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The General USB Process
5. The available bandwidth is then divided into frames, and the
host controls those frames which contain 1,500 bytes. Every
millisecond a new frame begins. Within the frame, slots are
reserved for isochronous and interrupt devices (up to 90%) so
that they are guaranteed the bandwidth they need. Bulk and
control transfers use whatever space is left.
Frame
Transfer 1
Transaction
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Transfer 2
Transaction 1
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t 3
Transaction 2
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Transaction 3
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Transfer 3
Transaction 1
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1500 bytes / 1 Ms
The Enumeration Process:
What happens when a device is connected?