Data structure overview
As shown in the figure above, the PD data includes preamble (preamble), sop, message, CRC, EOP. Only the message belongs to the protocol layer, and everything else is the physical layer.
So focus on the message.
See the figure below for specific functions:
PD message
Messages are divided into three categories: control messages, data messages, and extended messages.
Control message: contains a header data header.
Data message: contains a header data header + 0~7 objects.
Extended message: contains a herder data header + an extended message header + 0~6 objects (or up to 260 bytes of data) .
The above is the structure of the message contained in the PD. As shown below:
Control message
The subject of the control message is a haeder, which is also something that both data messages and extended messages have.
Then the header contains the following content:
bit15 indicates whether it is an extended message;
bit14~12 indicates the number of objects, and the control message has no object, so here are all 0;
bit11~9 indicate the message ID, which means It is the number of successful communications, that is, the number of goodcrc received, the maximum is 7.
When bit8 is used for end-to-end communication (source and sink), this bit represents the power role, and the source role is still the sink role. If it is communicating with a cable cable, this bit indicates the role of the cable.
Bit7~6 indicate the PD version.
Bit5 is similar to bit8. If it is only end-to-end communication, it means data role (DFP/UFP). If it is a cable, this bit is reserved.
Bit4~0 indicates the message type. This type does not refer to control messages or data messages. This refers to the type of control messages. For example, if the control message has goodcrc and accept messages, then bit4-0 indicates these, as shown in the following figure;
Control There are many types of messages, and what we can often see is the content of the above picture.
goodcrc indicates that the data packet sent from the other port is received.
Accept means to accept the other party's request.
psd_rdy indicates that the power supply on the source side is ready, the voltage and current limit have been adjusted, and the sink side can start to draw power.
Everything else is shown in the picture above, which means it literally.
This is the control message. More control messages can be found in the PD3.0 standard file, or you can leave a message.
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