SPRUJ53B April 2024 – September 2024 TMS320F28P550SJ , TMS320F28P559SJ-Q1
The USB module requires a fixed 60MHz clock for bit sampling. When the PLLSYSCLK equals 150MHz, the PLLCLK output is 300MHz, which can be divided down evenly by 5 to achieve the 60MHz requirement.
USB clock tolerances are very tight. As stated in section 7.1.11 of the USB 2.0 specification, low-speed devices (1.50 b/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 clock source for the device.