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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 5V@4mA (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