STR720 datasheet(266/401 Pages) STMICROELECTRONICS

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STR720 - USB SLAVE INTERFACE (USB)

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19.4.1

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.

•

Suspend Timer: This block generates the frame locked clock pulse for any external device

requiring Start-of-Frame synchronization and it 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 word until the end of packet, keeping track of the number of exchanged bytes

and preventing the buffer to overrun the maximum capacity.

•

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

number of registers is 8, allowing up to 8 double-buffer endpoints or up to 16

mono-directional/single-buffer ones in any combination.

•

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.

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 512

Bytes, structured as 256 words by 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

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