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USB 2.0 Hi-Speed Hub Controller
Datasheet
Revision 1.0 (3-11-09) 34 SMSC USB251x
DATASHEET
Note: When the CFG_SEL[2] pin is unavailable, then the logic is internally tied to ‘0’.
8.1.2.1 Power Switching Polarity
When the PRTPWR_POL pin is unavailable (3 and 4 port, 48-pin packages only), the hub only
supports “active high” port power controllers.
8.1.3 VBus Detect
According to Section 7.2.1 of the USB 2.0 specification, a downstream port can never provide power
to its D+ or D- pull-up resistors unless the upstream port’s VBUS is in the asserted (powered) state.
The VBUS_DET pin on the hub monitors the state of the upstream VBUS signal and will not pull-up
the D+ resistor if VBUS is not active. If VBUS goes from an active to an inactive state (Not Powered),
the hub will remove power from the D+ pull-up resistor within 10 seconds.
8.2 EEPROM Interface
The SMSC hub can be configured via a 2-wire (I
2
C) EEPROM (256x8). (Please see Table 8.1, "Hub
Configuration Options" for specific details on how to enable configuration via an I
2
C EEPROM).
The internal state-machine will (when configured for EEPROM support) read the external EEPROM for
configuration data. The hub will then “attach” to the upstream USB host.
Note: The hub does not have the capacity to write, or “Program,” an external EEPROM. The hub
only has the capability to read external EEPROMs. The external eeprom will be read (even if
it is blank or non-populated), and the hub will be “configured” with the values that are read.
1 0 0 Internal Default Configuration with the following over-rides
Dynamic power-switching enabled
Strap options disabled
LED mode = Speed (when available on package)
Individual power switching
Individual over-current sensing
1 0 1 Internal Default Configuration with the following over-rides
Dynamic setting enabled
Strap options disabled
LED mode = USB (when available on package)
Individual power switching
Individual over-current sensing
1 1 0 Internal Default Configuration with the following over-rides
Strap options disabled
LED mode = Speed (when available on package)
Individual power switching
Individual over-current sensing
1 1 1 Internal Default Configuration with the following over-rides
Strap options disabled
LED mode = USB (when available on package)
Ganged port power switching
Ganged over-current sensing
Table 8.1 Hub Configuration Options (continued)
CFG_SEL[2] CFG_SEL[1] CFG_SEL[0] DESCRIPTION