USB Frames

Last modified by Microchip on 2023/11/10 11:19

USB data transfers occur through a series of events called transactions. Transactions are conducted within a host-controlled time interval called a frame. The length and frequency of the transactions depend upon the transfer type being used for an endpoint.

The type of transfer which can be sent in a frame and the frame length is defined by the specified speed of the USB.

The four possible USB transfer types allowed are:

  • Interrupt
  • Bulk
  • Isochronous
  • Control

Frame Specifications

LOW SPEED Frame size: 1 ms

Supported Transfer TypesMaximum Size of TransferTransfers per FrameMaximum Theoretical Throughput
Control 18 K bytes/sec
Interrupt 18 K bytes/sec
Bulk-0-
Isochronous-0-

FULL SPEED Frame size: 1 ms

Supported Transfer TypesMaximum Size of TransferTransfers per FrameMaximum Theoretical Throughput
Control64 bytes164 kbytes/s
Interrupt64 bytes164 kbytes/s
Bulk64 bytesUp to 191.2 Mbytes/s
Isochronous1023 bytes11023 kbytes/s

HIGH SPEED Frame size: 125 μs

Supported Transfer TypesMaximum Size of TransferTransfers per FrameMaximum Theoretical Throughput
Control64 bytes164 kByte/s
Interrupt1024 bytesup to 324 MByte/s
Bulk512 bytesup to 1353 MByte/s
Isochronous1024 bytesup to 324 MByte/s