SPRUII0F May 2019 – June 2024 TMS320F28384D , TMS320F28384D-Q1 , TMS320F28384S , TMS320F28384S-Q1 , TMS320F28386D , TMS320F28386D-Q1 , TMS320F28386S , TMS320F28386S-Q1 , TMS320F28388D , TMS320F28388S
The USB module requires a fixed 60MHz clock for bit sampling. Since the main system clock is usually not a multiple of 60MHz, the correct frequency cannot be achieved with a simple divider. Instead, the USB clock is provided through an auxiliary clock path (AUXPLLCLK), which can use an independent clock source and PLL to generate the correct frequency.
USB clock tolerances are very tight. As stated in section 7.1.11 of the USB 2.0 specification, low-speed devices (1.50 Mb/s) have a tolerance of +/- 1.5%, while high-speed devices (12.000Mb/s) have a tolerance of +/- 0.25%. Typically these tolerances are achieved by using an external crystal or resonator as the source for AUXOSCCLK.