TM
Software – General Concepts
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
Start of Frame
Transfer 2
Transaction 1
Packet 1
Packet 2
Packet 3
Transaction 2
Packet 1
Packet 2
Packet 3
Transaction 3
Packet 1
Packet 2
Packet 3
Transfer 3
Transaction 1
Packet 1
Packet 2
Packet 3
1500 bytes / 1 Ms
The Enumeration Process:
What happens when a device is connected?