INTRODUCTIONCXPI is a communication protocol established as a standard by JASO that is currently also undergoing SAE and ISO standardization. Although communication in body systems currently relies on LIN, high expectations are being placed on CXPI as a protocol capable of meeting the stringent (severe) requirements of body system communication. However, EMC resistance is an issue since CXPI uses the same single wire communication as LIN, making it necessary to increase said resistance in the low frequency band used for communication.

The basic functions and characteristics of CXPI, as well as measures to raise EMC performance, are presented in the following sections.

BASIC CXPI FUNCTIONSThe basic functions of CXPI will be described first. This paper limits itself to the fundamentals of the communication protocol. For information on its details, the JASO standards [1][2][3] should be consulted.

CXPI is a dedicated vehicle communication protocol with a transfer rate of up to 20 kbit/s. Its basic characteristics include the use of a single wire connection, the possibility of attaching up to 16 nodes, compatibility with multi-master access using the CSMA/CR (Carrier Sense Multiple Access with Collision Resolution) protocol, and high error detection functionality.

Communication System ConfigurationFigure 1 presents an example configuration for a CXPI communication system that consists of one clock master node with a communication microcontroller, and a slave node with no microcontroller. A low-cost UART (Universal Asynchronous Receiver Transmitter) equipped microcontroller can be used as the communication microcontroller. The clock master node generates a constant clock signal on the bus. Here, the clock signal refers to a PWM (Pulse Width Modulation) waveform of a logical value of ‘1’ which always starts with a falling edge. That falling edge is considered as the clock signal. Using the clock signal transmitted on the communication bus makes it unnecessary to be equipped with an internal high precision oscillator in the slave node. The clock master and slave nodes superimpose the data to transmit with the clock signal for transmission on the communication bus.

Multiplex Communication Protocol for Switch/Sensor/Actuator Network: “CXPI”

Eiji Taki, Yoshiro Hirata, and Yoshifumi OhmoriToyota Motor Corporation

Naoji KanekoDENSO Corporation

Hiroya AndouToyota Motor Corporation

ABSTRACTThe growing functionality and complexity of recent vehicle electronic systems have made inter-device communication (on-board LAN) technology vital to vehicle design. By field of application, the LAN (Local Area Network) systems currently in use are LIN (Local Interconnect Network) used for body systems, CAN (Controller Area Network) used for control systems, and MOST (Media Oriented Systems Transport ) used for multimedia and camera systems, and work to standardize the next-generation communication technology for each of those fields is underway. This paper provides a technical overview of the CXPI (Clock Extension Peripheral Interface) communication protocol, which satisfies the body system requirements (rapid response, system extensibility, high reliability, and low cost). It also presents the progress made on standardization at SAE and other organizations. The current situation of standardization for CXPI is as follows: JASO standard [1][2][3]: May 2014 (1st edition), May 2015 (2nd edition), and May 2016 (final), SAE standard: November 2015 (IR), and ISO standard: May 2015 PWI, May 2016 (NWIP), August 2017 (WD), and August 2019 (IS).

CITATION: Taki, E., Hirata, Y., Ohmori, Y., Kaneko, N. et al., "Multiplex Communication Protocol for Switch/Sensor/Actuator Network: “CXPI”," SAE Int. J. Passeng. Cars – Electron. Electr. Syst. 9(1):2016, doi:10.4271/2016-01-0057.

2016-01-0057Published 04/05/2016

Copyright © 2016 SAE Internationaldoi:10.4271/2016-01-0057

saepcelec.saejournals.org

81

In case the clock master node sends data to the bus, receiving the UART NRZ (Non-Return to Zero) code from the Microcontroller, encoding the data into PWM code, the transceiver sends the encoding data onto the bus.

Figure 1. Example configuration for CXPI communication system.

Bus Access MethodThe communication bus PWM waveform can take one of two logical values, ‘0’ or ‘1’. Figure 2 shows how the PWM waveform is generated.

Figure 2. PWM generation method.

In wakeup mode, the clock master node always transmits a clock pulse as “0” or “1” data to the communication bus. Each node can synchronize for every bit by synchronizing with the falling edge of (1).

When a certain node transmits data “0,” the node outputs a low potential to the communication bus only for a defined time, after it detects a falling edge. When transmitting data “1,” it outputs nothing after it detects a falling edge.

This produces waveform (3) in the communication bus, allowing it to use a UART to transmit the data.

Arbitration ResolutionAs stated earlier, CXPI is a multi-master system implemented with-the CSMA/CR method, supposing the case that collisions between transmitted signals arise. In such cases, arbitration between nodes is performed by the way-that transmission by the node with the higher priority (transmitting a logical ‘0’) shall be continued-and transmission by the lower priority node (transmitting a logical ‘1’) shall be suspended. Figure 3 illustrates how arbitration is resolved.

Figure 3. Arbitration method.

All nodes are synchronized with the clock signal provided by the clock master node. When a collision between logical ‘0’ and logical ‘1’ data occurs on the communication bus, as the dominant signal is received with-priority. Logical ‘ 0’ shall be received with higher priority than logical ‘1’ on the communication bus. The transmitting node shall make a bit-by-bit comparison of the data it transmits and receives. If they match, it wins the arbitration and continues transmitting. Conversely, if they do not match, it loses the arbitration and stops transmitting from the next bit. In the figure, there is a mismatch between transmitted and received data at the second data bit, so transmission stops from the third bit.

Transmission MethodCXPI provides two methods of transmission. According to system requirements, either one method shall be-selected by an implementer.

• Method 1: event-triggered method • Method 2: polling method

Event-Triggered MethodA description of the sequence of operation for event-triggered method follows. When an ECU detects that the bus is idle (no ECUs currently transmitting data), it can transmit a frame (PID field).

Figure 4 provides an example of the event-triggered transmission sequence.

Figure 4. Example of event-triggered transmission.

ECU_A detects that the communication bus is idle and transmits request ID_B. ECU_B, its response is associated with request ID_B, transmits response B. Then, the internal event requesting transmission occurs in ECU_B and ECU_C at the same time. Immediately after the bus becomes idle next time, request ID_B from ECU_B and request ID_C from ECU_C are transmitted at the same time, resulting in a collision on the communication bus. After arbitration, request ID_B with higher priority from ECU_B is transmitted. Responding request ID_B, and ECU_B transmits a response associated with request_B. ECU_C retransmits request ID_C when the bus becomes idle again.

Figure 5 shows the normal frame for event-triggered method.

Taki et al / SAE Int. J. Passeng. Cars – Electron. Electr. Syst. / Volume 9, Issue 1 (May 2016)82

Figure 5. Normal frame in event-triggered method.

A normal frame includes a PID (Protected ID) field and a response field. The PID field is composed of a 7 bit frame ID and 1 bid parity. The response field consists of a 4 bit DLC (Data Length Code), a 2 bit NM (Network Management), a 2 bit counter (CT), up to 12 bytes of DATA, and an 8 bit CRC (Cyclic Redundancy Check). The DLC indicates the number of bytes of DATA in the frame. NM includes two bits of indication flags - wakeup.ind and sleep.ind. The wakeup.ind indidates the first wakeup node by setting ‘1’. The sleep.ind indicates whether the nodes can go to sleep. CT holds sequential number data representing frame continuity. The DATA fields stores data in accordance with the DLC. CRC is used for frame error detection.

Figure 6 shows the long frame for event-triggered transmission. The long frame is a format designed to increase transfer efficiency by allowing up to 255 bytes of DATA per frame.

Figure 6. Long frame in event-triggered method.

It differs from the normal frame in the use of DLC2 (1 byte) to specify data length, and the extension of the CRC field to 16 bits. A DLC (4 bits) with a value of 1111b is handled as a long frame.

Polling MethodNext, a description of the operation sequence for polling method is presented. In polling method, according to the predefined communication schedule, the master node issues two types of request - PID field or PTYPE(Protected TYPE) field. Responding to the request issued by the master node, the master node as well as slave node transmits the response associated with the request.

Figure 7 provides an example of the polling method sequence.

Figure 7. Normal frame in polling method.

After detecting that the communication bus is idle, master ECU_A transmits request ID_B in accordance with the communication schedule, and the corresponding slave ECU_B transmits response B. Before the master node issues next request, the internal event occurs in the slave ECU_B and slave ECU_C triggering request ID_B and request ID_C respectively. Next, a PTYPE field is transmitted by master ECU_A, and the slave ECU_B and slave ECU_C simultaneously transmits the request ID_B and request ID_C respectively on the communication bus. The resulting collision is arbitrated, and the request ID_B (Event) from salve ECU_B is transmitted. A response B(Event) is then transmitted from the corresponding slave ECU_B. Next, master transmits request ID_B (Periodic) requesting slave ECU_B response, and slave ECU_B transmits a response B (Periodic). ECU_C, queuing the event, can not respond until it receives request ID_C or PTYPE fields.

The normal and long frames for polling method are shown in Figures 8 and 9, respectively.

Figure 8. Normal frame in polling method.

Figure 9. Long frame in polling method.

Taki et al / SAE Int. J. Passeng. Cars – Electron. Electr. Syst. / Volume 9, Issue 1 (May 2016) 83

The difference with event-triggered method lies in the addition of a PTYPE field in the header. The PTYPE field is used by the master node to permit slave nodes to freely transmit a PID field.

Wakeup/Sleep ControlCXPI supports wakeup/sleep control. Figure 10 presents mode management for the master and slave ECUs.

Figure 10. Mode management.

ECU mode management includes 3 modes.

1. Sleep mode is a low power mode where the oscillator circuit is shut down and no PWM encoding or decoding is performed. However, since the ECU needs to be woken on receiving a wakeup pulse, input and output is carried out in NRZ form without PWM encoding or decoding.

2. Standby mode is the wakeup transition handling mode, where the master transmits a clock signal and a slave transmits a wakeup pulse.

3. Normal mode is the state where transmission and reception are ready to go, with the restriction that slave nodes can transmit data only when receive an arbitrary PID.

Wakeup Sequence (Triggered by Slave ECU)The wakeup sequence triggered by a slave ECU is described below. After detecting a wakeup event, slave ECU_B transmits a wakeup pulse on the communication bus. Master ECU_A transmits a clock signal on the communication bus after receiving the wakeup pulse. The slave ECUs enter standby mode after receiving the wakeup pulse, and enter normal mode when they receive the clock signal. Master ECU_A transmits an arbitrary PID, and slave ECUs can transmit data after receiving that PID.

Figure 11 provides an example of the wakeup sequence triggered by a slave ECU.

Figure 11. Slave ECU-triggered wakeup sequence.

Wakeup Sequence (Triggered by Master ECU)The wakeup sequence triggered by the master ECU is described below. After detecting a wakeup event, master ECU_A restarts transmission of a clock signal on the communication bus and enters normal mode. The slave ECUs transit into standby mode triggered by the clock signal detection transmitted by master ECU_A, stay in standby mode receiving the clock signal. Master ECU_A transmits an arbitrary PID, and slave ECUs in standby mode receives the PID and get into normal mode. The slave ECU_B, corresponding the PID (request ID_B), transmits the data (respons_B).

Figure 12 provides an example of the wakeup sequence triggered by the master ECU.

Figure 12. Master ECU-triggered wakeup sequence.

Sleep SequenceThis section describes the sleep sequence for ECUs connected to the CXPI bus. The master ECU_A can transmit a sleep frame on condition that the sleep conditions defined by the system requirements are satisfied and that none of slave ECUs have responded with “sleep inhibited” frame during some specified set period. After receiving its own sleep frame, master ECU_A stops the clock signal and enters sleep mode.

After receiving the sleep frame, the slave ECUs prepare for, and then enter, sleep mode.

Figure 13 provides an example of the sleep sequence.

Figure 13. Sleep sequence.

BENEFITS OF CXPITable 1 lists the characteristics (in terms of performance and cost) of the CXPI, CAN and LIN protocols for communication in body systems.

Taki et al / SAE Int. J. Passeng. Cars – Electron. Electr. Syst. / Volume 9, Issue 1 (May 2016)84

Table 1. Characteristics of protocols for communication in body systems.

Performance Considerations

ResponseThe multi-master approach used in CXPI allows all ECUs to transmit frame by event triggered, giving it superior response time compared to the master/slave approach used in LIN.

As a sample of ‘control response time’ calculations, Tables 2 and 3 compare the average response time and frame length for CXPI and LIN systems set up with 5 nodes and a total of 11 frames (5 of which included events) for each system. The sample calculations show that response is approximately 6 times faster with CXPI than with LIN.

Table 2. Frame lengths in CXPI and LIN.

Table 3. Response times in CXPI and LIN.

ExtensibilityCXPI uses the same CSMA/CR method as CAN, making it easy to add or remove functions. For example, in case of data security improvement is required, total data length becomes much larger for adding security layer without changing the existing control data. With CXPI, that is a simple matter because CXPI can transmit up to 255 bytes and handle large amounts of data.

ReliabilityCXPI is a highly capable system with the ability to detect errors up to 3 bits.

Cost ConsiderationsCXPI uses a single wire transmission line, lowering its cost compared to the CAN twisted pair wires. In addition, providing a clock signal in the bus makes it unnecessary to install oscillators in the slaves. (CAN requires oscillators. LIN systems that implement communication between slaves also require high-precision oscillators.)

The above shows that CXPI has the potential to achieve a lower cost than CAN or LIN.

IMPROVEMENT IN EMC PERFORMANCEOne special characteristic of the CXPI physical layer is connecting nodes using single wire same as LIN. This makes EMC (Electromagnetic Compatibility) performance issue more serious than a differential transceiver on twisted pair lines of CAN.

The CXPI physical layer is also characterized by the constant output of a clock signal in the bus by the master node when frame transmission is available, which creates issues in terms of emission performance.

Consequently, CXPI requires reducing emission noise, especially in the low frequency band used for communication. The two approaches taken to resolve those issues are the control of voltage characteristics and control of current characteristics.

Details of those measures are presented after the description of the basic behavior of the master and slave nodes during PWM waveform generation, using an open collector circuit as an example.

Figure 14. Generation of PWM waveform.

Figure 14 shows the output waveforms for the master and slave nodes, as well as the composite waveform they form on the bus. The next section presents the behavior of the physical layer for sections (1) to (6) of the bus waveform before any measures were applied.

Taki et al / SAE Int. J. Passeng. Cars – Electron. Electr. Syst. / Volume 9, Issue 1 (May 2016) 85

Behavior of the Physical Layer (before Measures Applied)

Cases (1) and (3) (Figure 14)Figure 15 shows the behavior of the physical layer for states (1) and (3) from Figure 14.

Figure 15. Physical layer behavior for states (1) and (3).

The master node transceiver is turned on, and the slave node transceiver is turned off. Current flow is from power supply to the master node ground via the master node pull-up resistance. Consequently, no current flows in the communication bus, setting the bus logical value to Lo.

Cases (2) and (6) (Figure 14)Figure 16 shows the behavior of the physical layer for states (2) and (6) from Figure 14.

Figure 16. Physical layer behavior for states (2) and (6).

The master and slave node transceivers are both turned off. Consequently, no current flows in the communication bus, setting the bus logical value to Hi.

Case (4) (Figure 14)Figure 17 shows the behavior of the physical layer for state (4) from Figure 14.

When both the master and slave nodes are on, assuming the slave side is lower due to the electric potential of the collector transceivers, the master-side pull-up resistance sends current to the slave via the communication bus. If the master-side collector potential is lower, current flow is the same as for states (1) and (3). In short, the direction of the current flow varies according to the individual transceivers and the vehicle environment.

Figure 17. Physical layer behavior for state (4).

Case 5 (Figure 14)Figure 18 shows the behavior of the physical layer for state (5) from Figure 14.

Figure 18. Physical layer behavior for state (5).

If only the slave-side transceiver is on, current flow is the same as for state (4).

Summary of Cases (1) to (6) (Figure 14)Summarizing the above, Table 4 indicates the presence of current in the communication bus.

Table 4. Presence of current in the bus.

The two current control measures described below were taken to reduce the amount of noise generated when current flows in the communication bus (columns 4 and 5 in the table).

Measures Applied to the Physical Layer

Approach 1 (Current Control Applied to Column 4 in the Table)Given that the master cannot determine the timing of the slave-side transceiver turning on, the slave-side ground potential was raised to prevent the flow of current in the bus, even with individual differences in the elements and the vehicle environment taken into account. This made it possible to consistently avoid current flow of current in the communication bus under state (4).

Taki et al / SAE Int. J. Passeng. Cars – Electron. Electr. Syst. / Volume 9, Issue 1 (May 2016)86

Figure 19 shows the behavior of the physical layer for state (4) from Figure 14 after this measure was applied.

Figure 19. Post-measure physical layer behavior for state (4).

Approach 2 (Current Control Applied to Column 5 in the Table)A current control circuit was added to the master-side drive circuit since master-side pull-up resistance makes current flow toward the slave side via the communication bus when the master transceiver switches from on to off. For state (5), this led to controlling the amount of current fluctuation in the communication bus from the master side.

Figure 20. Flow of current for state (5) before applying the measure.

Figure 21. Flow of current for state (5) after applying the measure.

Figure 22. Voltage and current waveform in Approach 2.

The results of measurements based on the above current control measures are presented below.

Measurement ResultsCompliance with the CISPR-25 [4] emission test (Class 5 standard) through the application of the current control measures in Approaches 1 and 2 was confirmed.

The results of the conducted emission test (using a CISPR-25 current probe) and radiated emission test (CISPR-25 ALSE) are shown in Figures 23 and 24, respectively.

Figure 23. Results for conducted emissions (CISPR-25).

Figure 24. Results for radiated emissions (CISPR-25).

STATUS OF STANDARDIZATION ACTIVITIESCXPI was proposed for standardization by JSAE in May 2012, and the use cases definition [1], the protocol specification [2], as well as the protocol conformance test specification [3] were debated and issued between then and May 2015. Since then, discussions on the diagnostic communication specification have begun, with that specification scheduled to be established in May 2016.

Taki et al / SAE Int. J. Passeng. Cars – Electron. Electr. Syst. / Volume 9, Issue 1 (May 2016) 87

At the same time, SAE agreed to engage in CXPI standardization activities in November 2013 and the CXPI TF started deliberations in March 2014. The J3076 Information Report (IR) was issued in October 2015, and work to issue the J3076 Recommended Practices (RP) is currently underway.

Moreover, a CXPI overview briefing was presented to the ISO (ISO/TC 22/SC 31) in October 2014, and a preliminary work item (PWI) was registered in May 2015 (ISO/PWI 20794). A new work item proposal (NWIP) is scheduled for May 2016, and an international standard (IS) should be issued in August 2019.

Figure 25. Schedule for standardization activities.

SUMMARY AND CONCLUSIONSCXPI meets the requirements for communication in body systems and also represents a promising technology that will contribute to reducing weight and saving space by making the wire harness smaller. Gradual deployment in vehicles is being planned. In addition, SAE and ISO are moving forward with CXPI standardization, and JAMA (Japan Automobile Manufacturers Association) is currently engaged in promoting the spread of CXPI. CXPI is introduced into a new system of the multiplexing communication and will be used together with CAN and LIN in future.

REFERENCES1. Society of Automotive Engineers of Japan, Inc. (JSAE), Automobiles

- Clock Extension Peripheral Interface (CXPI) -Part 1: General Information and Use Cases Definition, JASO D 015-1:2015, Version submitted at the div. meeting in Nov. 2014

2. Society of Automotive Engineers of Japan, Inc. (JSAE), Automobiles - Clock Extension Peripheral Interface (CXPI) -Part 3: Protocol Specification, JASO D 015-3:2015, Version submitted at the div. meeting in Nov. 2014

3. Society of Automotive Engineers of Japan, Inc. (JSAE), Automobiles - Clock Extension Peripheral Interface (CXPI) -Part 5: Protocol Conformance Test Specification, JASO D 015-5:2015, Version submitted at the div. meeting in Nov. 2014

4. International Electrotechnical Commission.(IEC), Vehicles, boats and internal combustion engines - Radio disturbance characteristics - Limits and methods of measurement for the protection of on-board receivers, CISPR 25 Edition 3.0

CONTACT INFORMATIONEiji TakiToyota Motor Corporation, E/E Architecture Dev. Div.eiji_taki@mail.toyota.co.jp

Hiroya AndohToyota Motor Corporation, E/E Architecture Dev. Div.hiroya_andou@mail.toyota.co.jp

ACKNOWLEDGMENTSThanks to all parties involved for making this paper possible.

DEFINITIONSsingle wire - the use of only one wire to connect ECUs

multi-master - communication protocol that allows transmission from any node when bus communication is available

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE International.

Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE International. The author is solely responsible for the content of the paper.

Taki et al / SAE Int. J. Passeng. Cars – Electron. Electr. Syst. / Volume 9, Issue 1 (May 2016)88