Datasheet

USB 2.0 Hi-Speed Hub Controller
Datasheet
SMSC USB251x 55 Revision 1.1 (04-26-10)
DATASHEET
8.3.9 SMBus Alert Response Address
The SMBALERT# signal is not supported by the hub.
8.3.9.1 Undefined Registers
The registers shown in Table 8.2 are the defined registers in the hub. Reads to undefined registers
return 00h. Writes to undefined registers have no effect and do not return an error.
8.3.9.2 Reserved Registers
Reserved registers should be written to ‘0’ unless otherwise specified. Contents read should be
ignored.
8.4 Default Configuration Option
To configure the SMSC hub in its default configuration, strap CFG_SEL[2:0] to 00h. This procedure
configures the hub to the internal defaults and enables the strapping options. Please see Section 8.2.1,
"Internal Register Set (Common to EEPROM and SMBus)" for the list of the default values. For specific
pin strapping options, please see Chapter 5, Pin Descriptions for instructions on how to modify the
default values. Options include port disable and non-removable pin strapping.
8.5 Default Strapping Options
The USB251x can be configured via a combination of internal default values and pin strap options.
Please see Ta ble 8.2 for specific details on how to enable the default/pin-strap configuration option.
The strapping option pins only cover a limited sub-set of the configuration options. The internal default
values will be used for the bits & registers that are not controlled by a strapping option pin. Please
refer to Table 8.2 for the internal default values that are loaded when this option is selected.
8.6 Strap Options
Table 8.7 Summary of Strap Options
8.6.1 Non-Removable Strap Option
The strap function of the NON_REM[x:0] pins are enabled through the internal default configuration.
The driver type of each strap pin is I/O (no internal pull-up or pull-down for the input function). Use
this type of strap option for NON_REM[1:0]. Figure 8.3 shows an example of Strap High and Strap
Low. Use the Strap High configuration to set the strap option value to a ‘1’. Use the Strap Low
configuration to set the strap option value to ‘0’.
Part
Number
USB251x
USB251xi
USB251xA
USB251xAi
USB251xB
USB251xBi
Resistor Value
(R)
Notes
Normal 3347 - 100 kΩ
Internal Pull-Down
3 10 kΩ
 Only applicable to port power pins.
 Contains a built-in resistor.
LED 3347 - 100 kΩ