Solution Guide to I2C BUS Communication
Contents
1. elementiu theknode www element14 com knode Solution Guide to IZ2C BUS Communication Yiying Sun Global Technology Centre Volume l October 2011 elementiu theknode 1 I2C Overview l2C short for Inter Integrated Circuits is a well known standard developed by Philips semiconductors now NXP Semiconductors in early 1980s As its name suggests it is used for communications between ICs resided on the same circuit board Sometimes it is also called IIC or I C Its original purpose was to provide an easy way to connect multiple peripheral circuits to a central processing unit inside a TV set Before that time the common method of inter IC communication was via a parallel 8 bit wide bus For example the parallel data and address bus structure is used to connect memory to MCUs for data transfer It results in high interconnection cost such as more board area and I O pins needed The 12C bus can eliminate that drawback It is a simple bidirectional 2 wire serial bus interface which supports moderate speed and short distance communication By switching from parallel to serial communication this 2 wire bus structure reduces the number of data lines and lowers general purpose input output GPIO usage on the central processor which enables to simplify designs and reduce costs Originally the I2C bus was limited to 100 Kbps operation Over time there have been several additions to the specification so that it is able to support2. According to I2C specification the number of devices connected to the same bus is limited by the total bus capacitive load of 400 pF and also by the address space Suppose the total capacitance of I2C devices in one system is less than 400 pF for a 7 bit address scheme the maximum number of I2C devices that are allowed to communicate in the same bus is 128 In a typical application the master device usually is a processor and every slave device is linked to the processor directly via SDA and SCL lines When designing out such a PCB all necessary signals have to be routed directly from the processor to the I2C devices This type of layout takes up a lot of board space becomes tedious in trying to correctly lay it all out and ends up being an expensive approach In order to ease PCB routing 12C I O expander is introduced between a master and multiple slaves One side of the I O expander is connected to the I2C master devices and the other side is connected to multiple slave devices shown in Figure 5 The I O expander can also easily enhance system IO functionality such as LED controls PWM and real time clock functions keypad controls and GPIOs Solution Guide to I2C BUS Communication Page 10 element theknode 12C Slave Devices sae Figure 5 The 12C System With an I O Expander Table 4 Selection Guide of 12C I O Expanders Order Code Max Farnell Frequency No of I Os Newark ajement14 KHz HEISE rssencrenr
3. cames eo oo O SE ce OMT RST es T oo e magones oeo 8 a E mam vaese ooe o C Fomsens oeao sssse ao me pomore seas sso aoo C RST eres wo me FeaorPW momoa sense ao me FeARSSPWie Tro sere aoo woo w ow w Manufacturer Part Number PCA9539PW 118 70R6557 1831205 Texas PCA9535DWR 63K6052 1575233 Texas PCF8575CDBE4 97K6980 1053571 Texas TCA6408APWR 5517325 1903303 Solution Guide to 12C BUS Communication Page 11 elementiu theknode Texas TCA6416APWR 10R6151 1754897 Texas 6 Special Function I2C Devices I2C bus is currently popular in electronics world and widely supported by IC manufacturers Engineers can easily find an MCU integrated with I2C interface from different IC vendors Apart from the processors more I2C bus compatible devices incorporate an on chip interface which allows them to communicate directly with each other via the I2C bus They include memories LED LCD drivers sensors DACs ADCs and so on 6 1 I2C Serial EEPROM Serial EEPROMs are widely used in many different applications where data retention during power off is essential These EEPROMs are designed to provide simple but flexible data storage for most popular MCU based solutions They need only two wires to communicate the host processor The MCU can use any tow available I O pins to emulate the operation of the SDA and SCL signals required for 12C communications shown in Figure 6 For a write operat
4. possible that a combination of different speed I2C devices exits in a system with the bus speed controlled by I2C master 2 3 ZC Bus Communication I2C is a master slave protocol Therefore the communication is initiated by a master only When a master on the 12C bus wants to communicate with a slave it first has to take control of the bus This is only possible when the bus is free Then the master device issues a START condition A START condition is determined by a HIGH to LOW transition on the SDA line while SCL is HIGH This condition informs all the slave devices to listen on the serial data line for their respective address The bus is considered to be busy after the START condition After a START condition has been signalled the bus master will then send the unique ADDRESS of the target slave device along with a Read or Write flag The slave with the corresponding address then responds with an acknowledgement ACK signal Now the communication proceeds between the master and the Slave on the data line The transmitter sends 8 bits of data to the receiver which replies with a 1 bit acknowledgement When the communication is complete the master issues a STOP condition indicating that everything is done A STOP condition is indicated by a LOW to HIGH transition on the SDA line while SCL is HIGH After the STOP condition is generated the I2C bus is considered free again Solution Guide to I2C BUS Communication Page 4 elementiu theknod
5. to easily connect to the PICkit Serial Analyzer DV164122 The PICkit Serial Analyzer provides the 12C master mode serial communications and power The PICkit Serial 12C Demo Board devices all operate in the I2C slave mode Figure 10 PICkit Serial 12C Demo Board Solution Guide to I2C BUS Communication Page 15 elementiu theknode 7 3 Total Phase I2C Development Kit TP120112 Total Phase s 12C development kit TP120112 allows developers to develop and debug an I2C bus on an embedded system With this kit developers can exercise target devices on an 2C bus as a master device simulate an I2C master or slave device program and verify 12C based memory devices passively monitor an 12C bus in real time The I2C development kit contains an Aardvark I2C SPI host adapter a Beagle I2C SPI Protocol Analyzer and one I2C SPI Activity Board The Aardvark I2C SPI host adapter can actively send and receive I2C message as a master or slave device With this interface a PC can communicate directly with I2C devices Because every I2C device is different this kit offers a free API that allows users to create custom applications to interface with I2C based systems The Beagle analyzer provides a real time bus monitoring and data capture for users systems Aardvark l2C SPI Host Adapter Beagle 12C PI Protocol Analyzer I2C SPI Activity Board 10 Pin Split Cables Figure 11 TP120112 I2C Development Kit 8 Reference Design 8 1 I2
6. C Bus Slave Reference Design Figure 12 shows the block diagram of the I2C bus interface slave reference design from Digital Core Design The reference design provides an interface between a target device e g memory LCD display and pressure sensors and an 12C bus It can work as a receiver or transmitter depending on the working mode determined by the master It supports slave transmit and receive operations speeds up to 400 Kbps and 7 bit addressing Meanwhile there is no programming required This Solution Guide to I2C BUS Communication Page 16 elementiu theknode reference design can be used in automotive audio video systems and embedded systems datail7 0 datao 0 Sahiti Register SDAO SDAI Register Register zx SCL Register Controller Figure 13 I2C Bus Interface Slave Block Diagram 8 2 I2C Bus Master Controller Reference Design Figure 13 shows the reference design for I2C bus master controller form Lattice Semiconductor It is intended to demonstrate how a fast and configurable I2C Bus Master Controller can be constructed and utilized in a Lattice CPLD FPGA device The I2C Master Controller accepts commands from a microprocessor These commands are decoded into I2C slave device read write cycle transactions The 12C bus transactions can be configured to be 1 to 256 bytes in length With the flexibility that this 12C Bus Master Controller offers a designer can communicate with up to 128 different 12C slave de
7. case if you use an FPGA in your design and you want to build an I2C technology then you need to purchase a slave address from NXP 5 Common Problems in 2C Applications 5 1 Mixed Voltage Levels Today s electronics systems require different supply voltages to power their various components With the increase demand of low power consumption the high performance of MCUs and DSPs are implemented advanced process technology to get the supply voltages as low as 1 8V On the other hand some 12C peripheral devices continue to operate at 3 3V or 5V supplies 12C bus devices currently in the market range in supply voltage from 2 V to 5 V with commensurate I O and threshold levels It results that 12C devices with different voltage levels might be connected to the same bus Since the I2C bus lines are bidirectional interfacing 12C devices with different voltage levels is not straightforward special level translation are necessary for this purpose One solution for this is using a bidirectional voltage level translator without the need of a direction control signal Here is one voltage level translation example the TCA9406 from TI to show how dual channel signal switches can be used to Solution Guide to I2C BUS Communication Page 6 elementiu theknode translate I2C signals SDA and SCL between mixed voltage levels The TCA9406 is a dual bidirectional 12C Bus Voltage Level translator with enable OE Input It can interface between processo
8. e Matching Slave Master issues I2C Bus idl j us Idle K Master sends slave Data transfer start Master issues Master flags READ WRTIE Slave Address Invalid Figure 2 12C Communication 2 4 2C Address Scheme Each device connected to the I2C bus has a unique 7 bit or 10 bit address 12C communication is 8 bit bytes transmission so the master will send out the 7 bit address and add on the 8 bit which is the Read Write flag In another words the last bit indicates if the master is going to send write or receive read data from the slave When the address byte is sent each slave device in the system compares the first seven receiving bits with its address If they match the device considers itself addressed by the master as a slave receiver or slave transmitter depending upon the value of the data direction bit 8 bit In order to prevent address clashes in a large 12C system due to the limited range of the 7 bit addresses a 10 bit address scheme is used to increases the available address range about ten times However the use of 10 bit address is not very common therefore it is not discussed here in details 3 Applications of I2C Bus The I2C bus is a de facto industrial standard that is now widely used in computing consumer electronics communications and industrial electronics These systems all have some intelligent control usually by a central control processor MCU o
9. h means more than one devices can be the master to control the bus at the same time without corrupting the message For example there are multiple MCUs connected to I2C bus If two or more MCUs try to talk on the bus concurrently they have to determine whether the bus is currently idle or not before I2C transferring The arbitration procedure is used to decide which master is granted access and which master should wait for the next idle time Vcc tral SDA cessing J ooo o deo deg J ec Unit E i i i Peripheral Peripheral Peripheral Peripheral Device 1 Device 2 Device 3 Device n Figure 1 Connection of 12C Bus Devices As for the total number of devices connected to the I2C bus is determined by the maximum bus capacitance According to 12C specification a maximum bus Solution Guide to IZC BUS Communication Page 3 elementiu theknode capacitive load is 400pF This 400pF load includes all I2C devices as well as trace lengths 2 2 I2C Bus Speed I2C bus is able to be operated at four speeds or modes standard mode up to 100 Kbps fast mode 400 Kbps fast mode plus up to 1 Mbps and high speed mode up to 3 4 Mbps Most of the I2C devices available today support 400Kbps operation Higher speed operation may allow I2C to keep up with the rising demand for bandwidth in multimedia and other applications The 12C devices are fully downward compatible so any device may be operated at a lower bus speed It makes it
10. he processor to focus on other tasks in the application Without the 12C LED drivers the host MCU needs to send out repeated commands to turn LEDs on and off This significantly increases the amount of traffic on the MCU and uses up one of the MCU timers The LED driver instead requires to only program the individual port with the frequency and duty cycle and then it only requires one command from the master MCU to control the LEDs Vcc SDA P1 0 aa d y SO Central Processor MPU MCU Figure 7 12C LED Driver Interface with an MCU Solution Guide to 12C BUS Communication Page 13 elementiu theknode 6 3 I2C Digital Sensors I2C compatible sensors can communicate with the master MCU via two wire bus The most common used I2C sensors are temperature sensors Many systems across a variety of applications require some implementation of temperature monitoring These sensors can monitor the temperature of the thermal diode inside the CPU or the diode connected to transistors and can trigger an interrupt or alert output They are also able to produce accurate digital readings of the ambient temperature and can be used to trigger interrupt shut down or over temperature alarms Figure 8 shows a typical connection of temperature sensors and the host MCUs master Temperature CONTROLLER Sensors Ad Al Az Vee Figure 8 Interface 12C Temperature Sensors with the MCU 7 Development Tools Fo
11. how it works Solution Guide to I2C BUS Communication Page 2 elementiu theknode 2 1 I2C Bus Hardware Architecture I2C bus defines a data signal line SDA and a clock signal line SCL which supports bidirectional data transfer between a master and several slaves Regardless of how many devices are attached to the I2C bus there are only two signals connected to all of them A master is the device which initiates a data transfer on the bus and generates the clock signals to permit that transfer The device being addressed by the master is called a slave In Figure 1 the typical microcontroller MCU based system use I2C bus for communication between the host MCU and multiple peripherals such as memory LCD drivers data conversion devices etc In this case the MCU is the master and the peripherals are slaves Both the master MCU and slave peripherals can receive and transmit data In I2C bus both SDA and SCL are connected to a positive supply voltage Vcc through pull up resistors see in Figure 1 Because SDA and SCL are open drain the devices connected to I2C bus can only drive these lines low or leave them open The pull up resistors Rp pull the line up to HIGH Vcc when the bus is free or there is no device on the bus Rp commonly ranges from 1 kQ to 10 kQ resulting in typical pull up currents of about 1 mA and less So far we discussed a single master implementation of 12C bus But I2C bus is in fact a multi master bus whic
12. ion the microcontroller first sends a start condition Then the control byte is sent The control byte consists of the device code for the specific device that is being addressed the address of the memory block being accessed and a bit that indicates a write operation is being performed The EEPROM then acknowledges receipt of the byte The MCU then sends the specific memory address of the byte being written to The EEPROM acknowledges again The MCU then sends the data to be written into the memory byte After the EEPROM acknowledges the data the MCU sends a stop condition Similar process is used to read from EEPROM The only difference is that the control byte is sent twice once before the MCU sends the address byte and a second time before the EEPROM returns the requested data byte The read write bit of the control byte is also set to a read operation Solution Guide to I2C BUS Communication Page 12 elementiu theknode A P1 0 P1 1 Central Processor MPU MCU Figure 6 12C Serial EEPROM Interface with an MCU 6 2 LED Drivers with I2C Interface LED lighting becomes more and more popular in current market with the advancement of LED technology LED drivers are required to ensure the proper operation of LEDs and able to add extra intelligence to the lighting system such as blinking dimming and color mixing The purpose of this device is to free the processor from the LED controls which allows t
13. is their ability to isolate a section on the I2C bus through Il2C software This saves valuable pins on the processor which can be used for additional functionality Table 2 Selection Guide for I2C Multiplexers Switches Order Code Max Manufacturer Part Number Configuration Farnell Frequency IN OUT element14 KHz PCA9540BD 70R6561 1831206 NXP PCA9544APW 70R6598 1831207 PCA9546APW 70R6610 1831208 Newark 5 2 I2C Address Conflicts As mentioned previously a 7 or 10 bit addressing scheme is used to identify each device on I2C bus This address is usually defined by device manufacturers Therefore it is possible that two different functions of devices with same address such as LCD driver and temperature sensor are connected to same 12C bus It will cause addressing conflicts One multiplexer switch is a solution for address conflicts The multiplexer splits the I2C bus into several sub branches and allow the 12C master to select and address one of multiple identical devices thus resolving address conflict issues The selection guide for multiplexers switches is shown in Table 2 5 3 Limited Distance Communication I2C is essentially a short distance bus protocol for on board communication The maximum distance depends on the load of the bus and the speed which it operates Solution Guide to I2C BUS Communication Page 8 elementiu theknode at The I2C bus capacitance limit of 400 pF restricts practical communication d
14. istances to a few meters Meanwhile if the designer needs to go far at high speed he has to sacrifice the communication distance Fortunately with the I2C repeaters buffers you can solve the dilemma without any degradation of system performance These repeaters can provide I2C bus expansion and can successfully extend the line Capacitance to greater than the 400pF capacitance Additionally these repeaters buffers also support voltage level translation and are able to resolve address conflicts Figure 4 is an example for the TCA9509 I2C repeater from TI to solve this issue The TCA9509 is 5 V tolerant so it does not require any additional circuitry to translate between 0 9 V to 5 5 V bus voltages and 2 7 V to 5 5 V bus voltages In this example the system master is running on a 1 1 V I2C bus and the slave is connected to a 3 3 V bus Both buses run at 400 kHz Master devices can be placed on either bus The TCA9509 buffers both the serial data SDA and the serial clock SCL signals on the I2C bus thus allowing 400 pF bus capacitance on the B side This device can also be used to isolate two halves of a bus for voltage and Capacitance iiv 3 3 V TCA9509 ie SDAB sc Pe SCLB L e Ic bus master bus slave 400 kHz 400 kHz 7 Bus 0 E Bus 1 p Figure 4 Texas Instruments TCA9509 I2C Repeater Table 3 Selection Guide for I2C Buffers Repeaters Order Code Max Manufacturer Part Number Farnell Frequency hh element14 KHz
15. mp PORISSAOP moea neman O oo CO roots satan ooo me poao wor tote oo C Solution Guide to I2C BUS Communication Page 9 Supply Voltage V elementiu theknode PCA9512ADP 70R6472 1831199 2 7 5 5 Texas PCA9515AD 80K2690 1792276 PE E Texas PCA9517D 71M0457 1703190 0 9 5 5 Texas Ponss1eDW 5eMaso2 40 30 36 Teas TCA9509RVHR 51T4809 1926947 400 27 5 5 Instruments i 5 4 Mixed Speed Operation As we know currently 12C bus supports four different data rates standard mode 100 Kbps fast mode 400 Kbps fast mode plus 1 Mbps and high speed mode 3 4 Mbps According to I2C specification if different frequency devices are connected in the same bus in an application the lowest frequency must be used to guarantee a safe communication For example devices designed for the 100 kHz specification are not suitable to work properly at 400 KHz while the opposite is true In applications where upgrades have been performed by using newer 400 kHz devices while keeping the 100 kHz legacy devices it may become necessary to separate the 400 kHz devices from the 100 kHz devices when a 400 kHz 2C transfer is performed An I2C bus repeater can be used for this purpose One side of the device will have all the devices running at one frequency while the other side will have all the devices running to another frequency The selection guide for I2C repeaters is shown in Table 3 5 9 Increased Complexity of PCB Routing
16. o l2C 3 12C Manual http Awww nxp com documents application note AN10216 pdf 4 I2C bus Specification and User Manual http www nxp com documents user_manual UM10204 pdf 5 12C Guide hitp www ti com lit sg sszc003c sszc003c pdf Solution Guide to 12C BUS Communication Page 18 elementiu theknode Disclaimer The information contained in this document is for general purposes only Thanks to the suppliers websites who provided the information and while we endeavour to keep the information up to date and correct we make no representations or warranties of any kind express or implied about the completeness accuracy reliability suitability or availability with respect to the website or the information products services or related graphics contained on the website for any purpose Any reliance you place on such information is therefore strictly at your own responsibility Solution Guide to I2C BUS Communication Page 19
17. r I2C Bus 7 1 NXP I2C 2002 1A Evaluation Kit The I2C 2002 1A Evaluation kit is a low cost I2C based platform that allows designers to use their Personal Computer PC to easily test and demonstrate 12C devices in a platform that allows multiple operations to be performed in a setting similar to a real system environment The evaluation board features 11 different 12C slave devices including 1 temperature sensor 1 12C repeater 3 12C I O expanders 2 12C LED drivers 1 12C EEPROM 1 12C buffer and 2 I2C switches It also supports for easy programming and evaluation of various general purposes I2C products from NXP This evaluation board can be use to learn how to program and Blink LEDs using the GPIOs and or the LED drivers learn how the multiplexer serial EEPROM and temperature sensor work learn how to program the multiplexed 12C EEPROM This kit also includes a PC graphical interface that allows easy manipulation of 12C devices Solution Guide to 12C BUS Communication Page 14 Tad elementiI the knode Figure 9 12C 2002 1A Evaluation Board Microchip PICkit Serial I2C Demo Board The PICkit Serial 12C Demo Board demonstrates I2C serial communications and operation of the following devices 24LC02B 2Kbit Serial EEPROM MCP9801 High Accuracy Temperature Sensor MCP3221 Low Power 12 Bit A D Converter TC1321 10 Bit Digital to Analog Converter MCP23008 8 Bit I O Expander The PICkit Serial 12C Demo Board was designed
18. r MPU A typical 12C system usually has at least one MCU peripheral memory hard disk A D or D A converters graphic chip and other I O ports Nowadays almost all MCUs are integrated a real 12C interface in hardware but this should not restrict the use of Solution Guide to I2C BUS Communication Page 5 elementiu theknode the 12C bus on other MCUs Any MCU can be made to I2C communicate via general purpose I Os using software routines In current market besides MCUs I2C compatible devices are available in a wide range of functions Some examples are listed including but not limited to e Serial EEPROM RAM FLASH e O Expanders e Real Time Clocks e Digital to Analog Converter Analog to Digital Converter e LCD LED Drivers e Sensors such as temperature sensors capacitive sensors IR sensors etc e MEMS Sensors such as accelerometers compass etc 4 12C Legal Information The use and manufacturing of I2C devices is subject to a copyright and patents from NXP This means that electronics system designers need to purchase a license from NXP to use I2C technology in their system Fortunately as of October 10 2006 no licensing fees are required to implement the 12C protocol However fees are still required to obtain IC slave addresses allocated by NXP If you use an IC component which incorporating the technology has been licensed by IC manufacturers then you can immediately use these components for free In another
19. rs operating from 1 8 V to 3 3 V and the I2C slave devices operating from 2 3 V to 5 5 V It supports standard and fast mode I2C devices and multiple masters 3 3 V 5V TCA9406 Voca Veces Sman 4 oOo n SCLA SCLB gt DDC SCL OF Backplane or HDMI Connector Figure 3 Typical Application Circuit of TCA9406 Table 1 Selection Guide for I2C Voltage Translators Level Order Code Translation Range V I2C Speed Newark Farell Supported Newark Farnell Port Port upp element14 A B Standard Maxim 2 qe5 Ode rasi Integrated MAX3373EEKA T 68K9426 1787504 1055 t055 mode Fast mode Plus Manufacturer Part Number Standard 10to 18to mode Fast PCA9306D 70R6428 1831194 5 mode i Fast mode Plus 2 0 3 5 0 Standard Texas PCA9306DCUTE4 97K6976 1053573 1 210 1810 node Fast Instruments 3 5 0 mode Standard Texas TCA9406DCTR 59T0097 1972384 169 23to mode Fast Instruments to 3 6 5 ode Solution Guide to 2C BUS Communication Page 7 elementiu theknode Another solution for this issue is to use a multiplexer switch An 12C multiplexer basically expands the I2C Bus as it splits the SCL and SDA signals from the master into one or more SCL and SDA Channels where each channel can support unique functions and needs As each channel in the multiplexer is supply independent each channel can be pulled to a different supply voltage Another advantage of these multiplexers
20. up to 3 4 Mbps today However unlike other serial communication the 12C bus is a multi master bus which means each device connected to the bus can control the bus Because all communication takes place on only two wires all devices must have a unique address to identify it on the bus Communication between master and slave is bi directional Collision detection and arbitration mechanisms are present to prevent data corruption if two or more masters simultaneously initiate data transfer Hence there is no need for chip selection or arbitration logic making it simple to implement in the design With the advancement of the technology the implementation of 12C bus is not limited to inside a single board only It can be extended across multiple boards via bus extensions and control devices For example a host microcontroller on a main embedded board uses I2C to communicate with user interface devices located on a separate front panel board Because of its simplicity and flexibility the I2C bus is widely used in microcontroller microprocessor base systems across all industry segments It is also adopted by a large and growing number of semiconductor manufacturers These companies offer a variety of electronic devices including memories input and output devices different types of sensors real time clocks displays and much more 2 I2C Bus Theory of Operation In order to use the I2C bus properly and efficiently it is important to understand
21. vices operating in standard or fast mode with transactions ranging from 1 to 256 bytes Furthermore the 12C Master Controller can operate in interrupt or polling mode This means that the microprocessor programmer can choose to poll the 12C Master for a change in status at periodic intervals or wait to be interrupted by the 12C Master Controller when data needs to be read or written Solution Guide to I2C BUS Communication Page 17 elementiu theknode lC Master Controller Top DA Microprocessor fC Controller State Machina Arbitration Microprocessor Lirit Interface Figure 14 12C Master Controller I O Interface 9 Summary There are thousands of I2C devices on the market today ranging from data converters serial EEPROM digital sensors to video processors Those devices are built in on chip programmable registers to provide an ideal low cost solution to establish communications between these intelligent devices and the central controller Nearly every microcontroller vendor has taken care of the physical implementation of the bus even if some of the MCUs do not have on chip I2C hardware interface the I2C bus still can be implemented using software In addition since silicon area and I O pins heavily influence the IC price 12C has a lot to offer in cost sensitive applications 10 References 1 www i2c bus org 2 Introduction to 12C http Awww eetimes com discussion beginner s corner 4023816 Introduction t
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