USB-C Protocol

ECEN 5692 Principles of Digital Communication

Nikhil Joshi


The need to unify and replace the communication protocols with one single protocol has been prevalent since mid 1990s. USB C is the latest of the family of industry standard developed called USB. USB was developed in the mid-1990s that defines the cables, connectors and communications protocols, which may be used as a bus for connection, communication, and power supply between computers and electronic devices. The goal of USB was to replace variety of interfaces such as parallel, serial ports and power chargers so that software configuration is simplified and greater data rates are possible for external devices.

USB C takes this to a limiting approach where it is possible to use the same port for power, high speed data transfer as well as video output. USB C utilizes USB 3.1 (also called as SUPERSPEED+) generation for data transfer and the comparison of which can be seen with legacy applications as follows.


Causes of development:

The devices are getting thinner and world is moving towards more mobile domain. However, the relative size of USB ports has been unchanged since its inception at 1994. This puts a great restraint on thickness of the devices. Also the micro USB plugs that are used in most mobile applications such as Tablets or Smartphones require special adaptors.




Alongwith this, USB C had to avoid certain legacy problems dealt with USB 3.0 typically with RFI(Radio frequency intererence).

USB C protocol goals

1.      Higher data rate

2.      Improved channel encoding

3.      New host receptacle

4.      Revised spread spectrum clocking

5.      Short & long cable support

6.      Enhance power and alternate mode functionality

7.      Reduced RFI and EMI


Standard Body

The standard body for USB is USB Implementers Forum, Inc. is a non-profit corporation founded by the group of companies that developed the Universal Serial Bus specification. These companies include  Apple Inc., Hewlett-Packard, NEC, Microsoft, Intel, and Agere Systems.  The USB-IF was formed to provide a support organization and forum for the advancement and adoption of Universal Serial Bus technology.  The corporation isn’t federally regulated however, most of the leading companies in the technology domain abide by the standard.

The documentation can be found at and for USB C power delivery the documentation can be found at

Products Using This Technology:

Among the first devices to accommodate a Type-C cable are the Nokia N1 tablet, Apple's 2015 MacBook and Google's second Chromebook Pixel. The first smartphone that accommodated a USB Type-C cable is from Chinese company LeTV. This technology is emergent and will soon replace all the cables with one type in near future.


Device Classes and Possible applications (legacy and USB C combined)

Base Class

Descriptor Usage




Use class information in the Interface Descriptors






Communications and CDC Control



HID (Human Interface Device)












Mass Storage









Smart Card



Content Security






Personal Healthcare



Audio/Video Devices



Billboard Device Class



Diagnostic Device



Wireless Controller





USB C Connector

i.            USB C is supported by legacy ports (viz. USB 3.0, 2.0 & 1.1) However, the data transfer speed would depend on the speed supported by the older of the devices connected

ii.            USB C delivers power through power delivery 2.0 protocol

iii.            USB C connects to both hosts and devices and is supported by alternate mode which is used for vendor defined messages and are primarily used for display port 1.3 (for laptops) and MHL 3.0 for tablets and smartphones.  Alternate mode means that properly configured devices will be able to use physical USB Type-C cables to transmit non-USB data.


USB C Pins

USB System Design

The system design of USB is asymmetrical in its topology, consisting of a host, a multitude of downstream USB ports, and multiple peripheral devices connected in a tiered-star topology. Additional USB hubs may be included in the tiers, allowing branching into a tree structure with up to five tier levels. A USB host may have multiple host controllers and each host controller may provide one or more USB ports. Up to 127 devices, including hub devices if present, may be connected to a single host controller.

                    i.            Each host controller may provide one or more USB ports.

                  ii.            Up to 127 devices (7-bits address)

                iii.            Device can be a composite (many functions on one device) or compound (distinctive devices built around a central hub)

                iv.            Device communication happen through pipes: stream or message

                  v.            Message pipe is bidirectional and is used for control transfers

                vi.            Stream pipe is a unidirectional pipe connected to a unidirectional endpoint that transfers data using an isochronous, interrupt, or bulk transfer:


Data Transfer types


Supported data transfer types in USB are listed as follows





Data rate guaranteed, data loss possible (real-time audio/video)


Guaranteed quick response
(Keyboard and mouse etc)


Best of effort transfer uses all remaining bandwidth latency not controlled
(bulk file transfers)


Used for status responses


USB Specifications and Physical layer

Since USB C uses USB 3.1 for high speed data transfer, the physical layer is identical to USB 3.1 layer and documentation about the physical layer can be found. Similarly for alternate modes USB C manual can be referred  

Power Delivery Capability of USB


USB power standards





USB 1.x and 2.0

500 mA[a]

5 V

2.5 W

USB 3.x

900 mA[b]

5 V

4.5 W

USB Battery Charging (BC 1.2)

0.5–1.5 A

5 V

2.5–7.5 W

USB 2.0 with type-C

500 mA

5 V

2.5 W

USB type-C

1.5 A

5 V

7.5 W

3 A

5 V

15 W

USB Power Delivery[f][e]

2 A

5 V

10 W

1.5 A

12 V

18 W

3 A

12 V

36 W

5 A

12 V

60 W

3 A

20 V

60 W

5 A

20 V

100 W


Use of USB as a bidirectional port for power had been envisaged long ago.

Power Architecture

The USB Power Delivery revision 2.0 specification has been released as part of the USB 3.1 suite. It covers the type-C cable and connector with four power/ground pairs and a separate configuration channel, which now hosts a DC coupled low-frequency BMC-coded data channel that reduces the possibilities for RF interference. Biphase mark coding transitions on every positive edge of the clock signal (when the clock goes from 0 to 1) and also transitions on the negative edge of the clock signal when the data is a 1. Power Delivery protocols have been updated to facilitate type-C features such as cable ID function, Alternate Mode negotiation, increased VBUS currents, and VCONN-powered accessories.


System Block Diagrams

Functionally the system could be split into a transmitter, and a receiver where transmitter can be further split into an encoder, a scrambler, and a driver. While a receiver will be comprised of clock recovery, a block aligner, an elastic buffer & a descrambler.



The scrambling polynomial is and can be represented as G(X) = X23 + X21 + X16 + X8 + X5 + X2 + 1.  Few system block diagrams are as follows.

Low Frequency Shift Register



Parallel To Serial Conversion & Vice Versa


The 128b/132b conversion


Error Control Codes

USB 3.1 uses CRC (Cyclic redundancy check) codes as part of error detection codes in the link management layer. There are two different generator polynomials, one each for token and data packets respectively. The token CRC is 5 bit pattern and data CRC is 16 bit pattern.

CRC used

Type of Packet


Link Control Word


Packet Header


Data Packet Payload



Apart from the above error correction codes, USB 3.1 also enforces a block header in the physical layer.

        i.            It is able to detect & correct upto 1 bit of error and detect more than 2 bits of error in block header which can be then reported to link layer.

      ii.            Use of bandwidth optimization techniques such as store and forward technique which ensures prioritized packet transmission for slower devices.

    iii.            USB 3.1 will support equal opportunity data transfer based on weighted sum round robin algorithm for devices connected to hubs via secondary hubs.

Because of these techniques discussed USB 3.1 hub to be more resource efficient.



A.    Input Pulse Spectrum

The input pulse spectrum used in USB C is trapezoidal and is given as follows


B.     Eye Diagram:

        i.            Conditions are more stringent than that of USB 3.0.

      ii.            Owing to the limitations of a physical channel, the bandwidth supported is finite.

    iii.            Increase in data rate results in loss of flatness of the curve which will result into closing of eye.


        i.            USB C complies with two different ongoing standards USB 3.1 and USB PD which support two different modulation standards for different applications.

      ii.            USB 3.1 uses triangular spread spectrum clocking modulation (SSC Modulation) which is linearly shifting the clock frequency in a band from DC to up to 5 GHz. This is done in order to comply within the FCC emission requirements. As per findings of USB 3.0 interference issues, USB 3.1 tries to minimize it by adding more ground pins.

  iii.            USB power delivery uses power configuration protocol uses 24 MHz BFSK coded transmission line channel on the VBUS line.  USB power delivery 2.0 covers type c cable including the configuration channel which now hosts a DC coupled low-frequency BMC-coded data channel that reduces the possibilities for RF interference.

Standardization and Major Players

        i.            USB C is upcoming standard. Due to its smaller size it is an ideal candidate for mobile devices such as laptops, 2-in-1, smartphones and tablets.

      ii.            Amongst laptops, Apple and Google have already adopted the USB C standard. Support from Windows will be expected soon.

    iii.            Support for Android has been already adopted for Nokia N1 tablet, support from other candidates is expected in near future.

    iv.            Asus & Biostar has announced use of adaptors supporting type A to USB C, other companies of USB-IF will be releasing supporting adaptors soon.






4.      USB C specifications: