TUSB2040A

4-PORT HUB FOR THE UNIVERSAL SERIAL BUS

WITH OPTIONAL SERIAL EEPROM INTERFACE

SLLS288B – FEBRUARY 1998 – REVISED MARCH 1998

2

POST OFFICE BOX 655303

description (continued)

The TUSB2040A supports bus-power and self-power modes. The power switching to the downstream ports can

either be controlled individually or ganged using external devices to switch power and to detect overcurrent

conditions. Outputs from the external power devices provide overcurrent inputs to the TUSB2040A OVRCUR

pins and in the case of an overcurrent condition, the corresponding PWRON pins will be disabled by the

TUSB2040A. In the GANGED operation, all PWRON signals transition simultaneously, and any OVRCUR input

can be used.

The hub requires a 48-MHz clock signal to sample data from the upstream port and generate a synchronized

12-MHz USB clock signal. The hub supports the flexibility to use any device that generates a 48-MHz clock.

Because the majority of oscillators are active devices, the low power suspend mode of the TUSB2040A will not

function because there is no way to stop the oscillator from driving the internal clock. An oscillator with a TTL

output not exceeding 3.6 V can be used by connecting its output to the XTAL1 terminal and leaving the XTAL2

terminal open. For crystal or resonator implementations, use the XTAL1 terminal as the input and the XTAL2

terminal as the feedback path. Because the crystal is required to resonate at 48 MHz, a tuning circuit as shown

in Figure 8 may be required.

The upstream port and all downstream ports are USB-compliant transceivers. Every downstream port supports

both full-speed and low-speed connections by automatically setting the slew rate according to the speed of the

device attached to the port.