Figure 500. Host-Mode Fifo Address Mapping And Ahb Fifo Access Mapping; Host Fifo Architecture - ST STM32L4x6 Reference Manual

USB on-the-go full-speed (OTG_FS)
Peripheral Tx FIFOs
The core has a dedicated FIFO for each IN endpoint. The application configures FIFO sizes
by writing the endpoint 0 transmit FIFO size register (OTG_DIEPTXF0) for IN endpoint0 and
the device IN endpoint transmit FIFOx registers (OTG_DIEPTXFx) for IN endpoint-x.
43.11.2

Host FIFO architecture

Figure 500. Host-mode FIFO address mapping and AHB FIFO access mapping

Host Rx FIFO
The host uses one receiver FIFO for all periodic and nonperiodic transactions. The FIFO is
used as a receive buffer to hold the received data (payload of the received packet) from the
USB until it is transferred to the system memory. Packets received from any remote IN
endpoint are stacked back-to-back until free space is available. The status of each received
packet with the host channel destination, byte count, data PID and validity of the received
data are also stored into the FIFO. The size of the receive FIFO is configured in the receive
FIFO size register (OTG_GRXFSIZ).
The single receive FIFO architecture makes it highly efficient for the USB host to fill in the
receive data buffer:
•
All IN configured host channels share the same RAM buffer (shared FIFO)
•
The OTG_FS core can fill in the receive FIFO up to the limit for any sequence of IN
tokens driven by the host software
The application receives the Rx FIFO not-empty interrupt as long as there is at least one
packet available for download. It reads the packet information from the receive status read
and pop register and finally pops the data off the receive FIFO.
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