Isochronous and Interrupt Transfers in Usb3 and Usb2

8/12/2019 Isochronous and Interrupt Transfers in Usb3 and Usb2

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Agenda

What is isochronous transfer? Use of isochronous transfer Format of isochronous transactions Comparison of transfer in USB 2.0 & USB 3.0

a a orma Transfer Direction Service Period Bandwidth Allocation

Transfer Packet Size Power Management

Summary


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Agenda

What is interrupt transfer? Principal Use Handshake Phase

Format of Interru t Transactions Characteristics of interrupt transfer Comparison of transfer in USB 2.0 & USB 3.0

Data Format Transfer Direction Maximum Latency Bandwidth Allocation Packet Size Transfer Completion


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Isochronous Transfers

Synchronous connection with other devices Timeliness is more important than accurate delivery of

data Occur at fixed intervals

Spec a pr or ty ur ng execut on o eac rame Guaranteed access to USB bandwidth Guaranteed constant data rate No Handshake Phase

No retrying attempts in case of delivery failure


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Principal Use

Typically contain time sensitive information,such as an audio or video stream

e.g. microphone, speaker, CD audio etc.


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Format of Isochronous IN & OUTTransaction


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Data Format

USB imposes no data content structure oncommunication flows for isochronous pipes

Streaming pipe


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Transfer Direction

Isochronous pipes are unidirectional pipes

Endpoint descriptor defines the direction of data

flow

Two endpoints are required for data transfer inboth directions

This feature is same for USB 2.0 and USB 3.0


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Service Period

Isochronous transfers is scheduled to beperformed regularly during the service period

In USB 2.0, service interval isms or u pee en po n s

125s for High Speed endpoints

Isochronous transfers does not support LowSpeed endpoints

In USB 3.0, service interval for isochronoustransfer is 125 s


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Bandwidth Allocation

Isochronous transfers along with interrupttransfers can be allocated up to 80% of totalbandwidth on SuperSpeed

USB 2.0 requires maximum 90% of totalavailable bandwidth for Full Speed endpointsand maximum 80% of total bandwidth forHigh Speed endpoints


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Packet Size

In USB 2.0, maximum data payload is 1023bytes during each frame for Full Speedendpoints and 1024 bytes for High Speed

Maximum data payload size is 1024 bytesduring each frame for SuperSpeed


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Power Management

USB 2.0 does not support low power USB busstate b/w service intervals for isochronoustransfers

SuperSpeed allows isochronous devices toautonomously enter low power link states b/wservice intervals. It defines a PING &PING_RESPONSE mechanism to activate thelow power link


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Summary

USB 3.0 Protocol preserves key characteristics of USB2.0 Isochronous Protocol i.e., no Retries , Simpleprotocol

isochronous devices

Software Backward Compatibility

Maximum data transferred in 1 service interval isincreased to 48K bytes


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Interrupt Transfers

Interrupt transfers are small data transfersused to communicate information from theUSB device to the client software.


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Principal Use

HID such as mouse, keyboard, joysticks etc.

All data transfer requests are initiated by hostinstead of interrupt requests from input

device Host polls the input device periodically


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Handshake Phase

ACK (acknowledgment)Sent by the host in IN Transactions and by thedevice in OUT Transactions on successful

NAK(negative acknowledgment)

Sent by the device only when it is busy or hasno data to send


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Handshake Phase

Only device sends the STALL

STALL can be due to Unsupported Control Transfer Request Control request Failed Endpoint Failed


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Format of an Interrupt IN & OUTtransaction


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Characteristics of Interrupt Transfers

High reliability to communicate is required Guaranteed maximum service interval Guaranteed retry of transfer attempts in next service

Guaranteed bandwidth in each service interval Host waits for ready notification to service the endpoint Handshake Phase exists


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Data Format

In both USB 2.0 and USB 3.0, no data contentstructure is imposed on communication flowfor interrupt pipes

Streaming pipes


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Transfer Direction

Interrupt pipes are unidirectional pipes

Endpoint descriptor defines the direction of dataflow

Two endpoints are required for data transfer inboth directions

This feature is same for USB 2.0 and USB 3.0


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Maximum Latency

Endpoint descriptor specifies the latency or pollinginterval.

For Low Speed devices, the maximum latency variesfrom 10 ms to 255 ms

For Full Speed interrupt transfers, the maximum

latency varies from 1 ms to 255 ms For High Speed devices, the maximum latency varies

from 125 s to 4 sec


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Bandwidth Allocation

In USB 2.0, interrupt transfers & Isochronoustransfers can use up to 90% of the totalavailable bandwidth

Isochronous & Interrupt transfers can use upto 80% of the total available bandwidth onSuperSpeed


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Packet Size

Endpoint specifies the maximum data payload size

In USB 2.0, data payload size is

8 bytes or less for Low Speed Devices 64 bytes or less for Full Speed Devices

1024 bytes for High Speed Devices

In USB 3.0, maximum data payload size is 1024 bytes

No data padding in case of less data


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Packet Size (contd)

Host ensures that data payload size of anypacket is not greater than endpoints


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Transfer Completion

Interrupt transfer is completed when Has transferred the exact amount of data

expected

Transfers a packet with a payload lessthan the maximum packet size

Responds with a STALL handshake


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Isochronous and Interrupt Transfers in Usb3 and Usb2

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