Low Speed Serial Interfaces

SPI and I2C

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The I2C Bus

• What is the I2C Bus and what is it used for?• Bus characteristics• I2C Bus Protocol• Data Format• Typical I2C devices• Example device• Sample pseudo code

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What is I2C• The name stands for “Inter - Integrated Circuit Bus” • A Small Area Network connecting ICs and other electronic

systems • Originally intended for operation on one

single board / PCB – Synchronous Serial Signal – Two wires carry information between

a number of devices – One wire use for the data – One wire used for the clock

• Today, a variety of devices are available with I2C Interfaces – Microcontroller, EEPROM, Real-Timer, interface chips, LCD driver, A/D

converter

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What is I2C used for?

• Data transfer between ICs and systems at relatively low rates – “Classic” I2C is rated to 100K bits/second – “Fast Mode” devices support up to 400K bits/second – A “High Speed Mode” is defined for operation up to 3.4M

bits/second• Reduces Board Space and Cost By:

– Allowing use of ICs with fewer pins and smaller packages – Greatly reducing interconnect complexity – Allowing digitally controlled components to be located

close to their point of use

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I2C Bus Characteristics• Includes electrical and timing specifications,

and an associated bus protocol • Two wire serial data & control bus implemented with the

serial data (SDA) and clock (SCL) lines – For reliable operation, a third line is required:

Common ground • Unique start and stop condition • Slave selection protocol uses a 7-Bit slave address

– The bus specification allows an extension to 10 bits • Bi-directional data transfer • Acknowledgement after each transferred byte • No fixed length of transfer

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I2C Bus Characteristics (cont’d)• True multi-master capability

– Clock synchronization – Arbitration procedure

• Transmission speeds up to 100Khz (classic I2C)

• Max. line capacitance of 400pF,approximately 4 meters (12 feet)

• Allows series resistor for IC protection • Compatible with different IC technologies

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I2C Bus Definitions

• Master: – Initiates a transfer by generating

start and stop conditions – Generates the clock – Transmits the slave address – Determines data transfer direction

• Slave: – Responds only when addressed – Timing is controlled by the clock line

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I2C Bus Configuration Example

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I2C Hardware Details • Devices connected to the bus must have an open drain or

open collector output for serial clock and data signal • The device must also be able to sense the logic level on

these pins • All devices have a common ground reference• The serial clock and data lines are connected to Vdd(typically

+5V) through pull up resistors • At any given moment the I2C bus is:

– Quiescent (Idle), or– in Master transmit mode or – in Master receive mode.

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I2C Electrical Aspects

• I2C devices are wire ANDed together.• If any single node writes a zero, the entire line is zero

I2C Bus Configuration

• 2-wire serial bus – Serial data (SDA) and Serial clock (SCL)• Half-duplex, synchronous, multi-master bus• No chip select or arbitration logic required• Lines pulled high via resistors, pulled down via open-drain drivers

(wired-AND)

I2C Protocol

1. Master sends start condition (S) and controls the clock signal2. Master sends a unique 7-bit slave device address3. Master sends read/write bit (R/W) – 0 - slave receive, 1 - slave transmit4. Receiver sends acknowledge bit (ACK)5. Transmitter (slave or master) transmits 1 byte of data

I2C Protocol (cont.)

6. Receiver issues an ACK bit for the byte received7. Repeat 5 and 6 if more bytes need to be transmitted.8.a) For write transaction (master transmitting), master issues stop condition

(P) after last byte of data.8.b) For read transaction (master receiving), master does not acknowledge

final byte, just issues stop condition (P) to tell the slave the transmission is done

I2C Signals

• Start – high-to-low transition of the SDA line while SCL line is high• Stop – low-to-high transition of the SDA line while SCL line is high• Ack – receiver pulls SDA low while transmitter allows it to float high• Data – transition takes place while SCL is slow, valid while SCL is high

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Bit Transfer on the I2C Bus • In normal data transfer, the data line only changes state

when the clock is low

SDA

SCLData line stable; Data valid

Change of data allowed

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Start and Stop ConditionsA transition of the data line while the clock line is high is defined as either a start or a stop condition.Both start and stop conditions are generated by the bus master The bus is considered busy after a start condition, until a stop condition occurs

Start Condition

Stop Condition

SCL SCL

SDASDA

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I2C Addressing

• Each node has a unique 7 (or 10) bit address • Peripherals often have fixed and programmable

address portions • Addresses starting with 0000 or 1111 have special

functions:- – 0000000 Is a General Call Address – 0000001 Is a Null (CBUS) Address – 1111XXX Address Extension – 1111111 Address Extension – Next Bytes are the

Actual Address

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MSB

ACK

LSB

7 – Bit Slave Address

R / Wr

First Byte in Data Transfer on the I2C Bus

R/Wr

0 – Slave written to by Master

1 – Slave read by Master

ACK – Generated by the slave whose address has been output.

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I2C Bus Connections

• Masters can be – Transmitter only – Transmitter and receiver

• Slaves can be – Receiver only – Receiver and transmitter

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Acknowledgements• Master/slave receivers pull data line low for one clock pulse

after reception of a byte • Master receiver leaves data line high after receipt of the last

byte requested • Slave receiver leaves data line high on the byte following the

last byte it can accept

Acknowledgement from receiver

Transmitter releases SDA line during 9th clock pulse.

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Acknowledgements

• From Slave to Master Transmitter: – After address received correctly – After data byte received correctly

• From Slave to Master Receiver: – Never (Master Receiver generates ACK)

• From Master Transmitter to Slave: – Never (Slave generates ACK)

• From Master Receiver to Slave: – After data byte received correctly

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Negative Acknowledge

• Receiver leaves data line high for one clock pulse after reception of a byte

Not acknowledgement (NACK) from receiver

Transmitter releases SDA line during 9th clock pulse.

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Negative Acknowledge (Cont’d.)

• From Slave to Master Transmitter: – After address not received correctly – After data byte not received correctly – Slave Is not connected to the bus

• From Slave to Master Receiver: – Never (Master Receiver generates ACK)

• From Master Transmitter to Slave: – Never (Slave generates ACK)

• From Master Receiver to Slave: – After last data byte received correctly

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Data Transfer on the I2C Bus• Start Condition • Slave address + R/W

– Slave acknowledges with ACK • All data bytes

– Each followed by ACK • Stop Condition

ACK from Slave ACK from Receiver

Remember : Clock is produced by MasterStart Stop

SCL

SDA

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Data FormatsMaster writing to a Slave

AAA

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Data Formats Cont’d.Master reading from a Slave :

Master is Receiver of data and Slave is Transmitter of data.

1

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Data Formats Cont’d.Combined Format

A repeated start avoids releasing the bus and therefore prevents another master from taking over the bus

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Multi-master I2C Systems

• Multimaster situations require two additional features of the I2C protocol

• Arbitration: – Arbitration is the procedure by which competing masters

decide final control of the bus – I2C arbitration does not corrupt the data transmitted by

the prevailing master – Arbitration is performed bit by bit until it is uniquely

resolved – Arbitration is lost by a master when it attempts to assert a

high on the data line and fails

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Arbitration Between Two Masters

• As the data line is like a wired AND, a ZERO address bit overwrites a ONE • The node detecting that it has been overwritten stops transmitting and

waits for the Stop Condition before it retries to arbitrate the bus

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Error Checking

• I2C defines the basic protocol and timing – Protocol errors are typically flagged by the interface – Timing errors may be flagged, or in some cases could be

interpreted as a different bus event • Glitches (if not filtered out) could potentially

cause: – Apparent extra clocks – Incorrect data – “Locked” bus

• Microprocessors communicating with each other can add a checksum or equivalent

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Bus Recovery

• An I2C bus can be “locked” when: – A Master and a Slave get out of synch – A Stop is omitted or missed (possibly due to noise) – Any device on the bus holds one of the lines low

improperly, for any reason – A shorted bus line

• If SCL can be driven, the Master may send extra clocks until SDA goes high, then send a Stop.

• If SCL is stuck low, only the device driving it can correct the problem.

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Type of I2C Implementations

• Byte Oriented Interface – Data is handled one byte at a a time – Processor interprets a status byte when an event occurs – For instance Philips 8xC554, 8xC591

• Bit Oriented Interface – Processor is involved in every bus event when the interface is not Idle

• “Bit Banged” – Implemented completely in software on 2 regular I/O pins of the

microcontroller – Works for single master systems – Not recommended for Slave devices or Multimaster systems

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Available I2C Devices• Analog to Digital Converters (A/D, D/A): MMI functions,

battery & converters, temperature monitoring, control systems• Bus Controller: Telecom, consumer electronics, automotive,

Hi-Fi systems, PCs, servers• Bus Repeater, Hub & Expander: Telecom, consumer

electronics, automotive, Hi-Fi systems, PCs, servers• Real Time Clock (RTC)/Calendar: Telecom, EDP, consumer

electronics, clocks, automotive, Hi-Fi systems, FAX, PCs, terminals

• DIP Switch: Telecom, automotive, servers, battery & converters, control systems

• LCD/LED Display Drivers: Telecom, automotive instrument driver clusters, metering systems, POS terminals, portable items, consumer electronics

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Available I2C Devices

• General Purpose Input/Output (GPIO) Expanders and LED Display Control: Servers, keyboard interface, expanders, mouse track balls, remote transducers, LED drive, interrupt output, drive relays, switch input

• Multiplexer & Switch: Telecom, automotive instrument driver clusters, metering systems, POS terminals, portable items, consumer electronics

• Serial RAM/ EEPROM: Scratch pad/ parameter storage• Temperature & Voltage Monitor: Telecom, metering systems,

portable items, PC, servers• Voltage Level Translator: Telecom, servers, PC, portable items,

consumer electronics

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End use

• Telecom: Mobile phones, Base stations, Switching, Routers

• Data processing: Laptop, Desktop, Workstation, Server

• Instrumentation: Portable instrumentation, Metering systems

• Automotive: Dashboard, Infotainment• Consumer: Audio/video systems, Consumer

electronics (DVD, TV etc.)

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Applications

• There are some specific applications for certain types of I2C devices such as TV or radio tuners, but in most cases a general purpose I2C device can be used in many different applications because of its simple construction.

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I2C designer benefits

• Functional blocks on the block diagram correspond with the actual ICs; designs proceed rapidly from block diagram to final schematic

• No need to design bus interfaces because the I2C-bus interface is already integrated on-chip

• Integrated addressing and data-transfer protocol allow systems to be completely software-defined

• The same IC types can often be used in many different applications

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I2C designer benefits

• Design-time improves as designers quickly become familiar with the frequently used functional blocks represented by I2C-bus compatible ICs

• ICs can be added to or removed from a system without affecting any other circuits on the bus

• Fault diagnosis and debugging are simple; malfunctions can be immediately traced

• Software development time can be reduced by assembling a library of reusable software modules

• The simple 2-wire serial I2C-bus minimizes interconnections so ICs have fewer pins and there are fewer PCB tracks; resulting in smaller and less expensive PCBs

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I2C Manufacturers benefits• The completely integrated I2C-bus protocol eliminates the

need for address decoders and other ‘glue logic’

• The multi-master capability of the I2C-bus allows rapid testing/alignment of end-user equipment via external connections to an assembly-line

• Increases system design flexibility by allowing simple construction of equipment variants and easy upgrading to keep design up-to-date

• The I2C-bus is a de facto world standard that is implemented in over 1000 different ICs (Philips has > 400) and licensed to more than 70 companies

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Example – EEPROM (Part 24WC32)• 400 KHz I2C Bus Compatible*• 1.8 to 6 Volt Read and Write

Operation• Cascadable for up to Eight Devices• 32-Byte Page Write Buffer• Self-Timed Write Cycle with Auto-

Clear• Zero Standby Current• Commercial, Industrial and

Automotive Temperature Ranges

Write Protection– Entire Array Protected When WP at VIH

1,000,000 Program/Erase Cycles

100 Year Data Retention

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• 32KBit memory organise as 4K x 8bit • 12 address bits (2^12 = 4K)• Device Address :

• Writing– Byte Write– Page Write– Write time 10mS maximum– Write acknowledge Polling

• Reading– Immediate/Current address reading– Selective/Random Read– Sequential Read

24WC32 Characteristics

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Writing a Single Data Byte

After the STOP bit is receive the device internally programs the EEPROM with the received data byte.

The programming can take up to 10ms (max.). The device will be busy during this period and will not respond to its slave address.

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Writing Multiple Bytes (Page Write)

The bytes are received by the device and stored internally in a buffer before being programmed into the EEPROM.

A maximum of 32 bytes (one page = 32 bytes) may be written at one time for the 24WC32 device.

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Reading EEPROM

Read current location

Read specified location – Note repeated start to prevent loss of bus during read process.

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Introduction - SPI What is it? Basic Serial Peripheral Interface (SPI) Capabilities Protocol Pro / Cons and Competitor Uses Conclusion

Serial Peripheral Interface

What is SPI?

• Serial Bus protocol• Fast, Easy to use, Simple• Everyone supports it

SPI Basics A communication protocol using 4 wires

Also known as a 4 wire bus Used to communicate across small

distances Multiple Slaves, Single Master Synchronized

Capabilities of SPI Always Full Duplex

Communicating in two directions at the same time

Transmission need not be meaningful Multiple Mbps transmission speed Transfers data in 4 to 16 bit characters Multiple slaves

Daisy-chaining possible

Protocol Wires:

Master Out Slave In (MOSI) Master In Slave Out (MISO) System Clock (SCLK) Slave Select 1…N

Master Set Slave Select low Master Generates Clock Shift registers shift in and out data

Advantages and drawbacks• SPI is a very simple communication protocol.

– It does not have a specific high-level protocol which means that there is almost no overhead.

• Data can be shifted at very high rates in full duplex mode– This makes it very simple and efficient in a

single master single slave scenario. • The exchange itself has no pre-defined protocol. This makes it ideal for

data-streaming applications. • Data can be transferred at high speed, often into the range of the tens of

megaHertz. • The flipside is that there is no acknowledgment, no flow control, and the

master may not even be aware of the slave's presence / or absence. – You could do “some” handshaking via software

Systems that use SPI

The question is of course, which peripheral types exist and which can be connected to the host processor. Peripheral types can be subdivided into the following categories:– Converters (ADC and DAC) – Memories (EEPROM and FLASH) – Real Time Clocks (RTC) – Sensors (temperature, pressure) – Others (signalmixer, potentiometer, LCD controller, UART, CAN

controller, USB controller, amplifier)

Concept of Master and Slave

• Master– The component

that initiates the transfer

– The component that controls the transfer

• Slave– The component

that responds to the transfer

Master / Slave conceptSlave Select (Chip Select)

• Master sends out active low chip select signal SS1, then slave 1 responds

• Master sends out active low chip select signal SS2, then slave 2 responds

FOR SAFETY – SELECT SIGNAL IS “ACTIVE LOW” NOT “ACTIVE HIGH”

Master / Slave conceptMaster to Slave data movement

• Master sends out information to slave on MOSI wire

• Slave receives information from the master on MOSI wire

• Information (bits) is clocked by SCLK signal. – 1-bit, 1 clock tick

MOSI --MASTER OUT – SLAVE IN

Master / Slave conceptSlave to Master data movement

• Master receives information from slave on MISO wire

• Slave sends information to the master on MISO wire

• Information (bits) is clocked by SCLK signal. – 1-bit, 1 clock tick

MISO --MASTER IN – SLAVE OUT

Wires in Detail MOSI – Carries data out of Master to Slave MISO – Carries data from Slave to Master

Both signals happen for every transmission SS_BAR – Unique line to select a slave SCLK – Master produced clock to

synchronize data transfer

Shifting Protocol

Master shifts out data to Slave, and shift in data from Slavehttp://upload.wikimedia.org/wikipedia/commons/thumb/b/bb/SPI_8-bit_circular_transfer.svg/400px-SPI_8-bit_circular_transfer.svg.png

Diagram

Master and multiple independent slaves

Master and multiple daisy-chained slaves

Some wires have been renamed

Clock Phase (Advanced) Two phases and two polarities of clock Four modes Master and selected slave must be in same

mode Master must change polarity and phase to

communicate with slaves of different numbers

Timing Diagram

Timing Diagram – Showing Clock polarities and phaseshttp://www.maxim-ic.com.cn/images/appnotes/3078/3078Fig02.gif

Pros and ConsPros: Fast and easy

Fast for point-to-point connections Easily allows streaming/Constant data inflow No addressing/Simple to implement

Everyone supports itCons: SS makes multiple slaves very complicated No acknowledgement ability No inherent arbitration No flow control

Uses Some Serial Encoders/Decoders,

Converters, Serial LCDs, Sensors, etc. Pre-SPI serial devices PPC

PPC implements SPI well The bus of choice for communicating with

small peripherals

Conclusion SPI – 4 wire serial bus protocol

MOSI MISO SS SCLK wires Full duplex Multiple slaves, One master Best for point-to-point streaming data Easily Supported

Universal Asynchronous Receiver/Transmitter (UART)

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UART (Universal Asynchronous Receiver/Transmitter)

• Most UARTS are full duplex – they have separate pins and electronic hardware for the transmitter and receiver that allows serial output and serial input to take place simultaneously.

• Based around shift registers and a clock signal.• UART clock determines baud rate• UART frames the data bits with

– a start bit to provide synchronisation to the receiver– one or more (usually one) stop bits to signal end of data

• Most UARTs can also optionally generate parity bits on transmission and parity checking on reception to provide simple error detection.

• UARTs often have receive and transmit buffers(FIFO's) as well as the serial shift registers

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UART - Transmitter

• Transmitter (Tx) - converts data from parallel to serial format– inserts start and stop bits – calculates and inserts parity bit if required– output bit rate is determined by the UART clock

Serial output

Parallel data

UART Clock from baud rate generator

Status information

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Asynchronous serial transmission1

0

Serial transmission is little endian (least significant bit first)

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UART - The Receiver– synchronises with transmitter using the falling edge of the start bit.– samples the input data line at a clock rate that is normally a multiple of

baud rate, typically 16 times the baud rate.– reads each bit in middle of bit period (many modern UARTs use a

majority decision of the several samples to determine the bit value)– removes the start and stop bits, optional calculates and checks the

parity bit. Presents the received data value in parallel form.

Serial input

Status information

Parallel data

UART Clock from baud rate generator

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Asynchronous serial reception

Idle

waiting for start bit

Start bit

1First data bit

etc.

0Start detected

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UARTs

• Usually used on simple systems• Typically point to point communications• Various different formats and protocols• Normally 8-bit data format with one start and one stop bit• Standards: E.g. RS232

– defines connector type, pin assignments, voltage levels, max bit rate, cable length etc.

– Min. 3 pins – TxD, RxD, Ground– Other pins for data flow control.

• Some common RS232 baud rates - 300,1200,9600,19200• Handshaking

– None– Hardware - RTS, CTS, etc - simple logic levels– Software - Xon/Xoff protocol

The LPC23xx UARTs

• UART1 is identical to UART0/2/3, but with the addition of a modem interface.

• 16 byte Receive and Transmit FIFOs.• Register locations conform to ‘550 industry

standard.• Receiver FIFO trigger points at 1, 4, 8, and 14

bytes.• Built-in baud rate generator.• Standard modem interface signals included

(CTS, DCD, DTS, DTR, RI, RTS).• Either software or hardware flow control can

be implemented.

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UART Registers

• Control registers• Transmit• Receive• FIFO control• Status• Interrupt• Interrupt enable• Format control• Baud rate control 7-73