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USB On-The-Go Implementation Trade-offs - PowerPoint PPT Presentation

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USB On-The-Go Implementation Trade-offs. Zong Liang WU TransDimension. Agenda. On-The-Go device vs. Dual role device Dual role device: top level architecture Basic issues: system constraints System constraints vs. Implementation choices Compliance & interoperability Product decision.

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Usb on the go implementation trade offs

USB On-The-Go Implementation Trade-offs

Zong Liang WU



  • On-The-Go device vs. Dual role device

  • Dual role device: top level architecture

  • Basic issues: system constraints

  • System constraints vs. Implementation choices

  • Compliance & interoperability

  • Product decision

On the go device vs dual role device
On-The-Go Device Vs. Dual Role Device

  • Dual role device (DRD):

    • Supports master negotiation protocol

    • Acts as master or slave, after MNP

    • Capable of supplying at least 4mA

  • On-The-Go device (OTG):

    • Dual Role Device

    • Slave-only device drawing less than 4mA from Vbus

Dual role device top level architecture
Dual Role DeviceTop-Level Architecture

Charge Pump

5V @4mA












Top level control

Basic issues system constraints
Basic IssuesSystem Constraints

  • Microprocessors

    • Wide range of performance (uP itself or the part available for DRD)

    • Different interfaces: often DRD is not allowed to be system bus master

  • System software

    • Latency of HW interrupt processing

      • Maybe critical for Isochronous applications

    • Different RTOS

System constraints vs implementation choices
System Constraints Vs.Implementation Choices

  • Performance of the master:

    • Throughput supported: full 12mbits/s (as good as or even better than a PC) vs. Very limited bandwidth

    • Endpoint types supported: all 4 types vs. A subset

    • Number of devices and endpoints supported: capable of supporting many devices/endpoints simultaneously vs. Onlya few endpoints

  • Difficulties:

    • How to get maximum master performance with a limited uP?

    • How to do in embedded applications where DRD is not allowed to be system bus master ?

    • How to support Isochronous applications within a RTOS having a large interrupt latency (like wince)?

System constraints vs implementation choices1
System Constraints Vs.Implementation Choices

  • uP requirements vs. Master performance:

    • Desired target for portable or STB applications:

      • Optimal performance

      • Light load on uP

      • Minimum interrupt frequency

      • Loose requirement on uP’s interrupt latency

      • Keep at low cost (HW+SW)

    • Trade-off vs. Smart SW/HW partitioning

      Learn from OHCI and UHCI partitioning

      • Call for major architecture innovation

System constraints vs implementation choices2
System Constraints Vs.Implementation Choices

  • Many applications:

    • Portable (PDA, mobile phone, MP3, pocket PC, digital camera etc)and less portable (set-top-box, game machine, etc):

      • Many RTOS on the market

  • RTOS-based stack vs. Dedicated system SW

    • RTOS is not always necessary

    • Think of a dedicated microprocessor

  • RTOS-based: how to design once for all?

    • Try to comply with and reuse OHCI/UHCI stack

    • Partner with specialty system software house

  • System constraints vs implementation choices3
    System Constraints Vs.Implementation Choices

    • Power management:

      • Portable appliances require low power

      • OTG spec introduces the concept of sessionand wakeup protocol

      • Manage power at chip architecture level, by introducing appropriate power management logic

    System constraints vs implementation choices4
    System Constraints Vs.Implementation Choices

    • Single chip vs. 2-chip solution

      • OTG master needs to supply [email protected] (minimum)

      • Analog transceiver’s signaling is 3.3V

      • Standard 0.18um process has trouble to implement

        • Put the charge pump and the analog transceiverinto a separate chip

        • Define a standard interface: ongoing effort

    Compliance interoperability
    Compliance & Interoperability

    • Compliance to WHAT:

      • A DRD must be a 100% compliant USB function

      • OTG master vs. Embedded master vs. StandardPC host (OHCI/UHCI):

        • OTG: one to one: simpler master

        • Embedded: Strong sales point if the master cansupport what a standard PC host can do(endpoint types, number, sizes)

    • An OTG compliance spec is in development

    Product decision
    Product Decision

    • What do you really want:

      • Slave-only with mini-connector and draws <=4ma

      • Dual role device (master or slave depending on MNP result)

      • Simultaneous master and slave

    • Do not forget your expectation on master’s performance

    • Discrete IC or integrate an IP into your system:

      • Time to market vs. Cost vs. Risk