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Figure 4.

RTC oscillator

2.3

USB clock

STR710 and STR711 series microcontrollers contain a USB 2.0 Full Speed device module

interface that operates at a precise frequency of 48 MHz. This clock is usually provided by an

external oscillator connected to the USB clock pin USBCLK. However, to save the board’s

space and cost, the 48MHz USB clock can also be generated by the internal PLL2 using one

single external oscillator for both system and USB module.

This part of the application note describes the hardware and software reference

implementation. USB Full Speed signal quality and jitter results can be measured using a

single external oscillator to generate not only the System PLL clock and Peripheral’s clocks, but

also the 48MHz USB clock.

2.3.1

Hardware implementation

The hardware implementation guidelines are described in the figure below.

STR71x

RTCXTI

GND

32kHz CRYSTAL

GND

RTCXTO

15pF*

15pF*

* these values are given only as examples, refer to the crystal manufacturer for more details