Universal serial bus full-speed device interface (USB) RM0367
926/1043 RM0367 Rev 7
• Endpoint-Related Registers: Each endpoint has an associated register containing the
endpoint type and its current status. For mono-directional/single-buffer endpoints, a
single register can be used to implement two distinct endpoints. The number of
registers is 8, allowing up to 16 mono-directional/single-buffer or up to 7 double-buffer
endpoints in any combination. For example the USB peripheral can be programmed to
have 4 double buffer endpoints and 8 single-buffer/mono-directional endpoints.
• Control Registers: These are the registers containing information about the status of
the whole USB peripheral and used to force some USB events, such as resume and
power-down.
• Interrupt Registers: These contain the Interrupt masks and a record of the events. They
can be used to inquire an interrupt reason, the interrupt status or to clear the status of a
pending interrupt.
Note: * Endpoint 0 is always used for control transfer in single-buffer mode.
The USB peripheral is connected to the APB bus through an APB interface, containing the
following blocks:
• Packet Memory: This is the local memory that physically contains the Packet Buffers. It
can be used by the Packet Buffer interface, which creates the data structure and can
be accessed directly by the application software. The size of the Packet Memory is
1024 bytes, structured as 512 half-words of 16 bits.
• Arbiter: This block accepts memory requests coming from the APB bus and from the
USB interface. It resolves the conflicts by giving priority to APB accesses, while always
reserving half of the memory bandwidth to complete all USB transfers. This time-duplex
scheme implements a virtual dual-port SRAM that allows memory access, while an
USB transaction is happening. Multiword APB transfers of any length are also allowed
by this scheme.
• Register Mapper: This block collects the various byte-wide and bit-wide registers of the
USB peripheral in a structured 16-bit wide half-word set addressed by the APB.
• APB Wrapper: This provides an interface to the APB for the memory and register. It
also maps the whole USB peripheral in the APB address space.
• Interrupt Mapper: This block is used to select how the possible USB events can
generate interrupts and map them to the NVIC.
32.5 Programming considerations
In the following sections, the expected interactions between the USB peripheral and the
application program are described, in order to ease application software development.
32.5.1 Generic USB device programming
This part describes the main tasks required of the application software in order to obtain
USB compliant behavior. The actions related to the most general USB events are taken into
account and paragraphs are dedicated to the special cases of double-buffered endpoints
and Isochronous transfers. Apart from system reset, action is always initiated by the USB
peripheral, driven by one of the USB events described below.
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