Host Mode; Usb System Performance - ST STM32F205 series Reference Manual

USB on-the-go full-speed (OTG_FS)
Transmit FIFO RAM allocation: the minimum RAM space required for each IN Endpoint
Transmit FIFO is the maximum packet size for that particular IN endpoint.
Note: More space allocated in the transmit IN Endpoint FIFO results in better performance on the
USB.
29.13.2

Host mode

Receive FIFO RAM allocation
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 channels 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 in the host channel, transfer complete status information is also
pushed to the FIFO. So one location must be allocated for this.
Transmit FIFO RAM allocation
The minimum amount of RAM required for the host Non-periodic Transmit FIFO is the
largest maximum packet size among all supported non-periodic OUT channels.
Typically, two Largest Packet Sizes worth of space is recommended, so that when the
current packet is under transfer to the USB, the CPU can get the next packet.
The minimum amount of RAM required for host periodic Transmit FIFO is the largest
maximum packet size out of all the supported periodic OUT channels. If there is at least one
Isochronous OUT endpoint, then the space must be at least two times the maximum packet
size of that channel.
Note: More space allocated in the Transmit Non-periodic FIFO results in better performance on
the USB.
29.14

USB system performance

Best USB and system performance is achieved owing to the large RAM buffers, the highly
configurable FIFO sizes, the quick 32-bit FIFO access through AHB push/pop registers and,
especially, the advanced FIFO control mechanism. Indeed, this mechanism allows the
OTG_FS to fill in the available RAM space at best regardless of the current USB sequence.
With these features:
•
The application gains good margins to calibrate its intervention in order to optimize the
CPU bandwidth usage:
–
–
•
The USB Core is able to maintain its full operating rate, that is to provide maximum full-
speed bandwidth with a great margin of autonomy versus application intervention:
–
980/1378
It can accumulate large amounts of transmission data in advance compared to
when they are effectively sent over the USB
It benefits of a large time margin to download data from the single receive FIFO
It has a large reserve of transmission data at its disposal to autonomously manage
the sending of data over the USB
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