ISO 20794-4:2020

INTERNATIONAL ISO
STANDARD 20794-4
First edition
2020-02
Road vehicles — Clock extension
peripheral interface (CXPI) —
Part 4:
Data link layer and physical layer
Véhicules routiers — Interface du périphérique d'extension d'horloge
(CXPI) —
Partie 4: Couches de liaison de données et physique
Reference number
ISO 20794-4:2020(E)
©
ISO 2020

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ISO 20794-4:2020(E)

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© ISO 2020
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ISO 20794-4:2020(E)

Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 2
4.1 Symbols . 2
4.2 Abbreviated terms . 3
5 Conventions . 4
6 Introduction to data link layer and physical layer . 5
6.1 Frames . 5
6.2 Frame collision avoidance . 5
6.3 Error detection and indication . 5
6.4 Clock transmission and detection . 5
7 Service interface parameters (SIP) . 5
7.1 SIP — General . 5
7.2 SIP — Data type definitions . 5
7.3 SIP — Ftype, frame type . 6
7.4 SIP — ReqId, request identifier . 6
7.5 SIP — ReqTypeId, request type identifier . 6
7.6 SIP — PDU, protocol data unit . 6
7.7 SIP — Length, length of PDU . 7
7.8 SIP — ev_wakeup_ind, event wake-up indication (optional) . 7
7.9 SIP — cmd_wakeup_req, command wake-up request . 7
7.10 SIP — NMInfo, network management information . . 8
7.11 SIP — SCT, sequence count . 8
7.12 SIP — Result, result . 8
8 Data link layer (DLL) . 8
8.1 SI — L_Data.req and L_Data.ind service interface . 8
8.2 SI — L_Data.req and L_Data.ind service interface parameter mapping . 9
8.3 DLL — Service interface with L_Ftype parameter mapping .10
8.3.1 DLL — L_Data.req and L_Data.ind with L_Ftype = NormalCom (L_
Length = NULL) .10
8.3.2 DLL — L_Data.req and L_Data.ind with L_Ftype = NormalCom (L_
Length ≥ 0016) . .11
8.3.3 DLL — L_Data.req and L_Data.ind interface with L_Ftype = DiagNodeCfg .12
8.4 DLL — Frame fields .13
8.4.1 DLL — Frame field definition .13
8.4.2 DLL — Request type identifier and request identifier fields.14
8.4.3 DLL — L_FI (frame information) field .15
8.4.4 DLL — L_DATA (data field) .16
8.4.5 DLL — L_CRC field .17
8.5 DLL — Internal operation .19
8.6 DLL — Timing parameters .20
8.6.1 DLL — IBS timing handling .20
8.6.2 DLL — IFS timing handling .20
8.6.3 DLL — Beginning condition of frame .21
8.6.4 DLL — Start of frame .22
8.6.5 DLL — Frame transmission time .22
8.7 DLL — Completion of frame .23
8.7.1 DLL — General .23
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ISO 20794-4:2020(E)

8.7.2 DLL — Completing condition of L_PTYPE field .23
8.7.3 DLL — Completing condition of L_PID field .23
8.7.4 DLL — Completing condition of frame .24
8.8 DLL — Byte arbitration .24
8.9 DLL — Function models .24
8.9.1 DLL — Transmission logic .24
8.9.2 DLL — Reception logic .25
8.10 DLL — Error detection .26
8.10.1 DLL — General .26
8.10.2 DLL — Byte error (Err_DLL_Byte) .26
8.10.3 DLL — CRC error (Err_DLL_CRC) .27
8.10.4 DLL — Parity error (Err_DLL_Parity) .27
8.10.5 DLL — Data length code error (Err_DLL_DLC) .27
8.10.6 DLL — Data length code extension error (Err_DLL_DLCext) .27
8.10.7 DLL — Framing error (Err_DLL_Framing) .28
8.10.8 DLL — Exception handling for L_FI_DLC = '1111 ' detection .28
2
9 Physical layer (PHY) .28
9.1 PHY — Overview .28
9.2 PHY — Concept of waveform generation .29
9.3 PHY — Physical signalling (PS) requirements .30
9.3.1 PHY — PS general .30
9.3.2 PHY — PS physical interface configuration .31
9.3.3 PHY — PS bit rate .31
9.3.4 PHY — PS bit sample timing .31
9.3.5 PHY — PS encoding and decoding logic .32
9.3.6 PHY — PS clock generation .35
9.3.7 PHY — PS node clock synchronization and bit synchronization .36
9.3.8 PHY — PS detection of clock existence .36
9.3.9 PHY — PS bit-wise collision resolution .37
9.3.10 PHY — PS AC parameters.37
9.3.11 PHY — PS node transmission of wake-up pulse .38
9.4 PHY — Physical medium attachment (PMA) requirements .39
9.4.1 PHY — PMA general .39
9.4.2 PHY — PMA electrical parameters.40
9.4.3 PHY — PMA AC parameters .42
9.4.4 PHY — PMA wake-up pulse and dominant pulse filter time .50
9.5 PHY — Physical media dependent (PMD) sub-layer requirements .53
9.5.1 PHY — PMD entity requirements .53
9.5.2 PHY — PMD device interface requirements .53
9.6 PHY — Physical media (PM) sub-layer requirements .54
9.7 PHY — Control and event services .54
9.7.1 PHY — Control and event service interface .54
9.7.2 PHY — Wake-up request .55
9.7.3 PHY — Wake-up event .55
Bibliography .56
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ISO 20794-4:2020(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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committee has been established has the right to be represented on that committee. International
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 22, Road vehicles, Subcommittee SC 31,
Data communication.
A list of all parts in the ISO 20794 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
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ISO 20794-4:2020(E)

Introduction
ISO 20794 (all parts) specifies the application (partly), application layer, transport layer, network
layer, data link layer, and physical layer requirements of an in-vehicle network called "clock extension
peripheral interface (CXPI)".
CXPI is an automotive low-speed single-wire network. It is an enabler for reducing vehicle weight and
fuel consumption by reducing wire counts to simple devices like switches and sensors.
CXPI serves as and is designed for automotive control applications, for example door control group,
light switch, and HVAC (Heating Ventilation and Air Conditioning) systems.
The CXPI services, protocols, and their key characteristics are specified in different parts according to
the OSI layers.
— Application and application layer
— application measurement and control data communication to exchange information between
applications in different nodes based on message communication;
— wake-up and sleep functionality;
— two kinds of communication methods can be selected at system design by each node:
i) the event-triggered method, which supports application measurement- and control-based
(event-driven) slave node communication, and
ii) the polling method, which supports slave node communication based on a periodic master
schedule;
— performs error detection and reports the result to the application;
— application error management.
— Transport layer and network layer
— transforms a message into a single packet;
— adds protocol control information for diagnostic and node configuration into each packet;
— adds packet identifier for diagnostic and node configuration into each packet;
— performs error detection and reports the result to higher OSI layers.
— Data link layer and physical layer
— provides long and short data frames;
— adds a frame identifier into the frame;
— adds frame information into the frame;
— adds a cyclic redundancy check into the frame;
— performs byte-wise arbitration and reports the arbitration result to higher OSI layers;
— performs frame type detection in reception function;
— performs error detection and reports the result to higher OSI layers.
— performs Carrier Sense Multiple Access (CSMA);
— performs Collision Resolution (CR);
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ISO 20794-4:2020(E)

— generates a clock, which is transmitted with each bit to synchronise the connected nodes on the
CXPI network;
— supports bit rates up to 20 kbit/s.
To achieve this, it is based on the Open Systems Interconnection (OSI) Basic Reference Model specified
[1]
in ISO/IEC 7498-1 and ISO/IEC 10731 , which structures communication systems into seven layers.
Figure 1 illustrates an overview of communication frameworks beyond the scope of this document
including related standards:
— vehicle normal communication framework, which is composed of ISO 20794-2, and ISO 20794-5;
[3]
— vehicle diagnostic communication framework, which is composed of ISO 14229-1, ISO 14229-2 ,
[4]
and ISO 14229-8 ;
[6]
— presentation layer standards, e.g. vehicle manufacturer specific or ISO 22901-1 ODX ;
— lower OSI layers framework, which is composed of ISO 20794-3, ISO 20794-4, ISO 20794-6, and
ISO 20794-7 conformance testing.
[4]
ISO 20794 (all parts) and ISO 14229-8 are based on the conventions specified in the OSI Service
[1]
Conventions (ISO/IEC 10731) as they apply for all layers and the diagnostic services.
Figure 1 — ISO 20794 documents reference according to OSI model
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INTERNATIONAL STANDARD ISO 20794-4:2020(E)
Road vehicles — Clock extension peripheral interface
(CXPI) —
Part 4:
Data link layer and physical layer
1 Scope
This document specifies the CXPI data link layer and the CXPI physical layer.
The DLL is based on:
— priority-based CXPI network access;
— non-destructive content-based arbitration;
— broadcast frame transfer and acceptance filtering; and
— node related error detection and error signalling.
The CXPI physical layer (PHY) requirements comprise of:
— physical signalling (PS) sub-layer, which specifies the requirements of the clock generation function,
the encoding and decoding of CXPI frames, and bit-wise collision resolution logic;
— physical media attachment (PMA) sub-layer, which specifies the requirements of the signal shaping
waveform logic;
— physical media dependent (PMD) sub-layer, which specifies the requirements of the CXPI network
termination, electrostatic discharge protection, etc., and device connector requirements; and
— physical media (PM), which specifies the requirements of the CXPI network cable/wiring harness.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO/IEC 7498-1, Information technology — Open Systems Interconnection — Basic Reference Model: The
Basic Model
ISO 20794-2, Road vehicles — Clock extension peripheral interface (CXPI) — Part 2: Application layer
ISO 20794-3, Road vehicles — Clock extension peripheral interface (CXPI) — Part 3: Transport and
network layer
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 20794-2, ISO 20794-3, and
ISO/IEC 7498-1 and the following apply.
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ISO 20794-4:2020(E)

ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
bit rate
transmission speed (bit/s) of the protocol within the specification range of transmit frames
3.3
higher OSI layer
OSI layer 3 or more than higher number of layers
3.4
idle state
state of the CXPI network when no frames are exchanged (idle state) and only the clock exists to
synchronise the nodes
3.5
inter frame space
IFS
time interval between frames on the CXPI network
4 Symbols and abbreviated terms
4.1 Symbols
C total bus capacitance
BUS
C capacity of CXPI network
LINE
C capacity of master node
MASTER
C capacity of slave node
SLAVE
kbit/s kilobit per second
LEN length of CXPI bus wire
BUS
R total bus-resistor including all slave nodes resistors and master node resistor
BUS
R master node resistor
MASTER
R slave node resistor
SLAVE
t bit time
bit
t bit time of reference bit rate
bit_ref
t difference of the dominant time between logical value '1' and logical value '0'
rx_dif_cont
t time that the receiving clock master detects the width of dominant level as the wake-
rx_wakeup_clk
up pulse.
t time that the receiving node detects each width of dominant level in the wake-up pulse.
rx_wakeup
t limitation time of acceptance second dominant pulse in the wake-up pulse from first
rx_wakeup_space
dominant pulse.
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ISO 20794-4:2020(E)

t time that the transceiver node transmits the dominant voltage of the wake-up pulse
tx_wakeup
t interval time between two of dominant level of transmitting wake-up pulse
tx_wakeup_space
t dominant time of logical value '0'
tx_0_lo
t dominant time of logical value '0' (TH = 30 % of V )
tx_0_lo_dom tx_dom SUP
t at the time of logical value '0' outputs, time from the LO level detection of the CXPI
tx_0_pd
network until falling the voltage TH = 30 % of V
tx_dom SUP
t dominant time of logical value '1'
tx_1_lo
t dominant time of logical value '1' (TH = 30 % of V )
tx_1_lo_dom tx_dom SUP
t dominant time of logical value '1' (TH = 70 % of V )
tx_1_lo_rec tx_rec SUP
TH dominant threshold voltage of the driver node
tx_dom
TH recessive threshold voltage of the driver node
tx_rec
TH dominant threshold voltage of the received node
rx_dom
TH recessive threshold voltage of the received node
rx_rec
V voltage of CXPI network
BUS
V centre recessive threshold voltage of the received node
BUS_CNT
V dominant voltage of the received node
BUSdom
V recessive voltage of the received node
BUSrec
V hysteresis voltage between the recessive threshold voltage and the dominant thresh-
HYS
old voltage of the received node
V maximum recessive level of logical value “1”
rec_master
V measured value of the dominant threshold voltage of the received node
th_dom
V measured value of the recessive threshold voltage of the received node
th_rec
4.2 Abbreviated terms
AC alternating current
CRC cyclic redundancy check
CT count
DLC data length code
DLL data link layer
ECU electronic control unit
FI frame information
Ftype frame type
HI high
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ISO 20794-4:2020(E)

IBS inter byte space
ID identifier
IFS inter frame space
ind indicator
L_ link
LO low
LSB least significant byte
MSB most significant byte
NM network management
NRZ non-return to zero
OSI open systems interconnection
PDU protocol data unit
PID protected identifier
PHY physical layer
PM physical media
PMA physical media attachment
PMD physical media dependent
PS physical signalling
PTYPE protected type identifier
PWM pulse width modulation
ReqId request identifier
ReqTypeId request type identifier
RX PMA receiver interface signal
PWM
RXD PS receiver interface signal
NRZ
SIP service interface parameter
TH threshold
TX PMA transmit interface signal
PWM
TXD PS transmitter interface signal
NRZ
5 Conventions
This document is based on the conventions discussed in the OSI Service Conventions as specified in
ISO/IEC 10731.
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ISO 20794-4:2020(E)

6 Introduction to data link layer and physical layer
6.1 Frames
Information on the CXPI network is sent in fixed format frames of different limited lengths. When the
CXPI network is in idle state, any connected node is allowed to start the tran
...