Several common device communication protocols in embedded development are summarized

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Embedded devices need to set parameters during operation , This job is often done by PC Machine to achieve , Need to design a protocol for communication between both parties , There are three representative agreements as follows ：

As can be seen from the table above , Generally, the memory and computing performance of embedded devices are limited , Therefore, fixed binary is the preferred communication protocol .

One ． simplicity

The guarantee agreement is a simple solution , Obscurity often means that implementation is difficult and error prone . The structure of the agreement should be plane , The scope of each domain is clear , The data field shall be designed with fixed length and position as far as possible , The notes are detailed , Documents are clear , Examples abound , Let people get started and understand as soon as possible .

Protocols generally require the following domains ： Frame head , length , The frame type , Destination address , source address , data , check , Frame tail .

Two ． Scalable

It must be ensured that the protocol is still competent after adding functions and changing hardware in the future , This is often achieved by reserving space , The change of agreement should only be an increase in quantity , So as not to cause changes in the protocol structure .

3、 ... and ． Low coupling

Ideally, each protocol packet is atomic information , That is, this agreement package is not involved with other agreement packages , To prevent errors caused by communication frame loss and setting involvement .

Four ． stability

The length of the protocol package is appropriate ： Too small contains too little information , There are many kinds of protocol packages , It is easy to cause communication confusion and implicated errors ; Too big contains too much information , Poor readability , Framing and de framing are difficult , It will also bring the defect that communication is vulnerable to interference , The general protocol length is based on the minimum atomic information .

The protocol must include a verification mechanism , So that the receiver can judge whether the protocol package is received correctly and completely , If there is an error, a better mechanism is needed to ensure successful communication （ Like retransmission ）.

5、 ... and ． high efficiency

Classify the protocol package according to the information type , Such as ： Set network information parameters , Set the current operating parameters , Can be distinguished , Facilitate program processing .

Encoding the same operations into a subset is an efficient means , Such as Read operation , Encoded as 0x0010,Write operation , Encoded as 0x0020.

Data should be designed as isomorphic as possible , If there is a difference , At least put data of the same type together , In this way, the program can make full use of pointers and linear addressing to speed up processing .

6、 ... and ． Easy to implement

Minimize the use of complex algorithms , Such as , The communication link is stable , The check code of the data frame can be determined by CheckSum Instead of CRC. Unless resources are very tight , Otherwise, don't squeeze too much information into one data , Because it will bring poor readability and implementation difficulties .

7、 ... and ． software development

Try to make the hardware ISR Complete the drive work , Don't let “ process ” Participate in complex temporal logic , Otherwise, the processor will stumble and the logic will be complex ！ Such as ：

Receive fixed length data frames , have access to DMA, Every frame received DMA_ISR Send a message to the process . Handle with care DMA Fault anomaly （ The received data frame length is normal, but the data is wrong , The data is the second half of the previous frame + The first half of this frame ）.

Receive data frames of variable length , You can use state machines , When receiving “ End of frame data ” Send a message to the process when . Be careful of data disorder and timeout exception （ In case of data disorder, the state machine needs to be reset in time , Timeouts are generally monitored by timers ）.

8、 ... and . Consider hardware

If the communication link is a high-speed bus （ Such as SPORT Can be up to 100Mbps）, It is generally designed to generate one interrupt per frame , It's triggered by length DMA To achieve , The protocol needs to be designed with a fixed length , See Appendix A. It has high efficiency , But less flexible .

If the communication link is a low-speed bus （ Such as UART commonly 100kbps）, Generally, receiving a byte generates an interrupt , The protocol can be designed into variable length frames , See Appendix B. It has high flexibility , But less efficient .

The figure above shows PC Format of sending data frame , The total length is 64 byte , yes 4 An integral multiple of a byte , Meet the vast majority 32 Features of bit processor structure alignment .

• 0x3C：INT8U, Frame head , Visible characters ’<’

• Len：INT8U, The total data length of this frame , In the figure 4 That is to say 64

• Dst：INT8U, Identifies the name of the target device ID Number

• Src：INT8U, Identify the source device ID Number

• Data：56 Byte storage area , The content depends on the specific communication frame （ Examples are shown in table 2）

• Cmd：INT16U, Category of data frame

• CS：INT8U, For all the data in front of it （62 byte ） Conduct 8 Bit accumulation and check

• 0x7D：INT8U, Frame tail , Visible characters ’｝’

Data Domain data structure instance ：

A method based on variable length format UART Communication protocol example ：

PC And iWL880A（ A wireless communication product , See www.rimelink.com） The communication frame adopts variable length format , As shown in the figure below . Most equipment （ It's common for PC machine ） For reception, use “ A carriage return ” The mechanism is easy to handle , In the agreement Tail Is equal to 0x0D( A newline ).

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