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
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APIX: High Speed Automotive Pixel Link … · automotive pixel link (APIX) technology, the article first explains the basic concepts of high-speed video transmissions and then focuses
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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,
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