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[General] how usb type c works

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Post At  2018-6-11 11:47:22  | Show all posts | Read mode
Since Apple released the new MacBook, a lot of people are talking about USB Type-C. Let me analyze the USB Type-C from the hardware point of view.


Small size, support positive and negative plug, speed (10Gb). This small is for the USB interface on the previous computer, actually the microUSB on the Android machine is a bit bigger.

USB Type-C:8.3mmx2.5mm;
And lightning:7.5mmx2.5mm;

So, I can't see the advantage of USB Type-C on handheld devices from the dimensions. Speed can only be seen if video transmission is needed.

Pin definition:

Pin definition

Pin definition

It can be seen that data transmission mainly consists of TX/RX two sets of differential signals. CC1 and CC2 are the two key pins.

1.        Detect connections, distinguish between positive and negative sides, distinguish between DFP and UFP, that is, master-slave.

2.        Configure Vbus. There are two modes of USB Type-C and USB Power Delivery.

3.        Configure Vconn, when a chip is in the cable, a CC transmission signal, a CC becomes a power supply Vconn.

4.        Configure other modes, such as DP, PCIe when audio accessories are supplied.

There are 4 sources of power and ground, which is why they can support 100W.

how usb type c works

how usb type c works

Don't look at USB Type-C as if it can support the highest 20V/5A, in fact this requires USB PD, and the support for USB PD needs additional PD chips, so don't think that USB Type-C interfaces can support 20V/5A.

Of course, there should be integrated chips in the future.

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

D+ and d- are compatible with the standards before USB.

In this case, USB3.0 has only one set of RX/TX, the speed is 5Gb, and the USB Type-C can be inserted in two groups to ensure that the positive and negative can be plugged, but the data transfer is actually only a set of RX/TX, and the speed has reached 10Gb. If the protocol is upgraded later, the two groups will be 20Gb like DisplayPort.




The DFP (Downstream Facing Port) above is also the main UFP (Upstream Facing Port). In addition to DFP and UFP, there is a DRP (Dual Role port), DRP can do DFP, also can do UFP. When DPR receives UFP, DRP is converted to DFP. When DRP receives DFP, DRP is converted to UFP. When the two DRP are connected, either party is DFP and the other side is UFP.

The CC pin in DFP has a pull-up resistor Rp, and a drop-down resistor Rd in UFP. When unconnected, the VBUS of DFP is no output. After connecting, the CC pin is connected. The CC pin of DFP will detect the drop-down resistance Rd of UFP, indicating that the connection is on, DFP will turn on the Vbus power switch and output the power to UFP. And which CC pin (CC1, CC2) detects the pull-down resistor to determine the direction of the interface insertion, and then switches RX/TX.

Resistance Rd=5.1k, resistance Rp is an uncertain value, according to the previous figure see the USB Type-C have several power supply modes, what to discriminate? Depending on the value of Rp, the value of Rp is different, CC pin detected voltage is different, and then to control the DFP terminal to perform which mode of power supply.

It should be noted that in the above picture, there are two CC. Actually, there is only one CC line in the cable without chip.

The cable containing the chip is not two CC lines, but a CC, a Vconn for power supply to the chip in the cable (3.3V or 5V), at this time there is no drop-down resistor Rd at the CC end, but the drop resistance Ra, 800-1200 Europe.

how usb type c works

how usb type c works

When the CC pin two are connected to the pull-down resistor <=Ra, DFP enters the audio accessories mode, the left and right channels, and Mic are all complete, as shown above.

USB Type-C and DisplayPort, PCIE

USB PD is a signal encoded by BMC, while the previous USB is FSK, so it is incompatible.

USB PD is transmitted on CC pin, and PD has a VDM (Vendor defined message) function that defines the device terminal ID, reads the device that supports DP or PCIe, and enters the alternative mode.

If DFP recognizes device as DP, then MUX/Configuration Switch is switched and Type-C USB3.1 signal is changed to transmit DP signal. AUX is assisted by Type-C's SBU1, SUB2. HPD is the test foot, the same as CC, so share it.

DP has four differential signals of lane0-3, and Type-C has RX/TX1-2, which is also the four set of differential signals. In addition, the alternative mode in the DP protocol can transmit USB signals and DP signals simultaneously, RX/TX1 transfers USB data, RX/TX2 is replaced by lane0,1 two sets of data transmission, and 4K can be supported at this time.

If DFP recognizes device as DP, then MUX/Configuration Switch is switched and Type-C USB3.1 signal is changed to transmit PCIe signal. Similarly, PCIe uses RX/TX2 and SBU1, SUB2 to transmit data, and RX/TX1 transmits USB data.

The advantage is that an interface uses two devices at the same time. Of course, the conversion line can be achieved without any chips.


USB Type-C terminated the long USB plug to plug in the defects, save a lot of time, at least 2S in one direction, 1 billion people around the world to insert a USB every day, 50% probability of inserting the wrong, a total of more than 277000 hours, about 31 years, too horrifying.
One interface has three kinds of audio and video data, and the volume is small. It is foreseeable that the future security machine can be changed to USB Type-C interface, if only USB2.0, only need to redo cable, no chip, the cost can be completely ignored.

As for Thunderbolt, lightning, what should we do? After all, a hundred flowers bloom is the multicolored world.

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