
EZ-USB FX2 Technical Reference Manual
Page 1-6 EZ-USB FX2 Technical Reference Manual v2.1
1.8 USB Transfer Types
USB defines four transfer types. These match the requirements of different data types delivered
over the bus.
1.8.1 Bulk Transfers
Figure 1-2. Two Bulk Transfers, IN and OUT
Bulk data is bursty, traveling in packets of 8, 16, 32 or 64 bytes at full speed or 512 bytes at high
speed. Bulk data has guaranteed accuracy, due to an automatic retry mechanism for erroneous
data. The host schedules bulk packets when there is available bus time. Bulk transfers are typi-
cally used for printer, scanner, or modem data. Bulk data has built-in flow control provided by
handshake packets.
1.8.2 Interrupt Transfers
Figure 1-3. An Interrupt Transfer
Interrupt data is like bulk data; it can have packet sizes of 1 through 64 bytes at full speed or up to
1024 bytes at high speed. Interrupt endpoints have an associated polling interval that ensures
they will be polled (receive an IN token) by the host on a regular basis.
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Data Packet
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