Introduction to Type-C Protocol (Part 2)

3.1 Pull-up resistor Rp of DFP

Both CC1 and CC2 signals of the DFP must have pull-up resistors Rp, which are pulled up to 5V or 3.3V. Or both CC1 and CC2 are pulled up by a current source. The ultimate goal is to detect the voltage on CC1 or CC2 after insertion, and then determine whether to flip and the current capability of the DFP. The following are all possible configurations. You can choose any of the three columns on the right as the pull-up method. For example, Fairchild’s FUSB300 uses 330uA pull-up, TI’s TUSB320LAI uses 80uA pull-up. Different pull-up methods have different voltages on the CC pin. Different voltages correspond to different current capabilities.

3.2 Rd of UPF

Both CC1 and CC2 pins of UFP must have a pull-down resistor Rd to GND (or use voltage clamp). The way Rd is handled is as follows.

The CC detection chip will detect this voltage and determine the next operation by judging the voltage range. The following table shows the current capability that the DFP can provide for different voltages on the CC pin. Each voltage range listed in the second column covers the voltage calculated in the above table. The calculation of Rp/Ra is the same.

3.3 Ra on the data line

For cables with electronic labels, one of the CC pins has been renamed VCONN to supply power to the electronic label chip. A Ra resistor is required between this VCONN pin and GND, and the range of this resistor value is 800Ω~1.2KΩ.

3.4 VCONN power supply

The allowable range of VCONN is 4.75V~5.5V, and the required power supply capability is 1W. DFP provides this power by default. If two DRPs are connected, the two parties can exchange VCONN power suppliers through USB PD protocol negotiation.

All USB3.0 interfaces supporting PD need to support VCONN, and VCONN power can be provided in one of the following two ways.

1. If a valid Rp/Rd connection is detected on one of the CC pins, the VCONN power supply can be connected to the other corresponding CC pin.

2. If a valid Rp/Rd connection is detected on one of the CC pins, first check whether the other CC pin also has an Rp/Ra connection, and then provide VCONN.

First check whether there is Ra, if it indicates that Vconn is needed for power supply, then Vconn is provided. The detection process does not require Vconn to exist.

Note that there is a switch inside each CC pin to train CC and VCONN functions in turn. The following figure is a typical connection:

4 Mobile phones are DRP

In reality, our mobile phones are all DRP, which can do both DFP and UFP, so how to switch?

DRP switches between DFP and UFP every 50ms in standby mode. When switching to DFP, the CC pin must have a resistor Rp pulled up to VBUS or output a current source. When switching to UFP, there must be a resistor Rd pulled down to GND on the CC pin. This switching action must be completed by the CC Logic chip. When the DFP detects that the UFP is inserted, it can output VBUS. When the UFP is unplugged, VBUS must be turned off. This action must be completed by the CC Logic chip.

5 USB Power Delivery 2.0

This is a single-wire protocol formulated by USB-IF. It is transmitted on the CC line to negotiate the power supply role, voltage, maximum power supply capacity, data role, standby mode, etc. The communication between the port and the power supply cable is carried out through the PD protocol. The agreement will not be expanded, please refer to the USB-IF official website for details. Here are a few features of the agreement:

1. All communications are through the CC line.

2. DFP is the bus master, used to initiate all communications.

3. All messages use 32bit 4b/5b coded bi-phase mark code (Bi-phase Mark Coded, BMC)

4.300K baud rate

5. CRC32 error check + message retry

6 Type-C cable specification

1. The cable supports at least 10,000 plug-in cycles.

2. The signal wire gauge is not specified, but the signal integrity of USB2.0 and USB3.0 must be guaranteed

3. The impedance of CC and SUB1/SUB2 lines is not more than 50Ω

4. The maximum IR voltage drop on the GND return path is 250mV

5. The maximum IR voltage drop on Vbus is 500mV

The USB Type-C specification does not clearly specify the cable length, but electrical requirements create some physical limitations. The insertion loss of the USB3.1 Type-C to Type-C cable assembly at 5GHz is specified as -6dB, which effectively limits the cable length to 1 meter. The insertion loss of the USB3.0 Type-C to Type-C cable assembly at 5GHz is specified as -7dB, which effectively limits the cable length to 2 meters.

 Summary of USB TYPE C cable length

7 Type-C data cable with electronic label

If the Type-C data line has a chip (we call it an electronic tag), this chip can communicate with the USB port through the USB power supply specification 2.0 BMC protocol. The electronic label cable can be powered by VCONN or directly by Vbus, and can consume up to 70mW of power. The following types of cables must have electronic labels:

1. USB Type-C cable compatible with USB3.1

2. 100W power supply cable. Any cable that can achieve a power carrying capacity of 60W or more must have an electronic label and be able to communicate with the DFP port.

If a cable with an electronic label is inserted into a socket that does not support the USB power supply specification 2.0, its behavior is exactly the same as that of a standard passive cable.

8 Audio accessory mode

8.1 Digital headset

The digital headset with the Type-C interface is a UFP (Device), and the mobile phone is a DFP. There must be Rd on the CC1 and CC2 pins of the headset. In fact, there is a 5.1K resistor on the CC pin of the LeTV digital headset.

8.2 Analog headset

The protocol requires that the two CC pins be directly connected to GND on the analog headset transfer line (must be less than Ra).