
Chapter 1. Introducing EZ-USB FX2 Page 1-7
1.8.3 Isochronous Transfers
Figure 1-4. An Isochronous Transfer
Isochronous data is time-critical and used to stream data like audio and video. An isochronous
packet may contain up to 1023 bytes at full speed, or up to 1024 bytes at high speed.
Time of delivery is the most important requirement for isochronous data. In every USB frame, a
certain amount of USB bandwidth is allocated to isochronous transfers. To lighten the overhead,
isochronous transfers have no handshake (ACK/NAK/STALL/NYET), and no retries; error detec-
tion is limited to a 16-bit CRC.
Isochronous transfers do not use the data-toggle mechanism. Full-speed isochronous data uses
only the DATA0 PID; high-speed isochronous data uses DATA0, DATA1, DATA2 and MDATA.
In full-speed mode, only one isochronous packet can be transferred per endpoint, per frame. In
high-speed mode, up to three isochronous packets can be transferred per endpoint, per microf-
rame.
1.8.4 Control Transfers
Figure 1-5. A Control Transfer
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SETUP
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DATA
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(optional)
STATUS
Stage
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