TUSB6250 datasheet(18/124 Pages) TI

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Device Parameter Information

4−5

SLLS535A − January 2004

TUSB6250

4.3

Device Operation

4.3.1 Device Master Reset

An external master reset signal, asynchronous to the TUSB6250’s internal clock, is needed to reset the

TUSB6250. This reset is referred to as the power-on reset throughout this document, which is connected to

the RSTI terminal of the TUSB6250. Since the TUSB6250 has built-in noise debouncing circuitry, it also

requires a valid clock signal present during the required active low master reset window. For the details of the

master reset timing requirement, see Section 13.2 Reset Timing Reference.

4.3.2 Clock Generation

The TUSB6250 requires an external 24-MHz crystal to be used. The integrated USB 2.0 UTMI-compliant PHY

generates all of the clock signals needed for the PHY analog, PLL, and digital logic. The PHY also generates

a 60-MHz clock used in the internal digital core of the TUSB6250.

4.3.3 Device Initialization

Since the TUSB6250 contains an integrated MCU, the device initialization process contains the following two

parts:

•

Hardware registers and state machines are cleared to their defined default reset state after power-on reset

initialization. The TUSB6250 powers up with a default USB function address of zero and is disconnected

from the USB bus.

•

The MCU executes a bootloader program in ROM (starting from address 0000 hex) to fetch the valid

application code from the external source and prepare for USB enumeration. The application code, once

in charge, may perform some initialization functions to configure the TUSB6250 to meet the requirement

of a particular end product application.

Since the application code (firmware) space is in the internal RAM, the TUSB6250 firmware needs to be

downloaded from an external source into the RAM space designated for code usage (see section 6.1 MCU

Memory Map for detailed information).

After power-on reset is applied to the TUSB6250, the integrated MCU executes the bootloader program (also

referred as boot code) residing in the on-chip 8K byte ROM mapped to the MCU’s program memory space,

this process is also referred to as booting.

The major tasks of the boot code are:

To fetch the descriptors required for itself or the firmware to perform USB enumeration

Download the application firmware from one of the two external sources available during booting: one from

an external I2C EEPROM connected to the I2C interface of the TUSB6250 and the other from the host

PC via the USB bus connection.

The MCU executes a read from an external I2C EEPROM and checks whether it contains the valid application

code by comparing the read value with the expected boot signature. If it contains the valid code, the MCU

executes follow-up reads from the EEPROM and writes the code into the TUSB6250’s internal 32K bytes of

default code RAM. If the external EEPROM does not contain any valid code, the MCU proceeds to boot from

the USB.

The I2C EEPROM normally is preprogrammed with the valid application code image. It also contains all the

configurable USB descriptors and other configurable descriptors or parameters for the mass storage device

connected to the TUSB6250’s ATA/ATAPI interface. For the option of booting from the USB host, the

application code may reside in the host PC. However, the external I2C EEPROM is still needed to store the

USB 2.0 specification required vendor ID and product ID specific to each individual end product manufacturer.

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