Cypress Semiconductor FX2LP Informacje Techniczne Pobierz pdf (Strona 80)

EZ-USB FX2 Technical Reference Manual

Page 3-2 EZ-USB FX2 Technical Reference Manual v2.1

1. If no off-chip memory (either on the I²C-compatible bus or on the address/data bus) is con-

nected to the FX2, it enumerates as the Default USB Device, with descriptors and VID / PID /

DID supplied by hardwired internal logic (Table 3-3). RENUM is set to 0, indicating that the

Default USB Device automatically handles device requests.

2. If an EEPROM containing custom VID / PID / DID values is attached to the FX2’s SCL and

SDA pins, FX2 also enumerates as the Default USB Device as above, but it substitutes the

VID / PID / DID values from the EEPROM for its internal values. The EEPROM must contain

the value 0xC0 in its first byte to indicate this mode to FX2, so this mode is called a “C0 Load”.

As above, RENUM is automatically set to 0, indicating that the Default USB Device automati-

cally handles device requests. A 16-byte EEPROM is sufficiently large for a C0 Load.

3. If an EEPROM containing FX2 firmware is attached to the SCL and SDA pins, the firmware is

automatically loaded from the EEPROM into the FX2’s on-chip RAM, and then the CPU is

taken out of reset to execute this boot-loaded code. In this case, the VID / PID / DID values

are encapsulated in the firmware; the RENUM bit is automatically set to 1 to indicate that the

firmware, not the Default USB Device, handles device requests. The EEPROM must contain

the value 0xC2 in its first byte to indicate this mode to FX2, so this mode is called a “C2 Load”.

Although the FX2 can perform C2 Loads from EEPROMs as large as 64KB, code can only be

downloaded to the 8K of on-chip RAM.

4. If a Flash, EPROM, or other memory is attached to the FX2’s address/data bus (128-pin pack-

age only) and a properly formatted EEPROM meeting the requirements above is not present,

and the EA pin is tied high (indicating that the FX2 starts code execution at 0x0000 from off-

chip memory), the FX2 begins executing firmware from the off-chip memory. In this case, the

VID / PID / DID values are encapsulated in the firmware; the RENUM bit is automatically set to

1 to indicate that the firmware, not internal FX2 logic, handles device requests.

Case (2) is the most frequently used mode when soft operation is desired, since the VID/PID val-

ues from EEPROM always bind the device to the appropriate host driver while allowing FX2 firm-

ware to be easily updated. In this case, the host first uses the FX2 Default USB Device to

download firmware, then the host takes the CPU out of reset so that it can execute the down-

loaded code. Section 3.8, "FX2 Vendor Request for Firmware Load" describes the USB Vendor

Request that the FX2 supports for code download and upload.

The Default USB Device is fully characterized in Appendices A and B, which list the built-in FX2

descriptor tables for full-speed and high-speed enumeration, respectively. Studying these Appen-

dices in conjunction with Tables 3-1 and 3-2 is an excellent way to learn the structure of USB

descriptors.

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