What is CXPI, the Japan-originated Automotive Communication Protocol Standard? Overview and Features Explained

This article describes the features of CXPI, a Japan-originated automotive communication protocol standard, and its advantages compared to other protocols.
It also introduces eSOL's AUTOSAR-compliant AUBIST CXPI products.

What is the Clock Extension Peripheral Interface (CXPI)?

CXPI is a Japan-originated automotive communication protocol standard developed by the Society of Automotive Engineers of Japan for the purpose of reducing wire harnesses (vehicle weight) and multiplexing between devices that connect one-to-one, which are increasingly common in the human-machine interface (HMI) domain.
CXPI is an Abbreviation for “clock extension peripheral interface”

In systems that require responsiveness from switches, sensor inputs, and so on, network communications such as CAN have been introduced because one-to-one connections between input/output devices would increase vehicle weight due to larger wire harnesses.
CXPI was developed as a next-generation subnetwork communication standard that is lighter and less expensive than the two-wire CAN because it uses a single-wire system. It also has responsiveness that cannot be achieved with LIN, which is also a single-wire communication standard.

CXPI Specification Established as ISO Standard in 2020

The CXPI specification was standardized in JASO D015 and SAE J3076 before becoming an international standard in 2020 as ISO 20794.

• PART 2: APPLICATION LAYER
• PART 3: TRANSPORT AND NETWORK LAYER
• PART 4: DATA LINK LAYER AND PHYSICAL LAYER
• PART 5: APPLICATION LAYER CONFORMANCE TEST PLAN
• PART 6: TRANSPORT AND NETWORK LAYER CONFORMANCE TEST PLAN
• PART 7: DATA LINK AND PHYSICAL LAYER CONFORMANCE TEST PLAN
  
  

Six Features

CXPI offers the following six features:

1. Two access methods: event trigger method and polling method
   - The event trigger method uses carrier-sense multiple access with collision resolution (CSMA/CR). Like CAN, this is a non-destructive arbitration method.
   - The polling method is similar to LIN in that the slave side, which is the source of the data, transmits the data with the ID indicated by the master.
   
   
2. CXPI communication can be realized using a UART as well as without a dedicated communication controller.
   
   
3. PWM is used for modulation
   Bit-by-bit synchronization is possible.
   The master node provides a clock to the communication bus and the slave nodes communicate using this clock.
   
   
4. Uses a single-wire bus
   Data including clock components is transmitted and received on a single-wire communication bus. Communication speeds up to 20 kbps are supported.
   
   
5. Can send and receive frames longer than 8 bytes, which is the upper limit in Classic CAN and LIN.
   (Easy to secure communication bandwidth even when data for message validation is added.)
   Data can be sent in frames of up to 12 bytes for normal frames and 255 bytes for burst frames.
   
   
6. Adoption of CRC for error detection
   Ensures communication reliability by adopting CRC.
   
   

Positioning with Other Protocols

LIN is not suitable for systems that require responsiveness because of its periodic transmission based on a schedule by a single master.
CXPI, on the other hand, allows each node other than the master node to send data at any given time.
Compared to LIN, CXPI offers greater flexibility for adding and removing nodes, higher data reliability through CRC checks, and the ability to communicate between slaves.

Comparison of protocols with respect to latency and communication speed

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Comparison of protocols for communication requirements in the HMI domain

*Can be used as functional replacement for LIN

Overview of AUBIST CXPI Provided by eSOL

The AUBIST products from eSOL are AUTOSAR-compliant automotive platform software and related products for automotive ECU products that enable real-time processing technology, high-performance computing, and safe and secure operation.

The AUBIST product line includes "AUBIST CXPI", a functional cluster (FC) that provides the CXPI communication service functions described above.

AUBIST CXPI Aims to Expand and Spread to Sub-Bus with Low-Cost Multiple Communication

Aims and Advantages

• Communication protocol from DENSO│Promotion to OEMs
  *This protocol has been adopted in mass production for TMC vehicles.
• Low-cost automotive LAN protocol
  Replaces hardwired multiplexing or LIN
• Intended for use as a sub-bus for CAN and other sophisticated automotive LANs
  * Combination switches, door switches, seat controls, rain sensors, wiper controls, mirror controls, air conditioner switches, etc.

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Development Concept of AUBIST CXPI

Provided as AUBIST without Affecting other Functions

• Connects to related modules as AUTOSAR Complex Driver (CDD)
• Configuration with AUBIST tool

Package Optimization Considerations

• Allows for resource-saving configuration that can operate without common communication (COM)
• A single package that combines CXPI-specific COM, MCAL, and transceiver drivers in a CXPI stack that is not included in the AUTOSAR CP architecture
• Conforms with relevant OEM-specific specifications
• Functional safety support

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AUBIST CXPI Software Structure

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AUBIST CXPI Function Overview (Basic Functions)re

Conclusion

This article has described CXPI and the AUBIST CXPI products supplied by eSOL.
eSOL has developed these products based on our world-class automotive technologies and know-how accumulated with our customers who have abundant automotive experience, and we provide scalable products that match your ECU development style.
We can make flexible proposals to meet your needs, so please feel free to contact us if you are considering developing automotive ECUs.