Serial Peripheral Interface [SPI]

Controller Area Network [CAN]

By: Instructor:Praneeth Kumar Bajjuri Dr.James M. Conrad

SPI

• Overview• Principle of operation• Configuration• Registers• SPI initialization

Overview• This name was coined by Motorola.• The Serial Peripheral Interface Bus or SPI bus is a

very loose standard for controlling almost any digital electronics that accepts a clocked serial stream of bits.

• The SPI is a synchronous serial interface in which data in an 8-bit byte can be shifted in and/or out one bit at a time.

• It is a synchronous serial data link that operates in full duplex

• SPI really gains efficiency in applications that take advantage of its duplex capability.

• SPI can also achieve significantly higher data rates than I2C .

Overview• During Data transfer the master always

sends a byte of data to slave and slave always sends a byte of data to master

• Less overhead in SPI than I2C due to lack of addressing .

• SPI requires more effort and more hardware resources than I2C when more than one slave is involved [due to lack of addressing]

• Hence for point to point this is simple and efficient whereas for multiple slaves more hardware is required.

SPI signals

• Data Signals1. MOSI2. MISO• Control Signals1. SCLK2. SS [slave sel]

Operation• In Master, the bits are sent out of the MOSI pin

and received in the MISO pin. • The bits to be shifted out are stored in the SPI

data register, SP0DR, and are sent out most significant bit (bit 7) first.

• When bit 7 of the master is shifted out through MOSI pin, a bit from bit 7 of the slave is being shifted into bit 0 of the master via the MISO pin.

• After 8 clock pulses or shifts, this bit will eventually end up in bit 7 of the master.

• The frequency of this clock can be controlled by the SPI baud rate register, SP0BR.

• The SS pin must be low to select a slave.• Data transfer is always initiated by the master.

Data Transfer Formats• 2 Parameters, Clock

Polarity (CPOL) and Clock Phase (CPHA), determine the active edge of the clock.

• Master and slave must agree on parameter pair values in order to communicate

CPOL,CPHA First Data

Driven

Other Data Driven

Data Sampled

00 Prior to first SCK

Rising Edge

SCK Falling Edge

SCK Rising Edge

01 First SCK Rising Edge

SCK Rising Edge

SCK Falling Edge

10 Prior to first SCK

Rising Edge

SCK Rising Edge

SCK Falling Edge

11 First SCK Rising Edge

SCK Falling Edge

SCK Rising Edge

Configuration• SPI interface defines only the communication lines and the

clock edge• SPI does not have an acknowledgement mechanism to

confirm receipt of data. • In fact, without a communication protocol, the SPI master has no

knowledge of whether a slave even exists. • Flow of Data is not controlled .If needed

External Flow control circuitry is designed outside SPI.

• Hardware Realization is just by using simple shift register.

• Supported by all AVR 8-bit μC except ATTiny and some AT90s

• ATmega323 1. PB6=MISO, PB5=MOSI, PB7=SCK, PB4=/SS2. SPCR – sets bit rate, CPOL, CPHA, M/S3. SPDR – used for data transfer to and from SPI shift register

Master operation• Set the SPI clock counter register to desired baud

rate.• Set the SPI control register for desired settings.• Data to be transferred is written in SPI Data register• Wait for status flag to be 1. This flag is set after all

the data is transferred from SPI Data register.• Always check the status flag.• Read the Received data from SPI data register [opt].• Constantly look for data availability to be

transferred.

Slave Operation• Set the SPI control Registers to the desired settings.• Write data to be transmitted in SPI Data Register.

Possible when the transfer is not in progress. [opt]• Wait for Status flag bit to be set to 1. this will be set

after the last sampling clock edge of SPI Data transfer.

• Poll the SPI status register.• Read the received data from SPI Data Register [opt].• Constantly look for data availability from master.

RegistersBit Control

Register[SPCR]

Status Register[SPSR]

Data Register[SPDR]

Clock counter Register[SPCCR]

Interrupt Register[SPINT]

[2:0] Reserved Reserved SPI Bi directional Data port

SPI Clock Counter Settings

0- SPI Interrupt

[1:7]-Reserved

3 CPHA ABRT

4 CPOL MODF

5 MSTR ROVR

6 LSBF WCOL

7 SPIE SPIF

SPI Initialization• Void config_spi() { PINSEL0 =

0x00005500; IODIR=0x0000000B; SPCR |= 0x20; SPCCR |= 0x08; SPDR = 0x44; SPINT = 0x00;}

• Void recv_spi(char *ptr, long count)

{ int i ; for(i=0;i<255;i++) { SPDR = 0x00; while(!(SPSR & 0x80)); ptr[i] = SPDR; count++; }}

Manufacturers OF SPIManufacturer Device Types

AKM EEPROM

Analog Devices DSP, ADC, digital Poti

Atmel EEPROM, digital Poti

Crystal ADC

Dallas RTC

EPSON RTC

Fairchild EEPROM

Infineon Pressure Sensor

Intel CAN Controller

Linear Technology ADC, DAC, Temperature Sensor + Voltage Monitor

Macronix FLASH

Maxim ADC, DAC, UART, Analog Switches

Microchip Micro controller, EEPROM, ADC, CAN controller

Motorola DSP, MCU

National Semiconductor LCD Controller, dig. Temperature Sensor, USB Controller

NeXFlash FLASH

RAMTRON FRAM

SanDisk FLASH, MultiMediaCard

SGS-Thomson EEPROM, Micro controller

Texas Instruments DSP, ADC, DAC

Xicor CPU Supervisor, EEPROMs, FLASH

Zilog DSP

CAN Protocol• History• Definition• Standards• Operation• Message Format• Data Link Layer• Physical Layer• Applications

History• CAN was first developed by Robert Bosch GmbH, Germany in

1986 when they were requested to develop a communication system between three ECUs (electronic control units) in vehicles by Mercedes.

• They found that an UART is no longer suitable in this situation because it is used in point-to-point communication. The need for a multi-master communication system became imperative.

• Is a high-integrity serial data communications bus for real-time applications

• Operates at data rates of up to 1 Megabits per second • Has excellent error detection and confinement capabilities • It is now being used in many other industrial automation and

control applications • International standard: ISO 11898 • sophisticated error detecting mechanisms and re-transmission

of faulty messages. This also guarantees data integrity.

Definition• CAN is a serial bus system with multi-master capabilities,

that is, all CAN nodes are able to transmit data and several CAN nodes can request the bus simultaneously

• A sender of information transmits to all devices on the bus. All receiving devices read the message and then decide if it is relevant to them. This guarantees data integrity as all devices in the system use the same information.

• Multi-Master Hierarchy: If one network node is defect the network is still able to operate.

• It is a two-wire, half duplex, high-speed network system and is well suited for high speed applications using short messages.

• CAN can theoretically link up to 2032 devices (one node with one identifier) on a single network.

• However, due to the practical limitation of the hardware (transceivers), it can only link up to110 nodes (with 82C250, Philips) on a single network.

Principle of operation• It uses Carrier Sense multiple Access with collision Detection

along with Arbitration on message priority [CSMA/CD +AMP].• Data messages transmitted from any node on a CAN bus do not

contain addresses of either the transmitting node, or of any intended receiving node.

• Instead, the content of the message is labeled by an identifier that is unique throughout the network .

• This identifier is the principle part of the CAN arbitration field, which is located in the beginning of each CAN message.

• All other nodes on the network receive the message and each performs an acceptance test on the identifier to determine if the message, and thus its content, is relevant to that particular node.

• If the message is relevant, it will be processed; otherwise it is ignored.

• The unique identifier also determines the priority of the message. • The higher priority message is guaranteed to gain bus access as

if it were the only message being transmitted. Lower priority messages are automatically re-transmitted in the next bus cycle

Principle of operation• The bits in a CAN message can be sent as either high or

low. • The low bits are always dominant, which means that if one

node tries to send a low and another node tries to send a high, the result on the buse will be a low.

• A node that sends a high in the arbitration field and detects a low knows that it has lost arbitration. It stops transmitting, letting the other node, with a higher priority message, continue uninterrupted.

• Two nodes on the network are not allowed to send messages with the same id.

• If two nodes try to send a message with the same id at the same time arbitration will not work. Instead, one of the transmitting nodes will detect that his message is distorted outside of the arbitration field. Error Handling Scheme will take care of this.

Frames in CAN• Data frames are used when a node wants to

transmit data on the network, and are the "normal" frame type.

• Remote frames can be described as a request for information. A frame with the RTR bit set means the transmitting node is asking for information of the type given by the identifier .

• Simpler CAN controllers (BasicCAN) can not respond automatically. In this case the host microcontroller is made aware of the remote request and has to send the data.

Message Format

• Standard CAN 2.0A format

• Extended CAN 2.0B format

Message Formats• 2.0B controllers are completely backward compatible

with 2.0A controllers and can transmit and receive messages in either format.

• There are 2 types of 2.0A controllers1. The first is capable of transmitting and receiving only

messages in 2.0A format. With this type of controller, reception of any 2.0B message will flag an error.

2. The second type of 2.0A controller (known as 2.0B passive) is also capable of sending and receiving 2.0A messages, but in addition, these devices will acknowledge receipt of 2.0B messages and then ignore them.

• The number of unique identifiers available to users, on a single 2.0A network, is 2,032 (2 to the power 11 - 2 to the power 4).

• The number of unique identifiers available on a 2.0B network is in excess of 500 million!

Data Link Layer [ISO 11898]

1. Bus Arbitration2. Error Detection3. Error Signaling4. Error Confinement

Error Detection• This Process is done automatically by a CAN controller.• The error is detected by the a CAN controller (a transmitter or a receiver). • An error frame is immediately transmitted. • The message is cancelled at all nodes• The status of the CAN controllers are updated. The message is re-transmitted. If

several controllers have messages to send, normal arbitration is used. Type of Errors:• Bit errors: 1. Bit stuffing error - normally a transmitting node inserts a high after five

consecutive low. This is called bit stuffing. A receiving node that detects violation 2. Bit error: A transmitting node always reads back the message as it is sending. If it

detects a different bit value on the bus than it sent, and the bit is not part of the arbitration field or in the acknowledgement field, and error is detected.

• Message errors: 1. Checksum error - each receiving node checks CAN messages for checksum errors. 2. Frame error - If a receiver detects an invalid bit in one of these positions of a

frame a Form Error will be flagged. 3. Acknowledgement Error - If a transmitter determines that a message has not been

Acknowledged then an ACK Error is flagged.

Error Modes• Error active - the normal operating mode for a

controller. Messages can be received and transmitted. On detecting an error an active error flag is sent (see error signaling).

• Error passive - a mode entered when the controller has frequent problems transmitting or receiving messages. Messages can be received and transmitted. On detecting an error while receiving, a passive error flag is sent.

• Bus off - entered if the controller has serious problems with transmitting messages. No messages can be received or transmitted until the CAN controller is reset by the host microcontroller or processor.

Bit Timings• Each bit is divided into four segments

1. Synchronisation segment (Synch_Seg): The synchronisation segment is used to synchronise the various nodes on the bus. When a bit is sent on the bus, the leading edge is expected to be within this segment.

2. Propagation segment (Prop_Seg):The Propagation Segment is needed to compensate for the delay in the bus lines.

3. Phase Segment 1 (Phase_Seg1), Phase Segment 2 (Phase_Seg2): These segments can be used lengthened or shortened by resynchronization.

Physical Layer• CAN transmits signals on the CAN bus

which consists of two wires, a CAN-High and CAN-Low.

• These 2 wires are operating in differential mode, that is they are carrying inverted voltages (to decrease noise interference)

• CAN High Speed [ISO 11898]• CAN Low Speed [ISO 11519]

CAN ImplementationBasic CAN Full CAN

Transmit It fills complete TX register including ID, RTR, data length , data

Transmit mailboxes initialized onceOnly data bytes written before transmission

Receive Every CAN message can be received

Only IDs defined in the receive mailboxes can be received

Remote Frame Handling Frames are answered by the application

Frames are answered automatically by the controller

Overrun Philosophy Keep the oldest message [new message will be lost]

Keep the newest message [old message will be lost]

References• SPI1. [http]: http://www.embedded.com/showArticle.jhtml?articleID=99004832. [http]: http://www.rpi.edu/dept/ecse/mpsd/SPI.pdf 3. [http]: http://www.answers.com/topic/serial-peripheral-interface-bus 4. [http]: http://www.mct.net/faq/spi.html 5. [http]: http://www.maxim-ic.com/appnotes.cfm?appnote_number=8026. [http]: http://controls.ame.nd.edu/~bill/microcontroller/main/node28.html 7. [http]: http://www.ece.osu.edu/ie/main/CurrentResearch/SPI_Nautilus_chip/ __________________________________________________________________________

• CAN1. [http]: http://www.can-cia.org/can/ 2. [http]: http://www.algonet.se/~staffann/developer/CAN.htm 3. [http]:

http://zone.ni.com/devzone/conceptd.nsf/webmain/264668060CFB196286257067007E57D4

Thank you

• Any Questions?

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