1.8.3 Isochronous Transfers
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Payload
Data
6
Token Packet
Data Packet
Figure 1-6. An Isochronous Transfer
Isochronous data is time-critical and used for streaming data like audio and video. 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), and no retries.
Error detection is limited to a 16-bit CRC. Isochronous transfers do not use the data tog-
gle mechanism; isochronous data uses only the DATA0 PID.
1.8.4 Control Transfers
S
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8 bytes
Setup
Data
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K
SETUP
Stage
6
Token Packet
Data Packet
H/S Pkt
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P
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DATA
Stage
(optional)
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Payload
Data
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Data Packet
C
Token Packet
H/S Pkt
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O
U
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STATUS
Stage
6
Token Packet Data Pkt
H/S Pkt
Figure 1-7. A Control Transfer
Control transfers are used to configure and send commands to a device. Being mission
critical, they employ the most extensive error checking USB offers. Control transfers are
delivered on a best effort basis by the host (best effort is defined by a six-step process in
the Universal Serial Bus Specification Version 1.1, “Section 5.5.4”). The host reserves a
part of each USB frame time for Control transfers.
Page 1-8
Chapter 1. Introducing EZ-USB
EZ-USB TRM v1.9
CYPRESS [ CYPRESS ]
