
EZ-USB FX2 Technical Reference Manual
Page 8-2 EZ-USB FX2 Technical Reference Manual v2.1
control of the FIFO interfaces described in Chapters 9 and 10, the CPU can access the large end-
points if necessary.
8.3 High-Speed and Full-Speed Differences
FX2 operates at both full speed (12 Mbps) and high speed (480 Mbps). The data-payload-size and
transfer-speed requirements differ between the two modes. FX2 architecture is optimized for high
speed transfers:
• Instead of many small endpoint buffers, FX2 provides a reduced number of large buffers.
• FX2 provides double, triple or quad buffering on its large endpoints (EP2, 4, 6, and 8).
•
The CPU need not participate in high-bandwidth transfers.
Instead, dedicated FX2 logic
and unified endpoint/interface FIFOs move data on and off the chip at USB 2.0 rates with-
out any CPU intervention.
FX2 endpoint buffers appear to have different sizes depending on whether the FX2 is operating at
full or high speed. This is due to the difference in maximum packet sizes allowed by the USB spec-
ification for the two modes, as illustrated by Table 8-1.
Although the EP2, EP4, EP6 and EP8 buffers are physically large, they appear as smaller buffers
when the FX2 is operating at full speed to account for the smaller maximum packet sizes.
When operating at high speed, firmware can configure the large endpoints’ size, type, and buffer-
ing; when operating at full speed, type and buffering are configurable but the maximum packet
size is always fixed at 64 bytes for the non-isochronous types.
Table 8-1. Maximum Packet Sizes for USB 1.1 and 2.0
Transfer Type Max Packet Size
USB 1.1 USB 2.0
CONTROL (EP0 only) 8,16,32,64 64
BULK 8,16,32,64 512
INTERRUPT 1-64 1-1024
ISOCHRONOUS 1-1023 1-1024
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