Cypress Semiconductor FX2LP Información técnica Pagina 79

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Chapter 3. Enumeration and ReNumeration™ Page 3-1
Chapter 3 Enumeration and ReNumeration™
3.1 Introduction
The FX2’s configuration is soft: Code and data are stored in internal RAM, which can be loaded
from the host over the USB interface. FX2-based USB peripherals can operate without ROM,
EPROM, or FLASH memory, shortening production lead times and making firmware updates
extremely simple.
To support this soft configuration, the FX2 is capable of enumerating as a USB device without firm-
ware
. This automatically-enumerated USB device (the
Default USB Device)
contains a set of inter-
faces and endpoints and can accept firmware downloaded from the host.
Two separate Default USB Devices actually exist, one for enumeration as a full speed (12 Mbits/
sec) device, and the other for enumeration as a high speed (480 Mbits/sec) device. The FX2 auto-
matically performs the speed-detect protocol and chooses the proper Default USB Device. The two
sets of Default USB Device descriptors are shown in Appendices A and B.
Once the Default USB Device enumerates, it downloads firmware and descriptor tables from the
host into the FX2’s on-chip RAM. The FX2 then begins executing the downloaded code, which
electrically simulates a physical disconnect/connect from the USB and causes the FX2 to enumer-
ate again as a second device, this time taking on the USB personality defined by the downloaded
code and descriptors. This patented secondary enumeration process is called “ReNumeration™.”
An FX2 register bit called RENUM controls whether device requests over endpoint zero are han-
dled by firmware or automatically by the Default USB Device. When RENUM=0, the Default USB
Device handles the requests automatically; when RENUM=1, they must be handled by firmware.
3.2 FX2 Startup Modes
When the FX2 comes out of reset, it can act in various ways to establish itself as a USB device.
FX2 power-on behavior depends on several factors:
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