TUSB3410 datasheet(89/92 Pages) TI

89 / 92 page

13–1

13 Application Notes

13.1 Crystal Selection

The TUSB3410 requires a 12-MHz clock source to work properly. This clock source can be a crystal placed across

the X1 and X2 terminals. A parallel resonant crystal is recommended. Most parallel resonant crystals are specified

at a frequency with a load capacitance of 18 pF. This load can be realized by placing 33-pF capacitors from each end

of the crystal to ground. Together with the input capacitance of the TUSB3410 and stray board capacitance, this

provides close to two 36-pF capacitors in series to emulate the 18-pF load requirement. Note, that when using a

crystal, it takes about 2 ms after power up for a stable clock to be produced.

TUSB3410

33 pF

12 MHz

33 pF

Figure 13–1. Crystal Selection

13.2 External Circuit Required for Reliable Bus Powered Suspend Operation

TI has found a potential problem with the action of the SUSPEND output pin immediately after power on. In some

cases the SUSPEND pin can power up asserted high. When used in a bus powered application this can cause a

problem because the VREGEN# input is usually connected to the SUSPEND output. This in turn causes the internal

1.8-V voltage regulator to shut down, which means an external crystal may not have time to begin oscillating, thus

the device will not initialize itself correctly.

TI has determined an on-chip fix for this problem, but has not determined a schedule on when the fix will be

implemented. In the meantime, the components R2 and D1 (rated to 25 mA) in the circuit shown below can be used

as a workaround. Note that R1 and C1 are required components for proper reset operation, unless the reset signal

is provided by another means. R2 and D1 can be left in place or removed once the silicon is modified.

Note that use of an external oscillator (1.8-V output) versus a crystal would avoid this situation, but it is not expected

that many applications would use an oscillator. Also note that self-powered applications would probably not see this

problem because the VREGEN# input would likely be tied low, enabling the internal 1.8-V regulator at all times.

TUSB3410 Arkusz danych(PDF) 89 Page - Texas Instruments