There is an accuracy parameter (ppm) for any external crystals, for example 12M+/-100ppm, and there are independent clock source from full speed USB device and hos. USB SPEC. has defined a frame timer range to be 500ppm.

Below is USB2.0 Spec. definition about frame timer range in ppm.

214-1

Nuvoton has no constrain of the crystal accuracy and user can calculate the effect to USB by the following calculation. (PFM-M487 has HXT with +/-30ppm)

There are two major errors contribute to USB clock, one is the crystal accuracy and the other is PLL jitter.

Crystal ppm contribution – For a 100ppm crystal pass through the PLL to the USB clock has still 100ppm at the end of USB clock.
PLL Jitter contribution – By datasheet, the maximum PLL jitter is 250ps (RMS, Figure1). PLL Jitter (192MHz) pass to USB (12M) has a divider of 16 timers and 250ps jitter will become 250ps/16 = 15.625ps (RMS) at USB clock. Transform RMS jitter to peak-to-peak period jitter by equation1 of Figure2, 7.44*15.625 = 116.25ps. For a full speed USB, it takes 1 ms for a single frame and jitter ppm calculation will be 116.26ps/1ms=116.25ppm
The final USB in ppm = 100ppm(by crystal) + 116.25ppm(by PLL jitter) = 226.25pps still less than 500ppm. By this calculation, we can make sure that a crystal with 100ppm is fine for a full speed USB devuce.

Figure1:

Figure2:

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Function:	Peripherals,Clock Control,HXT,PLL,Connectivity,SPI,USB

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How to calculate the necessary HXT accuracy(ppm) for a full speed USB device? Issue Date：2018-03-07

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