STR720 datasheet(265/401 Pages) STMICROELECTRONICS

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STR720 - USB SLAVE INTERFACE (USB)

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19.4 Functional Description

The USB Peripheral provides an USB compliant connection between the host PC and the

function implemented by the microcontroller. Data transfer between the host PC and the

system memory occurs through a dedicated packet buffer memory accessed directly by the

USB Peripheral. The size of this dedicated buffer memory must be according to the number of

endpoints used and the maximum packet size. In this implementation, the dedicated memory

of 512 Byte and up to 8 endpoints can be used. The USB Peripheral interfaces with the USB

host, detecting token packets, handling data transmission/reception, and processing

handshake packets as required by the USB standard. Transaction formatting is performed by

the hardware, including CRC generation and checking.

Each endpoint is associated with a buffer description block indicating where the endpoint

related memory area is located, how large it is or how many bytes must be transmitted. When

a token for a valid function/endpoint pair is recognized by the USB Peripheral, the related data

transfer (if required and if the endpoint is configured) takes place. The data 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).

Two interrupt lines are generated by the USB Peripheral : one IRQ collecting high priority

endpoint interrupts (isochronous and double-buffered bulk) and another IRQ collecting all

other interrupt sources (check the IRQ interrupt vector table for detailed interrupt source

mapping).

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

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 wake-up line to allow the system to immediately restart the normal

clock generation and/or support direct clock start/stop.

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