In the last blog, we have covered the basics of CAN communication. Now, we will see about some of the advanced concepts involved such as Bit Stuffing, frame types, error types, Synchronization etc. We will also look into some of the non-standard extensions available in modern CAN controllers.

Generally, all CAN modules support the classical CAN protocol. It can receive and transmit both CAN base and the CAN extended frames. The transmission and reception of CAN FD frames is optional. Classical CAN Implementation do not support 29-bit identifiers. CAN 2.0B passive nodes were compliant with ISO 11898-1:2003, but it used very rarely. In this context, let us explore some of other concepts in detail.

Bit Stuffing

Bit Stuffing is used to ensure the synchronization of all nodes even when transmitting consecutive information with same value either 1 or 0.
During the transmission of message, a maximum of five consecutive bits may have the same polarity. In this case, the transmitter will insert the one additional bit of opposite polarity into the bit stream before transmitting the further bits. This will ensure that there is always some activity in the bus with in 6-bit intervals and hence avoid DC Voltage build up as well as being in sync with the transmitter.

Stuffing and De-stuffing

On the receiving end, similarly the receiver also checks the number of bits of same polarity and removes the stuffed bits again from the bit stream in a process called de-stuffing.

CAN Frame Types

There are 5 types of frames in CAN protocol;

Data Frame (DF):

Carries Data from transmitting node to receiving node.

Remote Frame (RF):

Some times, a node might want to request some data from another which is made possible by Remote frame.
There are two differences between data and Remote frames.
RTR field of a data frame is dominant and RTR field of remote frame is recessive.
In data frame format data field is present, whereas in Remote frame format data field is absent.

The receiver will understand that transmitter is requesting some date and then prepares and sends the Data frame based on the protocol.

Error Frame (EF):

This type of frame is transmitted by any node to signal error.
The error frame consists of two different fields in CAN.
superposition of ERROR FLAGS (6–12 dominant/recessive bits)
ERROR DELIMITER (8 recessive bits).
There are two types of error flags:

Active Error Flag

When the Transmitting node transmitted six dominant bits, the error will be detected in network and the error sate called active error flag.

Passive Error Flag

When the Transmitting node transmitted six recessive bits, the error will be detected in network and the error sate called passive error flag.

Now let us see, how the CAN manages error states. In every CAN node, there are 2 error counters – Transmit Error Counter (TEC) and Receive Error Counter (REC). When the transmitter detects an error in the transmitted frame, it increments the TEC by 8. A receiver detecting an error will increment its REC by 1. On successful transmission/reception the error counters are reduced by 1.
Based on the error counts, the node behavior varies.

  • By default, the Active Error frame will be transmitted on the bus, when TEC and REC < 128. Thus, it will invalidate the frame globally.
  • But when 127 < TEC \ REC > 255, the passive Error frame will be transmitted on the bus, without affecting the bus traffic.
  • Finally, the node enters into the Bus off state, when TEC > 255. If node enters into the bus off state then no frames will be transmitted.

In any case, both transmitter and receiver reject the erroneous frames completely and do not process it any further.

Overload Frame (OF):

Overload frame contains two fields such as Overload flag and Overload Delimiter.
The over load frame will be generated, when the receiving node is overloaded – i.e. it is not able to detect and receive the incoming messages. The format is very similar to Error Frame but without the error counters incrementing. An Overload frame indicates that its transmitter require delay before receiving next data or remote frame and is mostly not used in modern CAN controllers.

Inter Frame Space (IFS):

Data frames and remote frames are separated from preceding frames and succeeding frame by a bit field called interframe space. It consists of three consecutive recessive bits. Following that, if a dominant bit is detected, it will be regarded as the “Start of frame” bit of the next frame.

Frame on CAN BUS

Error Types

There are 5 types of error in CAN protocol.

Bit error:

Every node reads back, bit by bit from the bus during transmitting the message and then compares the transmitted bit value with received bit value. If bit received does not match with bit sent, then Bit error is said to be occurred.

Stuff error:

Set when more than five consecutive bits of same polarity are received in receiving node.

CRC error:

A transmitted always transmits the CRC value in the CRC field of CAN frame. The receiving node also calculates the CRC value using same formula and compares with received CRC value. If receiving node detects mismatch between calculated CRC values and received CRC value then it is called CRC error.

ACK error:

Occurs when no acknowledgment is sent by receiving node or no acknowledgment received in transmitting node.

Form error:

Set when fixed format fields in receive frame is violated. No dominant bits are allowed in CRC delimiter, ACK delimiter, EOF and IFS.

Synchronization and Re-synchronization

As there is no separate clock signal on the CAN bus, the node itself need to synchronize on the bus. For that reason, the underlying transmission format is NRZ-5 coding.
When the transmitting node sends CAN frame it consists the first bit of SOF (start of frame). All the receivers align themselves to this falling edge (recessive to dominant) after the period of bus idle. This mechanism is called hard synchronization.
After subsequent falling edges on the CAN frame are used to re-synchronize the nodes on bus and it is called soft synchronization. This resynchronization happens continuously at every falling edge (recessive to dominant transition) to ensure transmitting and receiving nodes stay in sync.

Additional functions

Some CAN protocol implementations offer optional functions that may or may not be a part of CAN specification. These include, for example, the single-shot transmission of data frames. This means that the automatic re-transmission in case of detected errors is disabled. This is useful for TTCAN add-ons and some tool applications.
Another option generally available is the bus-monitoring mode. The node can receive data and remote frames, but doesn’t acknowledge them and also doesn’t send error and overload flags. Nevertheless, these dominant bits are communicated internally in the CAN module.
In another optional restricted operation mode, the CAN module behaves equally, but it acknowledges received data and remote frames. The error counters are not incremented and decremented in this mode. If a node is the TTCAN time master, it must be able to transmit the time-reference message; other frames must not be transmitted.
For some applications, message time stamping is required. ISO 11898-1:2015 specifies that the optional time-stamp function features resolutions of 8-bit, 16-bit, or 32-bit. The time-base value is captured at the reference point of each data frame and it is readable after EOF (end-of-frame). Other (not standardized) optional functions include readable error counters, configurable warning limits, interrupt request generation, and arbitration lost capture.
If the CAN implementation allows changing the configuration of a node by software, the configuration data (e.g. bit-time configuration or operating mode) needs to be locked against changes while CAN communication is ongoing.

Armed with details of CAN communication, we will now attempt to understand general configuration of a CAN node for transmission and reception with examples from a real controller.

About Embien

Embien Technologies is a leading provider of product engineering services for the Automotive, Semi-conductor, Industrial, Consumer and Health Care segments. Working with OEMs in Industrial segments, we have developed numerous gateways, sensory modes on top of CAN network and protocols such as DeviceNet, CANOpen etc. Our Automotive experience enabled us develop Telematic units and In-vehicle Infotainment systems, Instrument clusters with CAN interfaces.

Dhananjayan
31. May 2018 · Write a comment · Categories: Embedded Hardware · Tags: , , , ,

This blog is the sequel of the previous blog on Geo positioning system technologies. Here we will discuss more in detail about the important terms related to GPS receivers and guidelines for selecting suitable GPS receiver module for embedded system design.

GPS Receiver

Connection diagram for GPS module

GPS Module – Connection Diagram

The above picture depicts the typical connection diagram of GPS module with any host controller. There are multiple interface options available for a host controller to receive the NMEA data where UART, SPI and USB are most common. PPS signal is an output from GPS and it is discussed in detail in the upcoming sections. Most of the GPS modules have internal patch antenna but also supports external active antenna connection.

NMEA Data

 NMEA (National Marine Electronics Association) data is the detailed output from any GPS receiver that includes the current location data of the receiver such as Latitude, longitude, altitude etc. This data is provided in a standard format to the user for compatibility with various manufacturers much like ASCII standard for digital computer characters.

Following is the example of a NMEA message from a GPS receiver,

$GPGGA,181908.00,3004.6040718,N,07040.3900269,W,4,13,1.00,408.135,M,29.200,M,0.10,0000*40

All the NMEA message starts with a $ character where each field is separated by a comma.

“GP” in GPGGA – represent that it is a GPS position. For GLONASS, it will be GL instead of GP.

GPGGA is a basic GPS NMEA message and many other NMEA messages are also available providing similar or additional information beside GPS coordinates. There are 19 different NMEA messages and they are listed below,

  1. $GPBOD – Bearing, origin to destination
  2. $GPBWC – Bearing and distance to waypoint, great circle
  3. $GPGGA – Global Positioning System Fix Data
  4. $GPGLL – Geographic position, latitude / longitude
  5. $GPGSA – GPS DOP and active satellites
  6. $GPGSV – GPS Satellites in view
  7. $GPHDT – Heading, True
  8. $GPR00 – List of waypoints in currently active route
  9. $GPRMA – Recommended minimum specific Loran-C data
  10. $GPRMB – Recommended minimum navigation info
  11. $GPRMC – Recommended minimum specific GPS/Transit data
  12. $GPRTE – Routes
  13. $GPTRF – Transit Fix Data
  14. $GPSTN – Multiple Data ID
  15. $GPVBW – Dual Ground / Water Speed
  16. $GPVTG – Track made good and ground speed
  17. $GPWPL – Waypoint location
  18. $GPXTE – Cross-track error, Measured
  19. $GPZDA – Date & Time

181908.00 UTC Time stamp in hours, minutes and seconds.

3004.6040718 Latitude in DDMM.MMMMM format. Decimal places are variable.

N North latitude

07040.3900269 Longitude in DDMM.MMMMM format. Decimal places are variable.

W West Longitude

4 Quality Indicator. Other options are as follows,

0 = Fix not valid

1 = Uncorrected coordinate

2 = Differentially correct coordinate (e.g., WAAS, DGPS)

4 = RTK Fix coordinate (centimeter precision)

5 = RTK Float (decimeter precision)

13 Number of satellites used in the coordinate

1.00 denotes the HDOP (horizontal dilution of precision).

408.135 Altitude of the antenna

M unit of altitude (e.g. Meter or Feet)

29.200 geoidal separation. Subtracting this from the altitude of the antenna will provide the Height Above Ellipsoid (HAE)

M Unit of geoidal separation

0.10 age of correction

0000 correction station ID

*40 checksum

Time-To-First-Fix (TTFF)

 Another most important deciding feature of GPS receiver is the TTFF. TTFF of a receiver only decides how fast it can provide a valid NMEA data to the user. So the user must be very careful on the TTFF values while choosing the receiver. The TTFF values will be provided in the datasheet in seconds.

Any receiver can boot up in any one of the following three modes:

  1. Hot start
  2. Warm start
  3. Cold Start

The Time-To-First-Fix (TTFF) depends on the startup mode, with cold starts giving the longest TTFF. Following are the factoring affecting boot mode,

  1. Non availability of valid almanac and ephemeris data
  2. Level of incoming signals
  3. The unit is within 60 miles (100 Km) of location of previous fix
  4. Length of time since previous fix

Cold Start Mode

: Any receiver start in this mode when,

  1. Receiver has not been used for long time
  2. Moved several hundred kilometers
  3. Incoming signals are low or marginal. i.e. the predicted satellites are overhead of the receiver but cannot receive signals due to tall buildings, foliage etc.

Any of the above situation will make the receiver cannot predict which satellites are overhead. Then the receiver works with an internal list of satellites and tries to acquire each one in turn. This allows the receiver to discover the satellite which are in view and eventually establish a position. Normally the TTFF on cold start takes 2 to 4 minutes.

Warm Start Mode

: Any receiver start in this mode when,

  1. It has valid almanac data
  2. The current location of receiver is within hundred kilometers of the last fix location
  3. Receiver has been active in last three days and current time is known
  4. Ephemeris data that has not been stored or it has become stale
  5. Good signal strength and 4 or more satellites are visible

In this mode the receiver can predict which satellites are overhead but it needs to download the current Ephemeris data. TTFF for warm start mode is typically 45 seconds.

Hot Start Mode:

The receiver starts in this mode when the warm start conditions are met and,

  1. A fix has been established within the last 2 hours
  2. The receiver has stored valid ephemeris data for atleast 5 satellites

Receiver tracks the overhead satellites and needs to download minimum data to establish a position. TTFF for a hot start is typically 22 seconds. 

A-GPS

We have come across many startup modes for any receiver, but wonder how smartphone GPS units get a fix almost immediately?

They use Assisted-GPS (A-GPS) as a procedure of improving the TTFF or even allowing a fix in conditions where receiver might not be able to function.

A-GPS device will use a data connection available on the smartphone to contact an assistance server. This server will supply almanac and ephemeris data instead of waiting to receive them from the satellites. The server can also provide approximate location derived from the cell phone towers facilitating immediate fix.

PPS Signal

Most GPS receiver modules have an output called Pulse Per Second abbreviated as PPS. It is a digital output signal with much lower jitter than anything a MCU can do. PPS signal can be used          to time things very accurately at a precision in nanoseconds.

They are most commonly used to wake the MCU from deep sleep mode periodically at an interval of one second. In some applications, they are used to synchronize the system time and rectify the time drift due to the temperature effects of the RTC crystals. 

Selection guide for receiver

There is lot of options available while selecting a receiver module. Some of the main factors to be considered while selecting the receiver are as follows,

  1. Multi system support – Receiver module can be GNSS or GPS only. GNSS module provides simultaneous support to GPS, GLONASS, BeiDuo and Galileo systems. GNSS modules are better than GPS only modules and the cost of the GNSS module will be bit more comparative to the other.
  2. Size – Most important deciding factor for size constraint devices. Nowadays modules are getting very small but in general the antenna will also shrink to fit the module which will reflect in the lock time and accuracy.
  3. Number of channels – At a given time, there are so many satellites available in view but the number of channels a receiver module can track/acquire will affect the TTFF. The more a module can track/acquire many satellites, the faster it will find a fix.
  4. Update Rate – It is the time interval that how often a receiver module can recalculate and reports its position. The standard rate of a module is 1Hz (one report per second). Fast update rate means that there are more NMEA sentences coming out of the receiver module by which any microprocessor will be quickly overwhelmed trying to parse that much data.
  5. Power consumption – Another important factor that decides the success of a battery powered devices. There are many factors deciding the power consumption of a receiver module and in any case the typical power consumption should be low in few tenses of mA ranging between 25 to 30mA. Most of the receivers have various power saving modes which can be used during idle conditions.
  6. Antenna – Antenna defines the quality of the receiver module and it must be finely trimmed good enough to pick up the frequencies. Receiver modules available with a small patch antenna on top of it. It is made of ceramic. Some modules will also have dedicated antenna pin for connecting external active antenna. Receivers used in cars require external antenna support since the receiver module has to be placed inside the vehicle mostly connected to the OBD port which will be placed beneath the dashboard where a patch antenna will struggle to receive the signals.
  7. Accuracy – Varies between modules. Most modules can get it down to +/- 3m and sub meter or centimeter accuracy are also available but bit expensive.

About Embien

Embien Technologies is a product engineering service provider with handsome experience on automotive product developments. Embien has various solutions for time to market developments for automotive domain including Sparklet Embedded GUI library for 2D or 2.5D or 3D Instrument clusters, Flint IDE for GUI prototyping and eStorm-B1 – Automotive grade BLE Module.

Geo positioning system or GPS has become more or less a norm for smart phones. Geo positioning system was first created for the navigation of defense vehicles in any part of world. But over the period of time, this system is being used in many other purposes outside defense and has proved itself to be a revolutionary technology in today’s world. Apart of the smartphone, most of the premium cars and commercial vehicle do have inbuilt GPS for fleet tracking, vehicle Telematics, and driver assistance.

Apart from such fleet navigation use cases, GPS are now being used for many applications such as locating nearby restaurants, hotels and gas stations and finds huge applications in tourism industry. Personal navigation devices also employ GPS technology.

Also most of the IoT/M2M applications use GPS modules. Some of them are as follows

  • Smart utility metering
  • Connected health and patient monitoring
  • Smart buildings
  • Security and video surveillance
  • Smart payment and PoS systems
  • Wearable devices etc

While the term GPS in general represents the technology, there are numerous systems being used to achieve this. In this blog, we will briefly describe about the various such Geo positioning systems and related concepts.

Geo Positioning System – Technology

Any geo positioning system uses about three to four satellites from more than a dozen of satellites orbiting in a group (satellite constellation) to provide autonomous geo-spatial positioning. These satellites transmit 1500 bits of data such as the satellite health, its position in space, propagation delay effects, constellation status, the time of information being sent, etc. This allows a small electronic receiver to determine its location in terms of latitude and longitude based on triangulation of the data obtained from at least three satellites. With four or more satellites, the receiver can also determine the 3D position, i.e. Latitude, longitude and altitude. In addition, a GPS receiver can provide information about the speed and direction.

Anyone with the GPS receiver can access the system. Since it is an open source and providing almost accurate 3D position, navigation and timing 24 hours a day, 7 days a week, all over the world, it is used in numerous applications even in GIS data collection, mapping and surveying.

Geo Positioning System – Types

At present there are many options available for geo positioning system each of them owned and operated by countries such as US, Russia, European Union, China, etc. They are as follows

NAVSTART GPS – GPS, Global Positioning System is a one among the various satellite navigation system designed and operated by the U.S. Department of defense. Official name of GPS is Navigational Satellite Timing and Ranging Global Positioning System (NAVSTAR GPS).

GLONASS – Global Orbiting Navigation Satellite System, GLONASS developed by Russian, is an alternative to GPS and is the second global navigational system in operation providing global coverage with comparable precision. A GLONASS satellite design has various upgraded versions and the latest is GLONASS-K2 which is expected to operate in early 2018.

Galelio – Galelio is created by European Union with the aim to provide an independent high precision positioning system for European nations.

BeiDou – BieDuo Navigation Satellite System (BDS) is a Chinese satellite navigation system consisting of two separate satellite constellations BeiDuo-1 and BeiDuo-2. BeiDuo-1 is decommissioned and BeiDuo-2 also known as COMPASS offering services to customers in the Asia-Pacific region with a partial constellation of 10 satellites in orbit.

IRNSS – Indian Regional Navigation Satellite System also known as NAVIC (Navigation with Indian Constellation) is a regional satellite navigation system covering the Indian region extending 1500Km. This constellation is already in orbit and expected to operate in early 2018.

Satellite Based Augmentation System (SBAS)

All the above systems are autonomous and governed by the respective countries. Other than autonomous systems, other regional augmented systems are available that run with the aid of other autonomous satellites. These augmentation systems will provide reference signals (Signal in Space- SIS) via satellites to the receivers including correction information with the objective of increasing the accuracy of the position. In addition to the accuracy they also help to maintain the reliability and availability of the navigation system. The whole system is known as SBAS (Satellite Based Augmentation System) and satellite providing the SIS signal are known as SBAS GEO satellites. Some of them are as follows,

GAGAN – GPS-Aided Geo Augmented Navigation – It is the implementation of SBAS by Indian government. It supports pilots to navigate in the Indian airspace by an accuracy of 3m.

QZSS Quasi Zenith Satellite System is a project governed by Japanese government and operated in order to receive the US operated GPS in the Asia-Oceania regions with Japan as a primary focus.

Other commonly available SBASs are WAAS (US), EGNOS (EU) and MSAS (Japan).

GNSS

The above mentioned satellite systems such as global, regional and augmented systems are integrated together to form Global Navigation Satellite System, GNSS. It is a standard term for satellite navigation systems providing autonomous geo spatial positioning with global coverage. It is a satellite system that is used to pinpoint the geographic location of a user’s receiver anywhere in the world. Three GNSS systems are currently in operation: the United States’ Global Positioning System (GPS), the Russian Federation’s Global Orbiting Navigation Satellite System (GLONASS) and the Europe’s Galileo.

Most degrading factor of a receiver, i.e. Line of Sight degradation can be solved with the GNSS system due to its accessibility to multiple satellites and if one satellite system fails, GNSS receivers can pick up signals from other system.

Navigation messages

Any satellite in the constellation will transmit a detailed set of information such as each satellite position, network to receiver called the navigation messages. Following are available in the navigation message, 

  1. Date and time together with the satellite status and an indication of its health 
  1. Almanac data – Contains coarse orbit and status information of all the satellites in the constellation. It allows the GPS receiver to predict which satellites are overhead, shortening acquisition time. Almanac data can be received from any of the satellites. The receiver must have a continuous fix for approximately 15 minutes to receive a complete almanac data. Once downloaded it is stored in the non volatile memory.
  1. Ephemeris data – Contains precision correction to the almanac data necessary for the receiver to calculate the position of the satellite. It is continuously updated every 2 hours and so ephemeris data of a deactivated receiver will become stale after 3 to 6 hours.

Time-To-First-Fix (TTFF)

For a receiver to get a fix, it needs a valid almanac, initial location, time and ephemeris data. When a receiver is switched ON, it requires some time delay for the first fix. This delay depends on how long since the stored data’s being used. The time delay is commonly termed as Time To Fist Fix, TTFF and it is one of the main factor for receiver selection.

About Embien

Embien Technologies is a leading provider of embedded design services for the Automotive, Semi-conductor, Industrial, Consumer and Health Care segments. Embien has successfully designed and developed many products with GPS for various domains such as Wrist wearable based tracker device for healthcare, Vehicle Telematics device for automotive, Data acquisition/logger devices for industry etc.