TUSB7320 datasheet(101/115 Pages) TI1

101 / 115 page

TUSB7320, TUSB7340

www.ti.com

SLLSE76J – MARCH 2011 – REVISED MAY 2012

INPUT CLOCK

9.1

Clock Source Requirements

The TUSB73x0 supports an external oscillator source or a crystal unit. The frequency of the clock source

may be 20 MHz – 50 MHz. The FREQSEL pin is used to indicate the oscillator input frequency. If the

FREQSEL pin is pulled low, the oscillator input frequency is 48 MHz. If the FREQSEL pin is pulled high,

the value in the PLL_FREQ_SEL field controls the selected frequency. After the PLL_FREQ_SEL field is

set appropriately the FREQ_SEL_EN bit should be set. When the FREQ_SEL_EN bit is set high, the

oscillator is restarted with the PLL multiplier value and the oscillator frequency controls set according to

the value specified in the PLL_FREQ_SEL field. The register values may be programmed via EEPROM or

PCI Express access; refer to Table 4-100. Note: When FREQSEL is pulled high, system software should

ensure that the MEMORY_ENB and MASTER_ENB bits in the Command Register are not set until after

the PLL_FREQ_SEL and FREQ_SEL_EN bits are set appropriately. This will guarantee that the xHCI

memory mapped registers cannot be accessed until the USB clocks are stable at the specified frequency,

since accesses to the xHCI memory mapped register space are retried until the frequency change has

completed. If a clock is provided to XI instead of a crystal, XO is left open and VSSOSC should be

connected to the PCB ground plane. Otherwise, if a crystal is used, the connection needs to follow the

guidelines below.

Since XI and XO are coupled to other leads and supplies on the PCB, it is important to keep them as short

as possible and away from any switching leads. It is also recommended to minimize the capacitance

between XI and XO. This can be accomplished by connecting the VSSOSC lead to the two external

capacitors CL1 and CL2 and shielding them with the clean ground lines. The VSSOSC should not be

connected to PCB ground when using a crystal.

Load capacitance (Cload) of the crystal varying with the crystal vendors is the total capacitance value of

the entire oscillation circuit system as seen from the crystal. It includes two external capacitors CL1 and

CL2.

Figure 9-1. Oscillation Circuit

INPUT CLOCK

101

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