8.3.2 Clock Generation

The input clock configuration logic of TUSB2046x is enhanced to accept a 6-MHz crystal or 48-MHz on-the-board clock source with a simple tie-off change on TSTMODE (pin 31).

• A 6-MHz input clock configuration is shown in Figure 5.

In this mode, both TSTMODE and TSTPLL/48MCLK pins must be tied to ground. The hub is configured to use the 6-MHz clock on pins 30 and 29, which are XTAL1 and XTAL2, respectively, on the TUSB2046x. This is identical to the TUSB2046.

Figure 5. 6-MHz Input Clock Configuration

A.
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
• A 48-MHz input clock configuration is shown in Figure 7.

In this mode, both TSTMODE and XTAL1 pins 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 cannot be used for this mode, because the internal oscillator cell of the chip only supports the fundamental frequency.

Figure 7. 48-MHz Input Clock Configuration