Only 48MHz, 72MHz, 96MHz and 120 MHz system clocks derived from an
external crystal are suitable for the usb peripheral, as the internal
oscillator is not stable enough. Also the usb-prescaler only supports
division by 1 (48MHZ), 1.5(72MHz), 2(96MHz) and 2.5(120Mhz).
120Mhz is also out of spec and not added here.
Instead of using the HAL functions we can just use the defines from the
board support for the longan nano that comes with the nuclei-sdk. Also
we move some includes and defines to the header file.
The core of tinyusb must be as independent as possible, we previously
relied on nuclei-sdk or the GD32VF103 firmware library for the synopsys
driver to work with the GD32VF103. Fortunatly we needed very few parts
from them so we implement them here.
Without having __riscv_flen defined we get multiple warinings. But
defining it causes the startup code to contain floating point instructions.
This results in a exception right after booting.
See startup_gd32vf103.S lines 289-294 should open a PR at nuclei sdk
Stefan Kerkmann