Книга: Advanced PIC Microcontroller Projects in C

8.3.2 Data Flow Types

8.3.2 Data Flow Types

Data can be transferred on a USB bus in four ways: bulk transfer, interrupt transfer, isochronous transfer, and control transfer.

Bulk transfers are designed to transfer large amounts of data with error-free delivery and no guarantee of bandwidth. If an OUT endpoint is defined as using bulk transfers, then the host will transfer data to it using OUT transactions. Similarly, if an IN endpoint is defined as using bulk transfers, then the host will transfer data from it using IN transactions. In general, bulk transfers are used where a slow rate of transfer is not a problem. The maximum packet size in a bulk transfer is 8 to 64 packets at full speed, and 512 packets at high speed (bulk transfers are not allowed at low speeds).

Interrupt transfers are used to transfer small amounts of data with a high bandwidth where the data must be transferred as quickly as possible with no delay. Note that interrupt transfers have nothing to do with interrupts in computer systems. Interrupt packets can range in size from 1 to 8 bytes at low speed, from 1 to 64 bytes at full speed, and up to 1024 bytes at high speed.

Isochronous transfers have a guaranteed bandwidth, but error-free delivery is not guaranteed. This type of transfer is generally used in applications, such as audio data transfer, where speed is important but the loss or corruption of some data is not. An isochronous packet may contain 1023 bytes at full speed or up to 1024 bytes at high speed (isochronous transfers are not allowed at low speeds).

A control transfer is a bidirectional data transfer, using both IN and OUT endpoints. Control transfers are generally used for initial configuration of a device by the host. The maximum packet size is 8 bytes at low speed, 8 to 64 bytes at full speed, and 64 bytes at high speed. A control transfer is carried out in three stages: SETUP, DATA, and STATUS.

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