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DS000002248A-page 25

USB2502

The Green LED pins are sampled after RESET_N negation, and the logic values are used to configure the hub if the

internal default configuration mode is selected. The implementation shown below (see Figure 5-1) shows a recom-

mended passive scheme. When a pin is configured with a “Strap High” configuration, the LED functions with active low

signaling, and the PAD will “sink” the current from the external supply. When a pin is configured with a “Strap Low” con-

figuration, the LED functions with active high signaling, and the PAD will “source” the current to the external LED.

5.6

Reset

There are two different resets that the Hub experiences. One is a hardware reset (via the RESET_N pin) and the second

is a USB Bus Reset.

5.6.1

EXTERNAL HARDWARE RESET_N

A valid hardware reset is defined as, assertion of RESET_N for a minimum of 1us after all power supplies are within

operating range. While reset is asserted, the Hub (and its associated external circuitry) consumes less than 500

A of

current from the upstream USB power source (300

A for the Hub and 200A for the external circuitry).

Assertion of RESET_N (external pin) causes the following:

1.

All downstream ports are disabled, and PRTPWR power to downstream devices is removed.

2.

The PHYs are disabled, and the differential pairs will be in a high-impedance state.

3.

All transactions immediately terminate; no states are saved.

4.

All internal registers return to the default state (in most cases, 00(h)).

5.

The external crystal oscillator is halted.

6.

The PLL is halted.

7.

LED indicators are disabled.

The Hub is “operational” 500

s after RESET_N is negated.

Once operational, the Hub immediately reads OEM-specific data from the external EEPROM (if the SMBus option is not

disabled).

FIGURE 5-1:

LED STRAPPING OPTION

HUB

50K

Strap Low

GR2

50K

Strap High

GR1

+V