How to learn the protocols of CAN, PCI, USB, IIC, SPI and Ethernet?

The so-called agreement is the rules or regulations that the two must abide by in their communication.

To look at a protocol, we must understand the data format of the data exchanged by this protocol, why it is exchanged, what each bit means, and then the timing, chip selection, etc. So to look at a protocol, we must first understand the format of the frame code. Don't look at the CAN protocol yet, which is not suitable for beginners, because different layers of CAN protocol are different, which is very troublesome. Just look at SPI, IIC, or MODBUS protocol. These are relatively simple, and it is almost the same to understand the data structure and timing. This also takes time to accumulate, so don't rush for success.