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Universal Serial Bus Specification Revision 1.1

and decodes the Hub and Port Status Change Bitmap accordingly. The hub reports any changes in hub status in bit zero of the Hub and Port Status Change Bitmap.

The Hub and Port Status Change Bitmap size varies from a minimum size of one byte. Hubs report only as many bits as there are ports on the hub, subject to the byte-granularity requirement (i.e., round up to the nearest byte).

N

2

1

0

Port N change detected

 

 

 

 

Port 2 change detected

 

 

 

Port 1 change detected

 

 

 

Hub change detected

 

 

Figure 11-14. Hub and Port Status Change Bitmap

Any time the Status Change endpoint is polled by the host controller and any of the Status Changed bits are non-zero, the Hub and Port Status Change Bitmap is returned. Figure 11-15 shows an example creation mechanism for hub and port change bits.

 

 

Per-Port Logic

E

 

 

 

Port N

 

 

x

 

Logical OR

a

 

 

m

 

 

 

Change

Change

 

p

 

l

Detect Logic

Information

 

e

 

 

Hub and Port Status Change Bitmap

N

Figure 11-15. Example Hub and Port Change Bit Sampling

11.13.5 Over-current Reporting and Recovery

USB devices must be designed to meet applicable safety standards. Usually, this will mean that a selfpowered hub implement current limiting on its downstream ports. If an over-current condition occurs, it causes a status and state change in one or more ports. This change is reported to the USB System Software so that it can take corrective action.

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