I am trying to understand how the USB module works in the very low level on STM32F4 MCUs (ARM Cortex-M4 core). In the reference manual I read often the terms Periodic and Non-Periodic Rx/Tx FIFO (I am not sure if there is periodic and non-periodic RxFIFO), for example:
Periodic TxFIFO empty interrupt mask
Nonperiodic TxFIFO empty interrupt mask
Receive FIFO nonempty interrupt mask
I do not understand what does periodic and non-periodic Tx/Rx FIFO mean (I do know what a FIFO is), I found no really helpful online resources about this. Could someone explain these terms to me please? and are these terms only specific to Tx but not Rx FIFOs?