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APIX: High Speed Automotive Pixel Link

Markus Roemer

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Automotive Design Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Ground Offset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Electromagnetic Emissions and Immunity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Cable Characteristics and Aging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

APIX Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

AShell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

CML Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Optimized Chip Design for EMI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Signal Conditioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Lowering Emissions and Transmission Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Diagnostic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Summary and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Abstract

With the increasing demand on driver information, multimedia content, and

even Internet connectivity, displays and video signaling are receiving increasing

attention in the automotive industry. The requirement of transmitting video

signals includes applications like infotainment displays, dashboard and head-

up displays, and also driver assistance systems that require real-time video

streams.

A car environment has specific challenges and requirements that need to be

considered for video transport in terms of system design. This chapter provides

an overview of common challenges the designer of automotive display and

M. Roemer (*)

Inova Semiconductors GmbH, Munich, Germany

e-mail: [email protected]

# Springer-Verlag Berlin Heidelberg 2015

J. Chen et al. (eds.), Handbook of Visual Display Technology,DOI 10.1007/978-3-642-35947-7_41-2

1

mailto:[email protected]

camera applications needs to deal with. On the basis of the architecture of the

automotive pixel link (APIX) technology, the article first explains the basic

concepts of high-speed video transmissions and then focuses on considerations

and mechanisms to overcome issues involved in these.

List of Abbreviations

AGC Automatic gain control

APIX Automotive pixel link – High-speed serial interface standard devel-

oped by Inova Semiconductors GmbH

APIX2 2nd generation of APIX – High-speed serial interface standard

developed by Inova Semiconductors GmbH

AWG American wire gauge – Standardized wire gauge system used for

the diameter of wires

CID Central information display

CML Current mode logic

CMOS Complementary metal oxide semiconductor

DAB Digital audio broadcasting

DFE Decision feedback equalizer

DPI Direct RF power injection method

DVB Digital video broadcasting

DVI Digital video interface

EMC Electromagnetic compatibility

EMI Electromagnetic interference

FIR Finite impulse reponse

Gbit/Mbit Gigabit/Megabit (transmission speed)

GPS Global positioning system

GSM Groupe special mobile

IC Integrated circuit

LVDS Low-voltage differential signaling

PCB Printed circuit board

PLL Phase locked loop

RF Radio frequency

STP Shielded twisted pair

TEM Transverse electromagnetic cell

VDA Verband Der AutomobilindustrieVIA Vertical interconnect access – Used on PCBs to create through-

connections

Introduction

Since the last 20 years, the added value through electronics in cars has increased to

around 25 % and is forecast by the Verband der Automobilindustrie (VDA) to

further increase to around 40 % in 2015. The main innovation steps have, of course,

been in safety features like air bags, traction control, and braking control. Further,

2 M. Roemer

the driver is surrounded by sensors and cameras, monitoring the status of the car and

the environment in all kinds of situations, and assisting in parking and as navigation

systems or even managing the car in critical situations like lane departures.

The automotive pixel link (APIX) technology has been specifically designed to

address the different requirements for video and data transmission in automotive

applications. The latest technology standard APIX2 offers the ability to combine

real-time video data for up to two video streams, a full-duplex communication

channel for data or Ethernet, GPIO, and audio over a single cable. With the

transmission speed of 3 Gbps the technology supports the requirement for high-

resolution displays at maximum quality but also opens new challenges for the

complete transmission path in terms of cable characteristics and aging effects.

The APIX2 transmitter and receiver circuits incorporate mechanisms to optimize

the output driver for the given signal path at lowest EMI, and to ensure the operation

of the application over product lifetime.

Automotive Design Challenges

In comparison with video transmission standards used in consumer products (e.g.,

DVI), the video link used in car environments has to meet additional or more

stringent requirements. The technology needs to offer high-speed transmission over

a distance of up to 10 m but also the ability to be used at just 50 cm as on a

dashboard; it needs to be robust against electromagnetic emissions from mobile

phones or radios, and it needs to be designed for low emissions so as not to disturb

the surrounding environment.

With the growing demand for car manufacturers to reduce weight to meet the

regulatory requirements for emission, the transmission technology must be able to

provide maximum data rates for multiple services at minimum cabling effort.

The APIX2 technology combines multiple services at just two pairs of wires

offering a gross data rate of 3 Gbps. Sender and receiver incorporate features and

mechanisms to address the challenges of ground offset and electromagnetic com-

patibility (EMC) but also to compensate the tolerances and aging effects of the

cable and the PCB design specifically caused by the high frequency requirements of

the link.

Ground Offset

A critical challenge for electronic design in cars is the common ground. Since a car

is a “nongrounded” system, a typical approach is to use the car chassis as the

common ground for all electronic equipment. Therefore, only positive supply is

brought to the equipment; the ground connection is done locally to the chassis.

However, with the long ground distance between different devices and the

devices to the battery, the ground voltage level for the different components may

show a significant difference of up to several volts. The differences can be caused

APIX: High Speed Automotive Pixel Link 3

through different resistive circumstances for the equipment to the battery path as

well as local, high dynamic currents, for example, caused by control units or by

electric motors.

This ground offset may have a significant impact on systems with analog-to-

digital conversion like sensors or camera systems, requiring a stable reference for

the conversion. In the case of high-speed video interfaces, the ground offset may

have an impact on the clock and data recovery after the transmission.

Electromagnetic Emissions and Immunity

The area of EMC is one of the most challenging aspects in systems designs for the

automotive environment. The growing number of electronic or electromechanical

devices also increases the risk of electromagnetic interference (EMI).

Modern cars include a number of devices, each requiring highly sensitive

receivers for proper functionality. These include navigation systems (global posi-

tioning system, GPS), digital radios and televisions (DAB, DVB), or mobile phone

units (GSM). Due to high sensitivity, the level of acceptable emissions for the

automotive environment is well below the requirements specified for consumer

electronic devices. Table 1 illustrates the level of typical emission limits in the

automotive environment compared with the limits defined by the CE regulations

(Schwab 1996). Regulations valid for the automotive environment are, for example,

CISPR25 or EN55025, defining requirements for systems that are used in cars. For

example, CISPR25b defines a strip-line test, which verifies the emission of the

transmission line.

Critical sources of EMI are devices that require very high currents and, there-

fore, generate electromagnetic fields, for example, starter motors, comfort systems

like electric window lifts, electrical seat adjustment mechanisms, or seat-heating

elements. Another source of EMI is the board design, which may cause EMI by

parallel bus switching at the same clock rate and, therefore, adding up noise for one

or multiple specific frequencies (switching noise). Long traces, ground loops, or

oscillation circuits like PLLs, either on the board layout or even within the chip

itself, may also cause radiations.

Therefore, in addition to the above system tests, semiconductors need to be

tested at the device level to measure emissions and the immunity of the chip itself.

Figure 1 shows an example of the 150-Ω test setup as defined by IEC 61967-4

(2006-07), specifically testing the signal at a device output. The setup uses a 150-Ωantenna, which represents the emission characteristics of a typical cabling network.

The test procedure measures the emissions from the antenna on a spectrum ana-

lyzer. Another common test is the TEM cell test defined by IEC 61967-2, which

verifies the emission of the chip in an isolated chamber (IEC 61967-2 2005-09). The

TEM cell is also used to measure the immunity of the device.

In terms of immunity, the geometries of chip architectures typically are too small

to act as antennas for the reception of radio energy. Geometries more likely to be

affected are the traces or wires connected to the pins. Therefore, immunity tests as

4 M. Roemer

described by the IEC 62132 verify the immunity of the IC against RF energy, which

is brought in through the pins. The test as described in the IEC 62132-4 (also known

as DPI, direct RF power injection test) induces a frequency at a probe point on the

PCB, which is directly connected to the pin (IEC 62132-4 2006-02).

Especially the immunity tests show that EMI is not just a chip or a system

problem; it needs to be considered for all parts of a design, as every component,

trace, or even mechanical part may act as an antenna or as part of an oscillating

circuit.

Table 1 Comparison of consumer and typical automotive emission limits

RF noise level (dB μV) RF noise voltage (mV)

0 0.0010

3 0.0014

6 0.0020

Automotive limits 10 0.0032

15 0.0056

20 0.0100

30 0.0316

CE emission limits 35 0.0562

40 0.1000

45 0.1778

50 0.3162

60 1.0000

Items in bold represent the typical emission levels

Fig. 1 150-Ω emissions test as defined in the IEC 61967-4 ed.1.1 (Copyright # 2006 IEC,

Geneva, Switzerland. www.iec.ch)


http://www.iec.ch/

Cable Characteristics and Aging

A high-speed transmission system strongly relies on the quality of the signal path. A

typical APIX2 physical layer implementation consists of two twisted (differential)

pairs of a cable with 100 Ω impedance each. Figure 2 shows the complete signal

path for such a link which includes the transmission line design at the PCB, the

connectors, and the cable including in-line connectors. Especially for a transmis-

sion of 3 Gbps, the whole signal path needs to be designed for continuous 100 Ωimpedance. Any point of mismatch will cause signal reflections, which affect the

signal quality and with this result in bit errors.

At PCB level the quality of the signal path is influenced by the routing of the

transmission line and the selection and placement of components. See also chapter

“▶Biometrics and Recognition Technology” for further details on the design.

The next part of the signal path is the cable, including the connectors and

potential in-line connectors. This complete system needs to meet certain require-

ments in terms of characteristics like insertion loss or return loss.

These cable characteristics may change due to environmental influences like

temperature or mechanical stress. As a result, twisted pair cables are typically spec-

ified for certain frequency ranges and provide detailed information of insertion loss,

return loss or cross-talk over cable length, temperature, and a simulated aging period

(Fig. 3).

Cable length as such is not a specific design challenge on automotive applications;

however, it shall be discussed in this article, as the automotive environment requires

high flexibility in this area. Centralized systems like head units need to be able to

send or receive the video data in various distances. Assuming the head unit some-

where behind the instrument cluster, the interface technology should be able to serve

the Central Information Display (CID), displaying the radio menu or navigation

screen right above the head unit, as well as acting as providing the content to the

rear-seat displays. The difference therefore can be anywhere from 30 cm to 10 m.

Due to the requirement to support different cable lengths in combination with the

requirement to compensate for the different cable characteristics and aging effects,

the physical layer of transmitter and receiver need to offer the flexibility to adjust

PCBConnector

APIX2Tx

In-LineConnector

TwistedPair CablePCB

TransmissionLine

Cable Assembly

APIX2Rx

Fig. 2 High-speed transmission system

6 M. Roemer

Biometrics and Recognition Technology

the driver for different cable lengths but in the same way to offer enough margin to

ensure reliable video and data transmission over product lifetime.

APIX Technology

Architecture

The APIX architecture is designed to act as a single interface to a display or to a

remote digital camera solution, offering uncompressed, real-time video and data

communication over one cable. In order to support high transmission speeds at

distances of up to +10 m (3 Gbps) and up to +40 m (500 Mbps mode), the data are

serialized and transmitted via the current mode logic (CML) technology.

With the growing demand for higher video resolution, the APIX technology has

evolved from 1Gbps of downstream link in the first generation (APIX1) to up to

3 Gbps downstream link in the second generation (APIX2). The next generation for

the technology is already planned to support up to 6Gbps over a single pair of wires.

The APIX technology is a multichannel packet oriented architecture, which

allows independent transmission of several channels with different requirements

in bandwidth, integrity, and latency. APIX1 offers a high-speed downstream pixel

channel for up to 840 Mbps net video data rate and a side band channel for 26 Mbps

for communication data. The pixel channel and the downstream channel are

multiplexed and commonly transmitted over the downstream link (Fig. 4).

In upstream direction, the link offers 20 Mbps for communication data. The

Fig. 3 Example for the cable requirements for a 3 Gbps transmission (Inova Semiconductors

2014a)


communication data is implemented as two pins in each direction, which are

sampled at either side and transmitted to the other at lowest latency.

Offering up to 3 Gbps gross downstream bandwidth, APIX2 has been enhanced

to support multiple independent video streams in combination with GPIO, audio,

and data communication. In addition, the link can be used to directly establish a

100 Mbps Ethernet channel between sender and receiver.

The data communication of the link is based on the so-called Automotive Shell

(AShell), providing error-free transmission of application data (see chapter

“▶AShell”). The GPIOs are asynchronously sampled and transferred at lowest

latency.

In summary, the APIX2 products support the following features:

• Two independent video channels with up to 2.59 Gbps net data rate

• 8-channel audio support up to 30 Mbps

• Full-duplex 100 Mbps Ethernet

• Protected data channel using the AShell protocol

• Low-latency GPIO functionality (Fig. 5)

Fig. 4 APIX1 transmitter and receiver system

Fig. 5 APIX2 transmitter and receiver system

8 M. Roemer

AShell

APIX2 transmitter and receiver are driven by an external reference clock which

is used to generate a dedicated clock system for the high-speed serial link. With

this, the APIX technology features a fundamental advantage to pixel clock driven

devices (e.g., LVDS systems) as the serial link is not influenced by any variances or

jitter at the pixel clock which lowers the risk for uncontrolled electromagnetic

emissions.

AShell

The APIX Automotive Shell called “AShell” is an abstraction layer for the data

communication. The AShell allows a secure and error-free data exchange on the

bidirectional full-duplex communication channels of the APIX link.

Apart from the error control functions, the AShell is a wrapping layer, providing

the following services to the data communication:

• Transmission and reception of application data ensuring data integrity

• Supply of information about transmission link status as well as simple errors

The concept of AShell is that the receiving AShell will only offer error-free data

to the application. The error detection is based on a CRC sum, generated at the

transmit path and checked at the receive path that is part of the Protocol Data Unit

(PDU) exchanged between the AShells of both communicating islands (Inova

Semiconductors 2014c).

CML Technology

High-speed data transmission over long distances requires the use of differential

signaling technologies like low-voltage differential signaling (LVDS) or CML. In

comparison with single-ended and parallel interfaces, these technologies offer high

immunity against environmental noise and lower emissions, with the benefits of

lower voltage swings and low power consumption. The reason behind these fea-

tures lies in the fact that the data are transmitted via a pair of twisted cables, on

which the digital bit is transmitted as +VSwing on cable 1 and �VSwing on cable

2 or vice versa (logic “0” or “1”) (Fig. 6).

Since the information is transmitted differentially, the noise induced on the line

would affect both lines, therefore just influencing the DC level of the VSwing, but

not the relation of cable 1 to cable 2. In addition to immunity, the electromagnetic

emissions are kept at a minimum.

In order to ensure an efficient operation of differential signals, the layout, the

cable connectors, and the cable itself need to be designed to ensure a constant

distance between the differential cables and a constant impedance, to avoid reflec-

tions and, along with these, errors on the differential signal. Please see also chapter

“▶ Serial Display Interfaces” for more details on this subject.


Serial Display Interfaces

The APIX technology uses CML, as it operates with a constant current source,

which, when compared with LVDS, switches between the logical stages without

generating spikes on the power supply, which in turn generates high dynamic

currents, thus generating EMI. In addition, CML allows faster switching times.

Due to the architecture of CML, the twisted cables carry the same current in the

opposite direction, which compensates the electromagnetic field of the cable

(Fig. 7).

Optimized Chip Design for EMI

The principle and the benefits of CML, using a constant current source driving a

differential pair of wires to eliminate emissions, may also be used for chip design.

Especially at high switching frequencies and due to the high density of transistors,

semiconductor devices need to deal with supply and ground noises that influence all

components of the chip design. Traces within the chip may generate electromag-

netic fields, similar to any line on a PCB design.

The analog front ends of the APIX transmitter and the receiver ICs have been

designed using CML techniques. Chip signals are integrated using two wires for

each connection with the current mode switching for the signal, compensating the

electric field of the lines and reducing the switching noise to a minimum (Fig. 8).

Tek Run: 100 GS/s Et sample

3

DPO brightness: 60%

+VSwing

Ch3M 500 ps 0 V October 4, 200713:10:40

−VSwing

Ch3 100 m VΩ 100 m VΩ

DPO

Ch4

Fig. 6 Differential signal

10 M. Roemer

Signal Conditioning

A high-speed transmission system in the automotive environment has to offer good

EMI performance and needs to withstand environmental and mechanical stress at

minimum bit error rates (e.g., 10�12). All these requirements have to be fulfilled

over a product lifetime of more than 10 years and over million devices. Mapping

those requirements to the system, the link needs to be able to compensate for all

tolerances, temperature, and aging effects within the transmission path.

Besides the transmission line design at the PCB andwith this all the tolerances of the

PCBmaterial and components, the cable plays the important rolewithin the signal path.

Each cable has specific characteristics over frequency, which vary over cable

length and temperature. In addition, the cable characteristics may change due to

mechanical stress or by other aging effects.

In addition, these characteristics change with different cable lengths. Figure 9

shows the insertion loss versus frequency for different lengths of the same cable.

In order to address the different requirements for the link, the APIX physical

layer allows to optimize the signal output to match the given cable conditions. With

an ideal matching the APIX receiver input “sees” an optimal eye opening at the start

of product lifetime.

In APIX1 technology, each transmitter offers the adjustment of the nominal swing

(VSwing) and a preemphasis to optimize the signal for the given cable length and to

compensate for reflections. This optimized signal provides enough margin at the

receiver to cover all tolerances and aging effects at speeds up to 1 Gbps. Please see

also chapter “▶Serial Display Interfaces” for more details on the nominal swing.

Fig. 7 Current mode logic (CML) eye pattern



Fig.8

Difference

betweenstandardCMOSandCMLdesigns

12 M. Roemer

Figure 10 compares the signal on a 20-m cable with and without preemphasis

enabled. By using preemphasis, the signal is cleared from reflections, which can

disturb the pattern recognition.

Due to the increased frequency requirements of up to 1.5 GHz for a 3 Gbps

transmission, the APIX2 physical layer has been enhanced to allow most flexible

adjustment to the given conditions. The APIX2 transmitter front end is

implemented as five-tap FIR filter, generating the opposing transfer function to

the transmission path. This optimized setting ensures maximum margin against

0−1−2−3−4−5−6−7−8−9

−10−11

dB(S

)

−12−13−14−15−16−17−18−19−20−21−22−23

0,001 0,002 0,01 0,02 0,1

Frequency (GHz)

0,2

1m

3m

10m

1 2 3 4

Fig. 9 Insertion loss of a 1, 3, and 10 m cable

Run: 250 GS/s ET sample

Ch3 50.0 mVΩ 50.0 mVΩ

Δ: 316 mV@: 157 mV

Δ: 102 mV@: −54 mV

50.0 mVΩCh3 50.0 mVΩM 200 ps M 200 ps

No preemphasis With preemphasis

Ch4 Ch41.52 V 1.52 VCh2 Ch2

3

Run: 250 GS/s ET sample DPO brightness: 90%

3

DPO DPO

Fig. 10 Signal quality on a 20-m cable with and without preemphasis


temperature drift or cable aging but also reduces the electromagnetic emissions

from the cable (Fig. 11).

The FIR filter at the transmitter is supported by an adaptive equalizer in the

receiver, implemented with an automatic gain control (AGC) and a decision

feedback equalizer (DFE). The automatic AGC constantly compensates for

temperature-dependent attenuation of the chip and the cable. Based on a digital

filter, the DFE generates the transfer function to address the whole signal path

including transmitter line driver, connectors, cable, and package parasitics. Both

blocks can be controlled by a least mean square (LMS) algorithm, constantly

adjusting the filter to the incoming signal.

The combination of the signal produced by the FIR and the adaptive equalizer

provide the margin necessary to meet the high automotive requirements.

Lowering Emissions and Transmission Errors

Connectors and CablingBecause differential signaling has already been in use since many years and used by

various standards like Ethernet or digital video interface (DVI) (IEC 62132-4 2006-

02), the selection of cables and connectors available is quite large. However, the

final decision on the cable and the connector depends on the application require-

ments for EMI, distance, reliability, and cost.

In order to obtain optimum performance for EMI and transmission length, the

differential link needs to be optimized from the transmitter to the receiver. This

includes

1. Routing and layout of the signal from the pin to the connector (see chapter

“▶ Serial Display Interfaces”)

2. Quality of the plug in terms of EMI (shielding)

Fig. 11 APIX2 physical layer architecture

14 M. Roemer


3. Connection of the signals inside the plug and the connector (same length,

matched)

4. No change in impedance from the plug to the connector

5. A 100-Ω impedance cable

The main requirement for the line is to have a continuous impedance of 100 Ω.

Each “conversion” from the board to the connector or to the plug may induce an

impedance mismatch, generating reflections and, therefore, causing emissions.

Several cable and connector providers offer highly optimized solutions, which

fulfill these requirements for the cable and the connector. The APIX1 technology

has been tested with different connectors and cables like standard RJ45 connectors

and Cat5 shielded twisted pair (STP) cables, which are typically used in Ethernet

applications and also with specific automotive cables and connectors. However,

especially for the 3 Gbps requirement in APIX2 specific automotive cables and

connectors need to be used. An example for a robust connector is the

RosenbergerHSD® connector, optimized for a two-pair connection. The connector

is based on the star quad principle for minimized interference, has a controlled

impedance of 100 Ω across several interconnections, and includes a shield for high

EMI performance (Figs. 12 and 13).

LayoutSince the high-speed interface acts at frequencies of up to 1500 MHz, the design

needs to be treated as a high-frequency design. The main problems that generate

noise or radiations are caused by high current spikes, which are generated by strong

output drivers or by switching multiple outputs. Also important is to avoid ground

loops or long traces, which could resonate at undefined frequencies.

Fig. 12 Star quad principle

for optimized differential

signaling


The following recommendations can help reduce noise and avoid performance

issues. Of course, the list can just be seen as a simple starting point.

1. Power supply filteringThe chip supplies should be filtered with block capacitors, which help to reduce

the influence of high current requirements on the remaining system. The capac-

itor values depend on the requirements of the chip and should be calculated

based on the magnitude of the voltage ripple and the frequencies present. The

components need to be placed as close as possible to the devices for maximum

effect. Typical values can be found in the datasheets of the transmitter and the

receiver devices.

2. Loop filter designThe loop filter components of the APIX1 devices should be laid out as close as

possible to the input and output pins. The loop filter design should not include

any vertical interconnect access (VIA) to avoid the influence of the additional

inductance and capacitance.

3. Series resistorsThe video data are sampled into the serializer devices using a 27-bit parallel bus,

synchronous to a pixel clock signal. Graphics controllers with strong output

stages might generate radiations through very fast rise times. Series resistors in

the pixel data lines and the pixel clock limit the rise time and, therefore, act as a

filter for high frequencies. In addition, these resistors reduce the current flowing

into the chip.

4. Transmission linesThe transmission lines need to be designed for continuous differential imped-

ance of 100 Ω to avoid reflections. The following points should be considered:

• Reduce the length of the transmission line to a minimum to reduce the

influence of PCB tolerances.

• Keep the differential pair parallel at any time. In case the transmission lines

have to be connected to components like capacitors or ESD diodes, the

selected component should allow placing the pads directly at the

transmission line.

Fig. 13 RosenbergerHSD® connector

16 M. Roemer

• Do not use meanders at only one line of the pair to match the length of the

pair. Such meanders act as inductors creating impedance mismatch and

radiated emissions.

• In case filter components are used, ensure that the selected devices do not

cause impedance mismatches or signal attenuation.

• Avoid any additional stub at the transmission lines (like test points).

Pixel Clock JitterA key advantage of the APIX architecture is that the high-speed clock for the serial

interface is generated from an internal system clock, independent from the pixel

clock. As described in Sect. 3 of chapter “▶Panel Interfaces: Fundamentals,” the

parallel pixel interface between the graphics controller and the APIX transmitter is

a synchronous parallel interface, which may cause a significant peak in the emission

spectrum and disturb surrounding devices or receivers. Using series resistors can

help filtering the high-frequency components of the emission, but since all lines of

the parallel bus switch simultaneously, additional mechanisms are necessary.

In case the graphics controller hardware supports it, the problem can be

addressed using staggered outputs. With this, the output drivers are not switched

simultaneously but instead are driven with small time offsets. This method reduces

the switching currents and, therefore, the radiated power.

Another way is to spread the spectrum of the radiated emissions and, therefore,

flatten the EMI spectrum. This can be achieved by jittering the pixel clock, that is,

instead of using a constant frequency, for example, 40 MHz, the pixel clock is

provided with a continuously changing frequency around this central frequency.

The influence of the jitter to the APIX1 link has been tested in various config-

urations by varying the pixel clock, the frequency deviation, for example, 39 MHz

instead of 40 MHz, and the modulation frequency, for example, the offset being

supplied at a frequency of 1 kHz. The results have shown that the jitter added to the

pixel clock can be up to 10 % of the pixel clock with a modulation of 50 kHz,

without causing transmission errors on the video data (Inova Semiconductors

GmbH 2008a).

This immunity of the APIX link architecture against a wide range of jitter

provides the system designer a flexible tool to optimize the EMI performance of

his design.

Diagnostic

In the last chapters many methods and guidelines have been discussed, in order to

improve the design and the setup for getting maximum margin and robustness into

the system. Even though these methods can ensure the quality of the design, they

cannot prevent and react to unforeseen defects caused by mechanical stress or

environmental influence. Due to this, automotive systems require system diagnostic

to detect or even prevent such defects. During design phase, those features are

useful to speed up debugging and to determine the system margin.


Panel Interfaces: Fundamentals

In general, monitoring a link can be reached by inserting CRC values into the

data stream. As soon as the number of CRC errors increases, the link quality is

degraded. However, CRC information is only available in case the link may already

be degraded and the result is visible to the user. In addition, CRC information

consumes additional bandwidth and does not offer 100 % coverage.

In ideal case, the evaluation of the link quality is performed at the physical layer.

The automatically determined AGC and DFE values of the APIX2 receiver adap-

tive equalizer (see section “Signal Conditioning”) offer the information to detect

degradation. Reading and storing the parameters at first startup not only provides

immediate feedback on the actual status of the link at time of delivery, which can

even be used to avoid early life errors. The parameters also allow continuous

monitoring of the link over lifetime. A drastic change in the signal path like a

broken cable will be visible as significant change in those values. But also slow

degradation can be detected and may even be addressed by configuration changes or

at least reported to the system log.

On top of the physical layer, the APIX2 link offers information at each service

level. Based on the APIX frame layer, indicating general operation, the AShell

reports the protocol status and CRC information.

The combination of all diagnostic information allows the system provider to

constantly evaluate the link and to either create the respective error log or to even

take countermeasures (Fig. 14).

Summary and Outlook

The APIX high-speed display link offers a number of features and concepts that

addresses the different requirements for the car environment. As the APIX tech-

nology is optimized for low EMI, it can act as a single interface for a display or a

camera application to reduce cabling and system integration costs and to overcome

ground offset issues. Even though the technology is well defined, the designer still

needs to pay attention to the high-frequency component of such a design in terms of

the layout and in the selection of cables and connectors.

Looking into the future, the call for more bandwidth will also require higher

frequencies and, with this, continued optimizations for EMI. Large, high-resolution

displays as well as cameras with megapixel resolutions will require bandwidths of

up to 6 Gbps. The growing demand for more bandwidth combined with lowest EMI

and cost reduction requirements define the need for efficient, highly optimized

solutions.

The APIX architecture will continuously be enhanced in order to serve the

requirements of the automotive market.

Acknowledgments The author thanks the International Electrotechnical Commission (IEC) for

permission to reproduce Information from its International Standard IEC 61967-4 ed.1.1 (2006).

All such extracts are copyright of IEC, Geneva, Switzerland. All rights reserved. Further infor-

mation on the IEC is available from www.iec.ch. IEC has no responsibility for the placement and

18 M. Roemer

http://www.iec.ch/

context in which the extracts and contents are reproduced by the author, nor is IEC in any way

responsible for the other content or accuracy therein.

Further Reading

IEC 61967-2 (2005-09) Integrated circuits – measurement of electromagnetic emissions 150 kHz

to 1 GHz – part 2: measurement of radiated emissions, TEM-cell method and wideband TEM

cell method, 1st edn. International Electrotechnical Commission, Geneva

IEC 61967-4 (2006-07) Integrated circuits – measurement of electromagnetic emissions 150 kHz

to 1 GHz – part 4: measurement of conducted emissions – 1 Ω/150 Ω direct coupling method,

1.1 edn. International Electrotechnical Commission, Geneva

IEC 62132-4 (2006-02) Integrated circuits – measurement of electromagnetic immunity 150 kHz

to 1 GHz – part 4: direct RF power injection method, 1st edn. International Electrotechnical

Commission, Geneva

Inova Semiconductors GmbH (2008a) AN105 APIX video interface application note. Inova

Semiconductors GmbH, Munich

Inova Semiconductors GmbH (2008b) INAP125 datasheet

Inova Semiconductors GmbH (2014a) AN203 APIX2 transfer channel requirements

Inova Semiconductors GmbH (2014b) INAP375 datasheet

Inova Semiconductors GmbH (2014c) INAP375 user manual

Johnson H, GrahamM (1993) High speed digital design: a handbook of black magic. Prentice Hall,

Upper Saddle River

Schwab AJ (1996) Electromangnetische Vertraglichkeit. Springer, Berlin

Wadell BC (1991) Transmission line design handbook. Artech House, Norwood

Fig. 14 APIX2 layered diagnostic information


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