Automotive Interview Questions and CAN interview questions and important C programming interview Questions are asked in Automotive interviews.This are the very important questions for automotive interviews.
1. What is CAN?
Answer: 1. CAN is a multi-master broadcast serial bus standard for connecting electronic control
unit (ECU).
2. Controller–area network (CAN or CAN-bus) is a vehicle bus standard designed to allow
micro controllers a devices to communicate with each other within a vehicle without a host
computer.
3. CAN is a message-based protocol, designed specifically for automotive applications but now also
used in other areas such as industrial automation and medical equipment.
4. The Controller Area Network (CAN) bus is a serial asynchronous bus used in instrumentation
applications for industries such as automobiles.
2.CAN frame works?
Answer:
 |
| CAN Frame |
SOF – 1 Dominant
Arbitration Field – 11 bit Identifier, 1 bit RTR (or)
11 bit, 1SRR, 1IDE, 18 bit, 1RTR
Control Field – IDE, r0, 4 bits (DLC)
Data Field – (0-8) Bytes
CRC Field – 15 bits, Delimiter (1 bit recessive)
ACK Field – 1 bit, Delimiter (1 bit recessive)
EOF – 7 bits recessive
IFS – 3 bits recessive
Types of frames – Data, remote, Error frame and
Overload frame
Types of errors – ACK error, Bit error, Stuff error,
Form error, CRC error
Error frame – 0-12 superposition flags, 8 recessive
(Delimiter)
Overload frame – 0-12 superposition flags, 8
recessive (Delimiter)
3.Why CAN is having 120 ohms at each end?
Answer:To minimize the reflection
reference, to reduce noise. To ensure that
reflection does not cause communication failure, the transmission
line must be terminated.
4.Why CAN is message oriented protocol?
Answer:CAN protocol is a message-based
protocol, not an address based protocol. This means that messages are
not transmitted from one node to another node based on addresses.
Embedded in the CAN message itself is the priority and the contents
of the data being transmitted. All nodes in the system receive every
message transmitted on the bus (and will acknowledge if the message
was properly received). It is up to each node in the system to decide
whether the message received should be immediately discarded or kept
to be processed. A single message can be destined for one particular
node to receive, or many nodes based on the way the network and
system are designed. For example, an automotive airbag sensor can be
connected via CAN to a safety system router node only. This router
node takes in other safety system information and routes it to all
other nodes on the safety system network. Then all the other nodes on
the safety system network can receive the latest airbag sensor
information from the router at the same time, acknowledge if the
message was received properly, and decide whether to utilize this
information or discard it.
5.CAN logic what it follows?
Answer:Wired AND logic
6.What is CAN Arbitration?
Answer:CAN Arbitration is nothing but
the node trying to take control on the CAN bus.
7.How CAN will follow the Arbitration?
Answer:CSMA/CD + AMP (Arbitration on
Message Priority)
Two bus nodes have got a transmission request. The bus
access method is CSMA/CD+AMP (Carrier Sense Multiple Access with
Collision Detection and Arbitration on Message Priority). According
to this algorithm both network nodes wait until the bus is free
(Carrier Sense). In that case the bus is free both nodes transmit
their dominant start bit (Multiple Access). Every bus node reads back
bit by bit from the bus during the complete message and compares the
transmitted value with the received value. As long as the bits are
identical from both transmitters nothing happens. The first time
there was a difference – in this example the 7th bit
of the message – the arbitration process takes place: Node A
transmits a dominant level, node B transmits a recessive level. The
recessive level will be overwritten by the dominant level. This is
detected by node B because the transmitted value is not equal to the
received value (Collision Detection). At this point of time node B
has lost the arbitration, stops the transmission of any further bit
immediately and switches to receive mode, because the message that
has won the arbitration must possibly be processed by this node
(Arbitration on Message Priority)
For example, consider three CAN devices each trying to
transmit messages:
• Device 1 – address 433 (decimal or 00110110001
binary)
• Device 2 – address 154 (00010011010)
• Device 3 – address 187 (00010111011)
• Device 2 – address 154 (00010011010)
• Device 3 – address 187 (00010111011)
Assuming all three see the bus is idle and begin
transmitting at the same time, this is how the arbitration works out.
All three devices will drive the bus to a dominant state for the
start-of-frame (SOF) and the two most significant bits of each
message identifier. Each device will monitor the bus and determine
success. When they write bit 8 of the message ID, the device writing
message ID 433 will notice that the bus is in the dominant state when
it was trying to let it be recessive, so it will assume a collision
and give up for now. The remaining devices will continue writing bits
until bit 5, then the device writing message ID 187 will notice a
collision and abort transmission. This leaves the device writing
message ID 154 remaining. It will continue writing bits on the bus
until complete or an error is detected. Notice that this method of
arbitration will always cause the lowest numerical value message ID
to have priority. This same method of bit-wise arbitration and
prioritization applies to the 18-bit extension in the extended format
as well.
8.What is the speed of CAN?
Answer:
40m @1Mbps and if the cable
length increases will decrease the speed, due to RLC on the cable.
9.If master sends 764 and Slave sends 744 which
will get the arbitration?
Answer:Starts from MSB, first nibble
is same, Master sends 7, slaves also sends 7 the message with more
dominant bits will gain the arbitration, lowest the message
identifier higher the priority.
10.Standard CAN and Extended CAN difference?
Answer:Number of identifiers can be
accommodated for standard frame are 2power11.
Number of identifiers more compare to base frame, for
extended frame are 2power29.
IDE bit – 1 for extended frame.
IDE bit –0 for Standard frame.
11.What is bit stuffing?
Answer:CAN uses a Non-Return-to-Zero
protocol, NRZ-5, with bit stuffing. The idea behind bit stuffing is
to provide a guaranteed edge on the signal so the receiver can
resynchronize with the transmitter before minor clock discrepancies
between the two nodes can cause a problem. With NRZ-5 the transmitter
transmits at most five consecutive bits with the same value. After
five bits with the same value (zero or one), the transmitter inserts
a stuff bit with the opposite state.
12. What is the use of bit stuffing?
Answer:Long NRZ messages
cause problems in receivers
• Clock drift means that if there are no
edges, receivers lose track of bits
• Periodic edges allow receiver to resynchronize to
sender clock
13.What are the functions of CAN transceiver?
Answer:The transceiver provides
differential transmit capability to the bus and differential receive
capability to the CAN controller. Transceiver provides an advanced
interface between the protocol controller and the physical bus in a
Controller Area Network (CAN) node.
Typically, each node in a CAN system must have a device
to convert the digital signals generated by a CAN controller to
signals suitable for transmission over the bus cabling (differential
output). It also provides a buffer between the CAN controller and the
high-voltage spikes that can be generated on the CAN bus by outside
sources (EMI, ESD, electrical transients, etc.).
The can transceiver is a device which detects the
signal levels that are used on the CAN bus to the logical signal
levels recognized by a microcontroller.
14.Functionality of Data link layer in CAN?
Answer:LLC (Logical Link Control) – Overload
control, notification, Message filtering and Recovery management
functions.
MAC (Medium Access Control) - Encapsulation/
de-capsulation, error detection and control, stuffing and de-stuffing
and serialization/de-serialization.
15.What is meant by synchronization?
Answer:Synchronization is timekeeping
which requires the coordination of events to operate a system in
unison.
16.What is meant by Hard synchronization and soft synchronization?
Answer:Hard Synchronization to be
performed at every edge from recessive-to-dominant edge during Bus
Idle. Additionally, Hard Synchronization is required for each
received SOF bit. An SOF bit can be received both during Bus Idle,
and also during Suspend Transmission and at the end of Interframe
Space. Any node disables Hard Synchronization if it samples an edge
from recessive to dominant or if it starts to send the dominant SOF
bit.
Two types of synchronization are supported:
– Hard synchronizationis done with a
falling edge on the bus while the bus is idle, which is interpreted
as a Start of frame (SOF). It restarts the internal Bit Time Logic.
– Soft synchronizationis used to
lengthen or shorten a bit time while a CAN frame is received.
17.What is the difference between function and
physical addressing?
Answer:Functional addressing is an
addressing scheme that labels messages based upon their
operation code or content. Physical addressing is an addressing
scheme that labels messages based upon the physical address location
of their source and/or destination(s).
18.What happens if I have to send more than
8-bytes of data?
Answer:The J1939 standard has defined
a method of communicating more than 8 bytes of data by sending the
data in packets as specified in the Transport Protocol (TP). There
are two types of TP, one for broadcasting the data, and the other for
sending it to a specific address.
DTC consists of 4 components – SPN, FMI, OC and CM.
A DTC is a combination of four independent fields: the
Suspect Parameter Number (SPN) of the channel or feature that can
have faults; a Failure Mode Identifier (FMI) of the specific fault;
the occurrence count (OC) of the SPN/FMI combination; and the SPN
conversion method (CM) which tells the receiving mode how to
interpret the SPN. Together, the SPN, FMI, OC and CM form a number
that a diagnostic tool can use to understand the failure that is
being reported.
19.What is KWP2000?
Answer: KWP 2000(ISO14230) is a
Diagnostic communications standard. Specifies possible system
configurations using the K & L lines. As 9141-2 but limited to
the physical characteristics. Specifies possible system
configurations using the K & L lines.
-
5 Baud wake up as 9141- 2
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New fast initialization method
20.What is OBDII?
Answer:On-Board Diagnostics in an
automotive context is a generic term referring to a vehicle’s
self-diagnostic and reporting capability .
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