Cypress Semiconductor FX2LP Información técnica Pagina 87

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Chapter 3. Enumeration and ReNumeration™ Page 3-9
3.6 The RENUM Bit
An FX2 control bit called “RENUM” (ReNumerated) determines whether USB device requests over
endpoint zero are handled by the Default USB Device or by FX2 firmware. At power-on reset, the
RENUM bit (USBCS.1) is zero, indicating that the Default USB Device will automatically handle
USB device requests. Once firmware has been downloaded to the FX2 and the CPU is running, it
can set RENUM=1 so that subsequent device requests will be handled by the downloaded firm-
ware and descriptor tables.
Chapter 2, "Endpoint Zero"
describes how the firmware handles device
requests while RENUM=1.
If a 128-pin FX2 is using off-chip code memory at 0x0000 and there is no boot EEPROM to supply
a custom Vendor ID and Product ID, the FX2 automatically sets the RENUM bit to 1 so that device
requests are always handled by the firmware and descriptor tables in the off-chip memory. The
FX2 also sets RENUM=1 after a “C2 load” if the EA pin is low. In this case, firmware execution
begins in internal RAM using the code loaded from the EEPROM, with the firmware handling all
USB requests.
Another Use for the Default USB Device
The Default USB Device is established at power-on to set up a USB device capable of down-
loading firmware into the FX2’s RAM. Another useful feature of the Default USB Device is
that FX2 code can be written to support the already-configured generic USB device. Before
bringing the CPU out of reset, the FX2 automatically enables certain endpoints and reports
them to the host via descriptors. By utilizing the Default USB Device (i.e., by keeping
RENUM=0), the firmware can, with very little code, perform meaningful USB transfers that
use these pre-configured endpoints. This accelerates the USB learning curve.
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