USB Type-C Working Principle In-depth Analysis--Entry Level

Since Apple released the new MacBook, many people have been talking about USB Type-C. Let me analyze USB Type-C from a hardware perspective and clear up some confusion.

feature

Small size, reversible insertion, fast speed (10Gb). This small refers to the USB interface on computers in the past, but it is actually a bit larger than the microUSB on Android phones:

USB Type-C：8.3mmx2.5mm

microUSB：7.4mmx2.35mm

而lightning：7.5mmx2.5mm

Therefore, I don’t see the advantage of USB Type-C on handheld devices in terms of size. As for speed, it depends on whether video transmission is needed.

Pin Definition

As you can see, data transmission mainly consists of two sets of differential signals, TX/RX. CC1 and CC2 are two key pins with many functions:

• Detect connection, distinguish front and back, distinguish DFP and UFP, that is, master and slave

• Configure Vbus, with two modes: USB Type-C and USB Power Delivery

• Configure Vconn. When there is a chip in the cable, one cc transmits the signal and one cc becomes the power supply Vconn

• Configure other modes, such as when connecting audio accessories, dp, pcie

There are 4 power supplies and 4 grounds, which is why it can support up to 100W.

Don’t be fooled by the fact that USB Type-C seems to support up to 20V/5A. In fact, this requires USB PD, and supporting USB PD requires an additional PD chip, so don’t think that the USB Type-C interface can support up to 20V/5A.

Of course, integrated chips will appear in the future.

Auxiliary signals sub1 and sub2 (Side band use) are only used in some specific transmission modes.

D+ and D- are for compatibility with previous USB standards.

Here, USB3.0 has only one set of RX/TX, and the speed is 5Gb. USB Type-C uses two sets to ensure that it can be plugged in both directions, but in fact, only one set of RX/TX is used for data transmission, and the speed has reached 10Gb. If the protocol is upgraded later, both sets will be used, and the speed will be 20Gb, the same as DisplayPort.

Workflow

In the above figure, DFP (Downstream Facing Port) is the master, and UFP (Upstream Facing Port) is the slave. In addition to DFP and UFP, there is also a DRP (Dual Role port). DRP can be used as both DFP and UFP. When DPR is connected to UFP, DRP is converted to DFP. When DRP is connected to DFP, DRP is converted to UFP. When two DRPs are connected together, one of them is DFP and the other is UFP.

There is a pull-up resistor Rp on the CC pin of DFP and a pull-down resistor Rd on the UFP. When not connected, the VBUS of DFP has no output. After connection, the CC pins are connected, and the CC pin of DFP will detect the pull-down resistor Rd of UFP, indicating that it is connected. DFP will turn on the Vbus power switch and output power to UFP. Which CC pin (CC1, CC2) detects the pull-down resistor determines the direction of the interface insertion, and switches RX/TX by the way.

The resistor Rd=5.1k, and the resistor Rp is an uncertain value. According to the previous figure, USB Type-C has several power supply modes. How to distinguish them? It depends on the value of Rp. If the value of Rp is different, the voltage detected by CC pin will be different, and then the power supply mode executed by DFP end will be controlled.

It should be noted that two CCs are drawn in the above picture, but in fact there is only one CC wire in the cable without chip.

The cable containing the chip does not have two cc wires, but one cc and one Vconn, which are used to power the chip in the cable (3.3V or 5V). At this time, there is no pull-down resistor Rd at the cc end, but a pull-down resistor Ra, 800-1200 ohms.

When both CC pins are connected to pull-down resistors <= Ra, DFP enters audio accessory mode, and both left and right channels and mic are available, as shown above.

USB Type-C和DisplayPort，PCIE

USB PD is a BMC-encoded signal, while the previous USB is FSK, so there is incompatibility. I don’t know if there are any products on the market that can convert.

USB PD is transmitted on the CC pin. PD has a VDM (Vendor defined message) function that defines the device ID. When a device that supports DP or PCIe is read, DFP enters alternate mode.

If DFP recognizes the device as DP, it switches the MUX/Configuration Switch to change the Type-C USB3.1 signal pin to transmit DP signal. AUX auxiliary is transmitted by Type-C's SBU1 and SUB2. HPD is a detection pin, similar to CC, so it is shared.

DP has four differential signals in lanes 0-3, and Type-C has four differential signals in RX/TX1-2, so there is no problem with complete replacement. In addition, in the replacement mode of the DP protocol, USB signals and DP signals can be transmitted at the same time, RX/TX1 transmits USB data, and RX/TX2 is replaced by two groups of data transmission in lanes 0 and 1, which can support up to 4k.

If DFP recognizes the device as DP, it switches the MUX/Configuration Switch to change the Type-C USB3.1 signal pin to transmit PCIe signals. Similarly, PCIe uses RX/TX2 and SBU1, SUB2 to transmit data, and RX/TX1 transmits USB data.

The advantage of this is that one interface can use two devices at the same time. Of course, this can be done with a conversion cable without the need for any chips.

Summarize

USB Type-C puts an end to the long-standing defect of USB plugging and unplugging, saving people a lot of time. It takes at least 2 seconds to change the direction. Assuming that 1 billion people in the world plug and unplug the USB once a day, there is a 50% probability of plugging it wrong, which takes more than 277,000 hours in total, or about 31 years. It's terrifying.

One interface handles audio, video and data, and the size is relatively small. Now more and more mobile phone interfaces are gradually turning into

USB Type-C interface.