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Functional Principle and Technical Analysis of USB Type-C

The most widespread use of USB Type-C began with USB3.1, as the functionality of USB has since expanded.
USB type-c Working principle and technical analysis

Table of Contents

The most widespread use of USB Type-C began with USB3.1, as the functionality of USB has since expanded.

USB3.1 Basic Specifications

HistoryMax Transfer
USB 1.01.5 MbpsLow Speed2.5W500 mA1996.1
USB 1.112 MbpsFull Speed2.5W500 mA1998.9
USB 2.0480 MbpsHigh Speed2.5W500 mA2000.4
USB 3.05 GbpsSuper Speed Gen14.5W900 mA2008.11
USB 3.110 GbpsSuper Speed Gen2100W900 mA2013.8

Comparison of output power and flag of Serial USB

All previous dynasties USB
All previous dynasties’ USB
  1. Full functions: data, audio, video transmission, and charging are supported at the same time, all concentrated in one transmission line, which can solve the problem of too many transmission lines.
  2. Support direct and reverse connection: Type-C footprint is a mirror design, it can support direct and reverse connection, the hardware has a special detection mechanism to determine whether it is a direct or reverse connection, this part will be mentioned later. (Midnight phone to charge without having to turn on the light to see the direction of the plug!)
  3. Two-way transmission: Data and power can be transmitted and charged in both directions.
  4. Backward Compatibility: Supports USB Type-A and Micro-B interfaces via a dongle.
  5. Fast transmission rate: Support USB 3.1, which can support up to 10Gbps data transmission.

USB Type C pin function Overview


Pin Description:

  1. Tx/Rx: Two differential signal transmission pairs for data transmission.
  2. CC1/CC2 (configuration channel): detect forward and backward insertion, detect if the cable is connected, determine down-facing port (DFP) and up-facing port (UFP), configure Vbus, configure Vconn and alternative OR accessories set mode, PD communication, etc. In short, the CC manages the communication between the master and the slave end over a USB Type-C cabling.
  3. Vbus: The CC pin is connected to supply power to the Vbus.
  4. D+/D- : downward supports USB2.0.
  5. SBU1/SBU2: used to transmit auxiliary signals. DP Alt Mode can use this pin to transmit AUX data.
  6. GND: ground. Because there are four power supplies and grounding devices, it can support up to 100W.

CC working principle and model


USB CC pin working model

(1) The downstream facing port (DFP) is the host end and the upstream port (UFP) is the device end. The CC pin on the DFP has a pull-up resistor, Rp, and the UFP has a pull-down resistor, Rd. (Rp determines the host’s ability to power the device)

(2) If the DFP is not connected to the UFP, the DFP’s VBUS has no output. When the DFP is connected to the UFP and the CC pin is connected, the DFP CC pin detects the UFP pulldown Rd indicating it is connected to the device, the DFP turns on the VBUS-FET switch and sends VBUS power to the UFP, which is, before the CC pin configuration is detected, the VBUS does not power the UFP.

(3) It is clear from the figure above that apart from Ra, the rest of the resistance should not appear in the wire. The Ra value is a range value as described in the table below. Generally, 1K ohms is used, and if the cable is used as a power supply device it can be a little less than 800 ohms

(4) The value of Rp is regulated. The 56,000 ohms commonly used today is defined as standard USB power typically used to support traditional USB architecture. Hence the USB2 Type-A/B to Type-C cable. 0/USB3.0 is often added with this resistor at the Type-C end. Other resistance values ​​are listed in the following table:

Current Source
to 1.7 – 5.5 V
Resistor pull-up to
4.75 – 5.5 V
Resistor pull-up to
3.3 V ± 5%
Default USB Power80µA ± 20%56kΩ ± 20%
(Note 1)
36kΩ ± 20%
1.5A @ 5V80µA ± 8%22kΩ ± 5%12kΩ ± 5%
3.0A @ 5V330µA ± 8%10kΩ ± 5%4.7kΩ ± 5%

(5) Rd is only 5.1K ohms in the specification and will not be used in Cable.

(6) Cable with Ra must have E-Mark IC inside, so they all support PD protocol. The cable without Ra must be passive and has no IC inside. Therefore, it must not support PD protocol.

(7) CC PIN detection can be divided into the following results:

OpenOpenNothing attachedN/A
RdOpenSink attached
OpenRdSink attached
OpenRaPower cable without Sink attached
RaOpenPower cable without Sink attached
RdRaPower cable with Sink or Vconn-powered
Accessory attached
RaRdPower cable with Sink or Vconn-powered
Accessory attached
RdRdDebug Accessory Mode attached
(Appendix B)
RaRaAudio Adapter Accessory Mode attached
(Appendix A)
Summary table of CC PIN detection results at the Source end
StateSource BehaviorSink Behavior
Nothing attachedSense CC pins for attachment,
Do not apply VBUS or Vconn
Sense VBUS for attach
Sink attachedSense CC for orientation,
Sense CC for detach,
Apply VBUS and Vconn
Sense CC pins for orientation,
Sense loss of VBUS for detach
Power cable
without Sink
Sense CC pins for attachment,
Do not apply VBUS or Vconn
Sense VBUS for attach
Power cable with
Sink or Vconn-
Powered Accessory
Sense CC for orientation,
Sense CC for detach,
Apply VBUS and Vconn
If accessories are supported,
see Source Behavior with
the exception is that VBUS is not
applied, otherwise, N/A.
Debug Accessory
Mode attached
Sense CC pins for detachment,
Reconfigure for debugging
Sense VBUS for detach,
Reconfigure for debugging
Audio Adapter
Accessory Mode
Sense CC pins for detach
Reconfigure for analog audio
If accessories are supported,
see Source Behavior,
Otherwise, N/A
Summary table of CC PIN detection results at the Source end

(8) Detection of Direct & Reverse connection: Because Type-C supports forward and reverse connections, the CC pin is used to detect forward and reverse connections when CC1 receives pulldown (Rd), from the DFP point of view it is a direct connection when CC2 pulldown receives a reverse link. After the forward and reverse link detection, the corresponding USB signal is output, for example, CC1 corresponds to SSTX1 and SSRX1. The right side of the figure below integrates MUX because the data rate of USB 3.1 is up to 10Gbps, to avoid the PCB alignment bifurcation, the signals coming from the link are changed back and forth from MUX, on the forwarding link to SSRX1 and Switch SSTX1, switch to SSRX2 and SSTX2 on the reverse link.

Data transmission path diagram (the blue line is the forward insertion, and the red line is the reverse insertion)
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