DocID025202 Rev 7
RM0365
Universal serial bus full-speed device interface (USB)
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buffered by the USB peripheral is loaded in an internal 16-bit register and memory access to
the dedicated buffer is performed. When all the data has been transferred, if needed, the
proper handshake packet over the USB is generated or expected according to the direction
of the transfer.
At the end of the transaction, an endpoint-specific interrupt is generated, reading status
registers and/or using different interrupt response routines. The microcontroller can
determine:
•
which endpoint has to be served,
•
which type of transaction took place, if errors occurred (bit stuffing, format, CRC,
protocol, missing ACK, over/underrun, etc.).
Special support is offered to isochronous transfers and high throughput bulk transfers,
implementing a double buffer usage, which allows to always have an available buffer for the
USB peripheral while the microcontroller uses the other one.
The unit can be placed in low-power mode (SUSPEND mode), by writing in the control
register, whenever required. At this time, all static power dissipation is avoided, and the USB
clock can be slowed down or stopped. The detection of activity at the USB inputs, while in
low-power mode, wakes the device up asynchronously. A special interrupt source can be
connected directly to a wakeup line to allow the system to immediately restart the normal
clock generation and/or support direct clock start/stop.
32.4.1 Description of USB blocks
The USB peripheral implements all the features related to USB interfacing, which include
the following blocks:
•
Serial Interface Engine (SIE): The functions of this block include: synchronization
pattern recognition, bit-stuffing, CRC generation and checking, PID
verification/generation, and handshake evaluation. It must interface with the USB
transceivers and uses the virtual buffers provided by the packet buffer interface for
local data storage. This unit also generates signals according to USB peripheral
events, such as Start of Frame (SOF), USB_Reset, Data errors etc. and to Endpoint
related events like end of transmission or correct reception of a packet; these signals
are then used to generate interrupts.
•
Timer: This block generates a start-of-frame locked clock pulse and detects a global
suspend (from the host) when no traffic has been received for 3 ms.
•
Packet Buffer Interface: This block manages the local memory implementing a set of
buffers in a flexible way, both for transmission and reception. It can choose the proper
buffer according to requests coming from the SIE and locate them in the memory
addresses pointed by the Endpoint registers. It increments the address after each
exchanged byte until the end of packet, keeping track of the number of exchanged
bytes and preventing the buffer to overrun the maximum capacity.