USB on-the-go full-speed (OTG_FS) RM0008
822/1096 Doc ID 13902 Rev 12
28.12.2 Host Tx FIFOs
The host uses one transmit FIFO for all non-periodic (control and bulk) OUT transactions
and one transmit FIFO for all periodic (isochronous and interrupt) OUT transactions. FIFOs
are used as transmit buffers to hold the data (payload of the transmit packet) to be
transmitted over the USB. The size of the periodic (nonperiodic) Tx FIFO is configured in the
host periodic (nonperiodic) transmit FIFO size (HPTXFSIZ/HNPTXFSIZ) register.
The two Tx FIFO implementation derives from the higher priority granted to the periodic type
of traffic over the USB frame. At the beginning of each frame, the built-in host scheduler
processes the periodic request queue first, followed by the nonperiodic request queue.
The two transmit FIFO architecture provides the USB host with separate optimization for
periodic and nonperiodic transmit data buffer management:
● All host channels configured to support periodic (nonperiodic) transactions in the OUT
direction share the same RAM buffer (shared FIFOs)
● The OTG FS core can fill in the periodic (nonperiodic) transmit FIFO up to the limit for
any sequence of OUT tokens driven by the host software
The OTG_FS core issues the periodic Tx FIFO empty interrupt (PTXFE bit in
OTG_FS_GINTSTS) as long as the periodic Tx-FIFO is half or completely empty, depending
on the value of the periodic Tx-FIFO empty level bit in the AHB configuration register
(PTXFELVL bit in OTG_FS_GAHBCFG). The application can push the transmission data in
advance as long as free space is available in both the periodic Tx FIFO and the periodic
request queue. The host periodic transmit FIFO and queue status register (HPTXSTS) can
be read to know how much space is available in both.
OTG_FS core issues the non periodic Tx FIFO empty interrupt (NPTXFE bit in
OTG_FS_GINTSTS) as long as the nonperiodic Tx FIFO is half or completely empty
depending on the non periodic Tx FIFO empty level bit in the AHB configuration register
(TXFELVL bit in OTG_FS_GAHBCFG). The application can push the transmission data as
long as free space is available in both the nonperiodic Tx FIFO and nonperiodic request
queue. The host nonperiodic transmit FIFO and queue status register (HNPTXSTS) can be
read to know how much space is available in both.
28.13 FIFO RAM allocation
28.13.1 Device mode
Receive FIFO RAM allocation: the application should allocate RAM for SETUP Packets:
10 locations must be reserved in the receive FIFO to receive SETUP packets on control
endpoint. The core does not use these locations, which are reserved for SETUP packets, to
write any other data. One location is to be allocated for Global OUT NAK. Status
information is written to the FIFO along with each received packet. Therefore, a minimum
space of (Largest Packet Size / 4) + 1 must be allocated to receive packets. If multiple
isochronous endpoints are enabled, then at least two (Largest Packet Size / 4) + 1 spaces
must be allocated to receive back-to-back packets. Typically, two (Largest Packet Size / 4) +
1 spaces are recommended so that when the previous packet is being transferred to the
CPU, the USB can receive the subsequent packet.
Along with the last packet for each endpoint, transfer complete status information is also
pushed to the FIFO. Typically, one location for each OUT endpoint is recommended.
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