Datasheet
XTAL1
C1 C2
C
L
XTAL2
TUSB2046B
TUSB2046BI
SLLS413I –FEBRUARY 2000 –REVISED SEPTEMBER 2013
www.ti.com
USB Design Notes
The following sections provide block diagram examples of how to implement the TUSB2046B device. Note, even
though no resistors are shown, pullup, pulldown, and series resistors must be used to properly implement this
device.
Figure 6 is an example of how to generate the 6-MHz clock signal.
NOTE: This figure assumes a 6-MHz fundamental crystal that is parallel loaded. The component values of C1, C2, and R
d
are determined using a crystal from Fox Electronics – part number HC49U-6.00MHz 30\50\0-70\20, which means
±30 ppm at 25°C and ±50 ppm from 0°C to 70°C. The characteristics for the crystal include a load capacitance (C
L
) of
20 pF, maximum shunt capacitance (C
o
) of 7 pF, and the maximum ESR of 50 Ω. In order to insure enough negative
resistance, use C1 = C2 = 27 pF. The resistor R
d
is used to trim the gain, and R
d
= 1.5 kΩ is recommended.
Figure 6. Crystal Tuning Circuit
Input Clock Configuration
The input clock configuration logic of TUSB2046B is enhanced to accept a 6-MHz crystal or 48-MHz on-the-
board clock source with a simple tie-off change on TSTMODE (terminal 31).
• A 6-MHz input clock configuration is shown in Figure 7.
In this mode, both TSTMODE and TSTPLL/48MCLK terminals must be tied to ground. The hub is configured
to use the 6-MHz clock on terminals 30 and 29, which are XTAL1 and XTAL2, respectively, on the
TUSB2046B. This is identical to the TUSB2046.
Figure 7. 6-MHz Input Clock Configuration
• A 48-MHz input clock configuration is shown in Figure 8.
In this mode, both TSTMODE and XTAL1 terminals must be tied to 3.3-V V
CC
. The hub accepts the 48-MHz
clock input on TSTPLL/48MCLK (terminal 27). XTAL2 must be left floating (open) for this configuration. Only
the oscillator or the onboard clock source is accepted for this mode. A crystal can not be used for this mode,
since the chip’s internal oscillator cell only supports the fundamental frequency.
10 Submit Documentation Feedback Copyright © 2000–2013, Texas Instruments Incorporated
Product Folder Links: TUSB2046B TUSB2046BI