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Philips Semiconductors
ISP1122
USB stand-alone hub
Product specification Rev. 03 — 29 March 2000 22 of 48
9397 750 07002
© Philips Electronics N.V. 2000. All rights reserved.
[1] Default value at reset if no external EEPROM is present.
[2] Modifies the Hub Descriptor field ‘bPwrOn2PwrGood’, see Table 14.
[3] Modifies the Hub Descriptor field ‘MaxPower’, see Table 14.
11. Hub power modes
USB hubs can either be self-powered or bus-powered.
Self-powered — Self-powered hubs have a 5 V local power supply on board which
provide power to the hub and the downstream ports. The
USB Specification Rev. 1.1
requires that these hubs limit the current to 500 mA per downstream port and report
overcurrent conditions to the host. The hub may optionally draw 100 mA from the
USB supply (V
BUS
) to power the interface functions (hybrid-powered).
Bus-powered — Bus-powered hubs obtain all power from the host or an upstream
self-powered hub. The maximum current is 100 mA per downstream port. Current
limiting and reporting of overcurrent conditions are both optional.
Power switching of downstream ports can be done individually or ganged, where all
ports are switched simultaneously with one power switch. The ISP1122 supports both
modes, which can be selected using input INDV (see Ta ble 4).
11.1 Voltage drop requirements
11.1.1 Self-powered hubs
Self-powered hubs are required to provide a minimum of 4.75 V to its output port
connectors at all legal load conditions. To comply with Underwriters Laboratory Inc.
(UL) safety requirements, the power from any port must be limited to 25 W (5 A at
5 V). Overcurrent protection may be implemented on a global or individual basis.
Table 23: Configuration bits
Bit Function Value
(Bin)
Description
C0 OPTION see Table 4 “Mode selection”
C1 INDV see Table 4 “Mode selection”
C2 reserved 0
[1]
must always be programmed to logic 0
C3 PwrOn2PwrGood
[2]
0
[1]
100 ms (bPwrOn2PwrGood = 32H)
1 500 ms (bPwrOn2PwrGood = FAH)
C4 string descriptor enable 0
[1]
string descriptors disabled
1 string descriptors enabled (strings:
“Philips Semiconductors”, “ISP1122”)
C5 internal analog overcurrent
detection enable
0 internal analog overcurrent detection
circuit disabled; overcurrent pins
OCn
function as digital inputs (TTL level)
1
[1]
internal analog overcurrent detection
circuit enabled
C7, C6 MaxPower
[3]
00
[1]
100 mA (MaxPower = 32H)
1X 500 mA (MaxPower = FAH)
01 0 mA (MaxPower = 00H)