How do the power adapter and the phone communicate during fast charging?

Abstract: Consumer electronics have entered the era of fast charging, so how do the power adapter and the mobile phone communicate during fast charging? Today, we will take you to understand fast charging from the protocol level and appreciate the secrets of fast charging protocol!

Introduction to Fast Charge Protocol

Fast charging, as the name implies, is to charge the phone quickly. Through software/hardware technical means, the voltage and current input values ​​of the phone are adjusted, thereby shortening the charging time of the phone and breaking the traditional 5V/1A mode. There are three ways to increase the charging speed: keep the current unchanged and increase the voltage; keep the voltage unchanged and increase the current; increase both the voltage and the current.

With the expansion of market demand, the three modes of high voltage constant current, low voltage high current, and high voltage high current have appeared and improved on the market. For these three modes, each manufacturer has different choices, so a variety of fast charging protocols have been derived. The more mainstream ones are PD fast charging, Qualcomm's QC2.0/3.0, MediaTek's PE protocol, OPPO and vivo's flash charging, Huawei's SCP, etc. In order to standardize the fast charging standard, USB-IF (USB Standardization Organization) released an important update of USB PD 3.0, aiming to unify the fast charging technical specification scheme, and does not allow the USB interface to adjust the voltage through non-USB PD protocols. Google also announced that the fast charging protocol carried by Android7.0 and above mobile phones must support the PD protocol, which has accelerated the unification of PD in the fast charging protocol world. Let's take USB-PD as an example to show you a comprehensive understanding of mobile phone fast charging.

USBPD charging principle

USB-Power Delivery (USBPD) is one of the current mainstream fast charging protocols developed by the USB-IF organization. It can increase the current default maximum power of the type-c interface of 5V/2A to 100W. It can also perform bidirectional and even networked power transmission, and has a system-level power supply solution.

Figure 1 USB PD communication cable

USBPD communication performs half-duplex communication through modulation (24MHz) of the AC-coupled FSK signal on VBUS, thereby realizing the charging process of the mobile phone and the charger.

The SOURCE end and SINK end represent the adapter end and the SINK controller of the chip inside the mobile phone respectively. From the perspective of USB communication transmission, they can be understood as USBHOST (master device) and USBOTG (slave device).

When the cable is connected, the SOP communication of the PD protocol begins on the CC line (type-c interface communication configuration channel) to select the power transmission specification. This part is done by the SINK end asking the SOURCE end for the power configuration parameters that can be provided (5V/9V/12V/15V/20V).

Figure 2. Type-c system charging principle block diagram including USBPD protocol

Taking the 9V charging of the mobile phone and the adapter as an example, the overall process is as follows:

The USB OTG end (slave device: adapter end) monitors the voltage status on VBUS. If there is a 5V voltage on VBUS and the OTG ID pin is detected to be a 1K pull-down resistor, it means that the cable supports USBPD communication, and the communication process begins.

Figure 3 PD communication waveform level change

The SINK end initiates SOP (starting segment), starts the USBPD device manager on the SOURSE end, and applies for the specification information that the SOURSE end can provide;

The SOURCE end replies with a list of specifications that can be provided, that is, the message is parsed according to the USBPD specification to obtain a list of all voltage and current pairs supported by the adapter;

The SINK terminal replies with the selected voltage specification, that is, it selects a voltage and current pair and sends a corresponding request along with the required current parameters;

The SOURSE adapter receives the request after internal decoding and conversion, adjusts the adapter output, and raises the VBUS cable from 5V to 9V;

After the mobile phone receives the message, the SINK end will adjust the charging voltage and current, and wait until the VBUS cable on the SOURSE end reaches 9V and reaches stability for charging;

During the charging process, the mobile phone can dynamically send messages to request the charger to change the output voltage and current, thereby achieving a fast charging process.

USB PD protocol analysis plan

The communication code of the PD protocol is Bi-phase Mark Coded (BMC), and communication is carried out through the CC pin, as shown in the figure.

Figure 4 BMC communication cable

BMC code is a single-line communication code. The transmission of data 1 requires a switching process between high/low levels, while the transmission of 0 is a fixed high or low level. Each data packet contains a 0/1 alternating preamble. All PD transmission processes start with SOP Packet, start code (SOP), message header, data bit, CRC and end code (EOP).

Figure 5 PD transmission data

BMC-encoded communication, starting from the test node of the data stream, can be analyzed using an analyzer or directly decoded using an oscilloscope with protocol decoding function to capture each data packet and obtain the message parameters of the data packet.

Figure 6 Protocol Planning

As shown in the figure, the PD communication waveform on the CC pin is obtained by using an oscilloscope at the test node. It can be seen that the BPD protocol has more bits and the decoding is more complicated. However, through the protocol decoding function of the oscilloscope, the complete message can be quickly decoded in a short time, which greatly improves the work efficiency and intuitive experience of engineers.

Figure 7 ZDS oscilloscope USBPD decoding

Figure 8 Voltage raising process under PD protocol control

Figure 9: Analyzing the decoding protocol of each section of PD using dual ZOOM mode

Currently, the ZDS series oscilloscopes not only support USBPD protocol decoding, but also support QC2.0/3.0 protocol decoding, which can meet the decoding requirements of the current mainstream fast charging protocols. In addition, with its large storage mechanism of up to 512M, it can support ultra-long decoding to restore the real waveform and fully monitor the communication process. It also has a dual ZOOM analysis function, which can use the main time base to capture the waveform that needs statistical data, locate the characteristic value of a period of time through Zoom1, and then zoom in on the waveform details with Zoom2 to observe instantaneous signal changes, greatly improving the testing convenience of engineers.