Datasheet

Architecture Overview
5−4
SLLS535E − April 2008TUSB6250
5.2.6 I
2
C Interface Controller
The master-only I
2
C interface controller is responsible for acquiring the user-configurable descriptors and
other configurable feature parameters from the external I
2
C EEPROM during initial power up. It is also used
to download the application firmware from the external I
2
C EEPROM. The behavior of the I
2
C interface
controller is controlled by the boot code (the microcode embedded in boot ROM) or application firmware.
5.3 Other Major Features
5.3.1 Unique Power-On Sequencing to the Storage Device
The TUSB6250 provides unique power-on sequencing features to the storage device. When the TUSB6250
is powered up during the reset period, it turns off all the output buffers and activates all of the internal pulldown
resistors on the ATA/ATAPI bus. After reset, when the TUSB6250 controller is enumerated and configured,
the application firmware in operation decides when to power up the connected ATA/ATAPI drive and
reconfigure all the input, output, and bidirectional buffers, and the pullup and pulldown resistors on the
ATA/ATAPI bus based on their functionality defined in the ATA/ATAPI-5 specification.
This function is critical for implementing a truly bus-powered USB 2.0 mass storage device, because the disk
start-up spinning normally results in a high-current surge that is harmful to the USB device during enumeration.
According to the USB 2.0 specification, a USB device is only allowed to consume up to 100 mA before it is
configured.
This feature is also useful when the TUSB6250 controller interfaces to ATA/ATAPI mass storage devices that
do not implement fail-safe buffers on their ATA/ATAPI interface. In such conditions, this well controlled
power-on sequencing feature protects the connected storage device without fail-safe I/O buffers from damage
that might be caused by the bridge controller driving the signal lines when the power supply of the storage
devices is not present.
5.3.2 Die-ID Based USB Device Serial Number
The TUSB6250 supports unique USB device serial numbers by using the 48-bit die-ID number unique to each
silicon die. It also allows end-product developers to specify their own custom serial number in the external I
2
C
EEPROM to override this default die-ID serial number.