How to upgrade the interface of embedded system from the perspective of USB-C interface design?

Last Update Time: 2021-09-03 10:39:07

RaspBerry 4 Pi model B (Raspberry Pi 4B) has been officially released. It has been upgraded in all aspects from processing power, communication methods, and external interfaces, bringing a boon to embedded developers. After receiving the goods, many developers began to try it with excitement. As a result, it was found that the USB-C interface had serious problems in the design norm.

After actual testing, it is found that the USB-C interface on the Raspberry Pi 4 has CC1 and CC2 connected together and shares a 5.1k resistor to the ground. This design seems to be very clever. The control of the USB-C interface is extremely simple, requiring only a 5.1k pull-down resistor. When the external USB-C Cable is not equipped with Emark chip, it can work normally. Because CC2 of this type of USB-C Cable is floating, only CC1 is connected to the opposite end, so this Cable is connected with the USB-C interface socket of RaspBerry 4B, and it conforms to the design specifications of the Sink side very well. That is, on CC1, there is a 5.1k resistor pulled down to ground.

However, the USB TYPE-C specification also specifies a Cable with an Emark chip. The CC2 of this Cable has a 1K pull-down resistor to inform the CC identification chip on the DFP side that it needs to provide VCONN Source to CC2 . Once connected with such a cable, RaspBerry 4 Pi model B will have serious problems. Because CC1 and CC2 are connected, they will be connected in parallel with the 1K to ground resistor on the Cable to form an impedance that is smaller than the 1K resistor, which meets the USB Adapter specification of Audio Adapter Accessory Mode in the USB-C specification, and is mistaken for the power supply. It is an analog earphone device and thus refuses to supply power.


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The 1k resistor on the Emark connection line will cause CC1 to fail to establish, and the parallel connection of 1k resistor and 5.1k resistor will cause RaspBerry 4B to be considered an Audio Adapter Accessory Mode. The method to solve this problem is also very simple, only need to connect a 5.1K resistor to CC1 and CC2 to ground, independent of each other. You can search for this author's original article "Do you really need a TYPE-C chip" in 2015? This article provides three principles and two implementation methods for judging whether the system needs to use the USB-C control chip.

The design of RaspBerry 4B on the USB-C interface is actually an entry-level design, because this interface is only used for 5V power supply and a USB2.0 communication, without complicated audio and video and USB3.0 functions. In actual embedded development, the function of a USB-C interface may be much more than that. Next, we will explain three points of high-power power supply, high-speed signal transmission, and dual C-port DRP control.

First, you need to use the USB-C interface to obtain a 9V / 12V / 15V / 20V supply voltage. Many embedded systems have very complex functions, and the power supply of only 5V cannot meet the requirements. Then, at this time, it is not enough just to set 5.1k pull-down resistors on CC1 and CC2 separately, but you must use a USB PD control chip, preferably a USB PD control chip that can flexibly configure various voltages, such as LDR6015 LDR6021 can achieve this function. In some system designs, it is even hoped that the USB PD control chip will automatically determine the highest power level of the adapter and allow the power adapter to directly supply the highest power to the embedded system. At this time, the LDR6015Max can be used without any control. Maximum power.

Second, the application development of high-speed video signal transmission needs to use the USB-C interface. USB-C interface can support 10G / b USB 3.1Gen2 data transmission and 4K high-definition video transmission at the same time. But for the Sink to enter DP ALT mode, a USB PD Controller must be used at this time, such as LDR6282. This type of USB PD control chip acts as a traffic manager. Through USB PD communication, the high-speed differential pair path in the USB-C Cable is configured to adapt the data signal and video signal to the appropriate differential pair.

Third, dual C-port DRP function control. Many embedded applications not only use a single USB-C port, but also may have two USB-C ports, one of which is used for power supply, and the other C-port is used for high-speed Data and video signal transmission. However, the user is not sure which of the two ports will be plugged into the power supply or multimedia device, so it needs to meet the dual C port blind plug recognition and control. The most typical application is the display screen and the USB-C interface projector. This is a relatively complicated USB PD control function. Currently only LDR6282 can meet this demand.

In summary, we can see that for an embedded system where the USB-C interface is only used for power supply and Debug function, the USB-C interface does not need to use any chip control, and a 5.1k resistor is independently pulled down through CC1 and CC2. Just go to the ground. For embedded designs that require high-power power supply or high-definition video transmission, a USB PD control chip must be used.

 

If you want to know more, our website has product specifications for USB-C interface, you can go to ALLICDATA ELECTRONICS LIMITED to get more information