ST STM32F102 Series Reference Manual page 473

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clock is fixed by the requirements of the USB standard at 48 MHz, and this can be different
from the clock used for the interface to the APB1 bus. Different clock configurations are
possible where the APB1 clock frequency can be higher or lower than the USB peripheral
one.
Note: Due to USB data rate and packet memory interface requirements, the APB1 clock frequency
must be greater than 8 MHz to avoid data overrun/underrun problems.
Each endpoint is associated with two packet buffers (usually one for transmission and the
other one for reception). Buffers can be placed anywhere inside the packet memory
because their location and size is specified in a buffer description table, which is also
located in the packet memory at the address indicated by the USB_BTABLE register. Each
table entry is associated to an endpoint register and it is composed of four 16-bit words so
that table start address must always be aligned to an 8-byte boundary (the lowest three bits
of USB_BTABLE register are always "000"). Buffer descriptor table entries are described in
the Section 20.5.3: Buffer descriptor
Isochronous nor a double-buffered bulk, only one packet buffer is required (the one related
to the supported transfer direction). Other table locations related to unsupported transfer
directions or unused endpoints, are available to the user. isochronous and double-buffered
bulk endpoints have special handling of packet buffers (Refer to
transfers
between buffer description table entries and packet buffer areas is depicted in
and Section 20.4.3: Double-buffered endpoints
USB full speed device interface (USB)
table. If an endpoint is unidirectional and it is neither an
respectively). The relationship
Section 20.4.4: Isochronous
Figure
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