Renesas M16C Series User Manual page 137

M30245 Group
Interrupt Transfer
This transfer is used to notify the host of aperiodic and low-frequency data from the device. For
example, they include the notification of out of paper in printer and data concerning devices such as
the mouse and the keyboard.
Isochronous Transfer
This transfer is used for continuous and periodic communication. Once the communication path is
established, a transfer rate is guaranteed with a limited delay. The maximum size of transfer data is
specified by the endpoint, which is read by the host as the configuration data of the device. Based on
this data, transfer within the frame is scheduled and the bus time required for transfer of the maxi-
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mum size of data is secured with preference. Although the band width and the transfer rate of data
transfer are guaranteed, retransmission is not executed even if an error exists in the transfer. This is
used for streaming data such as animation and audio data which requires real time.
(2) Communication Protocol
Host CPU has the initiative for the entire USB communication. Even when data are transmitted to the
host from the device, the host gives the right of use to the device before data are transmitted. The
host, in order to process multiple transfers simultaneously, schedules each transfer in packet unit
within a frame of 1ms interval. The frames image is shown below:
SOF Audiomouse
Isochronous trasnfer
Interrupt trasnfer
Bulk trasnfer
Figure 2.8.1. Frame image
Packet
A packet is the unit in which the host CPU or the device secures a bus. In the USB, data are transmit-
ted/received in the packet unit.
A packet is a group of bit data strings (fields), which starts with the SOP (Start-of-Packet) as a part of
the SYNC (synchronous data) field. This is followed by the PID field which identifies a packet type
and, then, each data field of a frame number/ address/ data field, etc., finally ending with EOP (End-
of-Packet) which indicates the end of a packet. The packet types and formats are shown below:
Rev.2.00 Oct 16, 2006
REJ09B0340-0200
1ms
SOF
printer/scanner
page 128 of 354
1ms
printer/scanner
Audio
2. USB function
1ms
SOF Audio mouse
printer/scanner