TUSB6250 datasheet(64/124 Pages) TI

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8−23

SLLS535A − January 2004

TUSB6250

8.10.16 OEPBCNHY_n: Output Endpoint Y-Buffer Byte Count High-Byte Register (n = 1 to 4) (XDATA at

F01F, F02F, F03F, F04F)

The OEPBCNHY_n register contains the NAK bit and the higher 3-bit value in Y buffer that is used to specify

the amount of data received in a data packet from the USB host. See the procedure described in the

OEPBCNHX_n register when using the NAK bit for flow control handshake.

765

43210

NAK

RSV

C10

R/W

R/O

BIT

NAME

RESET

FUNCTION

2−0

C [10:8]

000

Y-buffer byte count higher 3 bits. These bits, in combination with C[7:0], provide the byte count of a

given transaction (count = 0 to 2047).

6−3

RSV

Reserved = 0

NAK

NAK bit is used as flow control handshake for the Y buffer.

NAK = 0 Buffer is empty and ready for a Host-OUT request.

NAK = 1 Buffer contains a valid packet from the host (will NAK Host-OUT request)

8.11 Serial Number Registers

The USB 2.0 specification encourages end-product vendors to support the unique USB device serial number.

The USB Mass Storage Cass—Bulk Only Transport specification also requires that the serial number shall

contain at least 12 valid digits, represented as a UNICODE string and the last 12 digits of the serial number

shall be unique to each USB idVendor and idProduct pair.

The TUSB6250 supports the unique USB device serial numbers. The serial numbers can be either specified

by the end product developer in the header block of the external I2C EEPROM with the custom format or

generated automatically by the TUSB6250 from its 48-bit on-chip unique die id number.

Each TUSB6250 chip has a unique 48-bit serial die id number, which is generated during the semiconductor

manufacturing process. The die id may not increment sequentially, however it is assured that the complete

48-bit die id is unique to each chip within 9 years.

The procedures below are followed by the TUSB6250 to identify and report the required serial number back

to the USB host:

•

After a power-up reset, the boot code performs a read to the SERNUM0 to SERNUM5 registers and

initializes its device serial number variable field stored in the XDATA memory space with the read value

from these registers.

•

The boot code then checks if the external I2C EEPROM is present on the I2C interface of the TUSB6250.

If the EEPROM is present and contains a valid device serial number as part of the USB device descriptor

information stored in the EEPROM, the boot code overwrites the serial number value stored in the XDATA

memory space with the one found in the EEPROM. Otherwise, the TUSB6250 serial number value stored

in the XDATA memory space stays unchanged from the previous step.

•

In summary:

−

The serial number value specified in the external EEPROM has the highest priority to be loaded into

the XDATA memory space, which is used as part of the valid device descriptor information to be

reported back to the USB host during USB device enumeration.

−

When responding to the Get_DeviceDescriptor command from the USB host, if the external I2C

EEPROM does not contain the valid serial number, the TUSB6250 converts the 48-bit unique serial

number stored in the serial number registers into a string of 12 UNICODE characters and returns the

string to the host.

For detailed information regarding how to specify the custom serial number in the header block of the external

I2C EEPROM, see the TUSB6250 Boot Code application note (SLLA126).

Little endian is used when describing the serial number registers with SERNUM0 as the least significant byte.

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