APIX: High Speed Automotive Pixel Link … · automotive pixel link (APIX) technology, the article...
Transcript of APIX: High Speed Automotive Pixel Link … · automotive pixel link (APIX) technology, the article...
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
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,
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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
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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)
APIX: High Speed Automotive Pixel Link 5
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
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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)
APIX: High Speed Automotive Pixel Link 7
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
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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.
APIX: High Speed Automotive Pixel Link 9
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
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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
APIX: High Speed Automotive Pixel Link 11
Fig.8
Difference
betweenstandardCMOSandCMLdesigns
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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
APIX: High Speed Automotive Pixel Link 13
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
APIX: High Speed Automotive Pixel Link 15
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.
APIX: High Speed Automotive Pixel Link 17
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
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
APIX: High Speed Automotive Pixel Link 19