Arbitration mechanism of various buses
Principle analysis of various bus arbitration mechanisms , There are many principles on the Chinese Internet , Here are some relatively comprehensive blogs IIC Bus arbitration ,CAN A comprehensive introduction to the bus ,SPI Bus Introduction , among SPI Communicate as a master-slave device , The protocol does not specify the bus arbitration mode .
In short ,CAN The arbitration of the bus is non-destructive , Similar to Ethernet , Belong to CSMA／CA （ Carrier detection , Multi master control , Conflict avoidance ,Carrier Sense Multiple Access with Collision Avoidance） Mode of communication . The principle is that data frames with higher priority , The identifier location has more 0 position , Dominant position , Multiple data frames send out data processing lines ‘ And ’ operation , Keep high priority data frames . The timing comparison between these data frames depends on the initial synchronization bit and the arbitration domain .
Communication requirements of real-time system
Real time system part reference Kopetz, H. (1997) "Real-Time Systems: Design Principles for Distributed Embedded Applications", Springer International Series in Engineering and Computer Science. ISBN 978-0792398943 Download link ：Real-time system principle
- Protocol transmission delay , Including the jitter predictability of the delay itself and the simultaneity under multicast , The delay between arriving nodes should be predictable
- The composability of the system , In this book Kopetz It is emphasized that composability is an important attribute of distributed real-time system ; The testability and real-time performance of the subsystem should be guaranteed in terms of system attributes ; Please see the following quotation for the definition of the original text ：
Composability: An architecture is said to be composable with respect to a specified property if the system integration will not invalidate this property once the property has been established at the subsystem level. Examples of such properties are timeliness or testability. In a composable architecture, the system properties follow from the subsystem properties.
- flexibility , Characterize the need for system customization
- Error detection , Including errors in communication information , Communication node errors and end-to-end authentication （End-to-End Acknowledgment）, A classic certification accident is the accident of Three Mile Island nuclear power plant PORV Not closed , And the indicator light shows the event that has been turned off , Design equipment reliability is a very important principle . Three Mile Island accident equipment cause
- Physical layer architecture design , Bus type design / Circular design , Physical isolation of the smallest replaceable unit （ In order to avoid part of the damage leading to overall failure ）
Network flow control
There are two main ways of flow control ： Set explicit flow control of timeout retransmission mechanism and do not set timeout retransmission , Implicit flow control that sends information according to priority 
Implicit flow control may lead to “event shower” The phenomenon , That is, a large number of event, As a result, there is no time for the data to be processed , Slow response of the system .
Event Trigger & Time Trigger System
overall , There are many trade-offs and design considerations in real-time communication systems , The book introduces the popular network structure of the last century and the comparison between several buses , See the picture below ：
among ,CAN As Event Triggered Protocol , It has the characteristics of event triggered Protocol . Between buses trade-off It is more reflected in the difference between time trigger and event trigger , As shown in the figure below ：
When the communication channel capacity is low and the communication demand is high ,Time Triggered System It can better meet the communication needs .
summary
In order to ensure the real-time performance of the system , It is necessary to design the software and hardware communication protocol so that useful information can be quickly transmitted to the required nodes . Such as the arbitration method and flow control mechanism in the agreement , Make the system achieve high communication efficiency . In a distributed communication architecture , The system delay should be considered at the beginning of the design 、 Reliability and fault tolerance mechanism 、 Information processing efficiency and other factors , Ensure the safety and reliability of the system .
