Philips Semiconductors The I2C-bus specification 13.5.1 F/S-mode transfer in a mixed-speed bus system 13.5.2 Hs-mode transfer in a mixed-speed bus system 13.5.3 Timing requirements for the bridge in a mixed-speed bus system CONTENTS 1 PREFACE3 1.1 1.2 1.3 Version 1.0- 1992 Version 2.0- 1998 Purchase of Philips I2C-bus components 2 THE I2C-BUS BENEFITS DESIGNERS AND MANUFACTURERS 2.1 2.2 3 14 10-BIT ADDRESSING Designer benefits Manufacturer benefits 14.1 14.2 14.3 INTRODUCTION TO THE I2C-BUS SPECIFICATION Definition of bits in the first two bytes Formats with 10-bit addresses General call address and start byte with 10-bit addressing 15 ELECTRICAL SPECIFICATIONS AND TIMING FOR I/O STAGES AND BUS LINES I2C-BUS 4 THE CONCEPT 5 GENERAL CHARACTERISTICS 6 BIT TRANSFER 15.1 15.2 Standard- and Fast-mode devices30 Hs-mode devices 6.1 6.2 Data validity START and STOP conditions 16 ELECTRICAL CONNECTIONS OF I2C-BUS DEVICES TO THE BUS LINES 7 TRANSFERRING DATA 16.1 7.1 7.2 Byte format Acknowledge Maximum and minimum values of resistors Rp and Rs for Standard-mode I2C-bus devices 8 ARBITRATION AND CLOCK GENERATION 17 APPLICATION INFORMATION 17.1 8.1 8.2 8.3 Synchronization Arbitration Use of the clock synchronizing mechanism as a handshake Slope-controlled output stages of Fast-mode I2C-bus devices Switched pull-up circuit for Fastmode I2C-bus devices Wiring pattern of the bus lines42 Maximum and minimum values of resistors Rp and Rs for Fast-mode I2C-bus devices Maximum and minimum values of resistors Rp and Rs for Hs-mode I2C-bus devices 9 FORMATS WITH 7-BIT ADDRESSES 10 7-BIT ADDRESSING 10.1 10.1.1 10.1.2 10.1.3 Definition of bits in the first byte General call address START byte CBUS compatibility 11 EXTENSIONS TO THE STANDARDMODE I2C-BUS SPECIFICATION 12 FAST-MODE 13 Hs-MODE 13.1 13.2 13.3 13.4 13.5 High speed transfer Serial data transfer format in Hs-mode Switching from F/S- to Hs-mode and back Hs-mode devices at lower speed modes Mixed speed modes on one serial bus system 17.2 17.3 17.4 17.5 18 BI-DIRECTIONAL LEVEL SHIFTER FOR F/S-MODE I2C-BUS SYSTEMS 18.1 Connecting devices with different logic levels 18.1.1 Operation of level shifter 42 19 DEVELOPMENT TOOLS AVAILABLE FROM PHILIPS 20 SUPPORT LITERATURE Philips Semiconductors The I2C-bus specification 1 1.1 1.2 PREFACE Version 2.0 - 1998 I2C-bus The has become a de facto world standard that is now implemented in over 1000 different ICs and licensed to more than 50 companies. Many of today's applications, however, require higher bus speeds and lower supply voltages. This updated version of the I2C-bus specification meets those requirements and includes the following modifications: Version 1.0 - 1992 This version of the 1992 I2C-bus specification includes the following modifications: * Programming of a slave address by software has been omitted. The realization of this feature is rather complicated and has not been used. * The "low-speed mode" has been omitted. This mode is, in fact, a subset of the total I2C-bus specification and need not be specified explicitly. * The High-speed mode (Hs-mode) is added. This allows an increase in the bit rate up to 3.4 Mbit/s. Hs-mode devices can be mixed with Fast- and Standard-mode devices on the one I2C-bus system with bit rates from 0 to 3.4 Mbit/s. * The Fast-mode is added. This allows a fourfold increase of the bit rate up to 400 kbit/s. Fast-mode devices are downwards compatible i.e. they can be used in a 0 to 100 kbit/s I2C-bus system. * 10-bit addressing is added. This allows 1024 additional slave addresses. * The low output level and hysteresis of devices with a supply voltage of 2 V and below has been adapted to meet the required noise margins and to remain compatible with higher supply voltage devices. * Slope control and input filtering for Fast-mode devices is specified to improve the EMC behaviour. * The 0.6 V at 6 mA requirement for the output stages of Fast-mode devices has been omitted. NOTE: Neither the 100 kbit/s I2C-bus system nor the 100 kbit/s devices have been changed. * The fixed input levels for new devices are replaced by bus voltage-related levels. * Application information for bi-directional level shifter is added. 1.3 Purchase of Philips I2C-bus components Purchase of Philips I2C components conveys a license under the Philips' I2C patent to use the components in the I2C system provided the system conforms to the I2C specification defined by Philips. 43 Philips Semiconductors The I2C-bus specification 2 * The number of ICs that can be connected to the same bus is limited only by a maximum bus capacitance of 400 pF. THE I2C-BUS BENEFITS DESIGNERS AND MANUFACTURERS In consumer electronics, telecommunications and industrial electronics, there are often many similarities between seemingly unrelated designs. For example, nearly every system includes: Figure 1 shows two examples of I2C-bus applications. 2.1 * Some intelligent control, usually a single-chip microcontroller Designer benefits I2C-bus compatible ICs allow a system design to rapidly progress directly from a functional block diagram to a prototype. Moreover, since they `clip' directly onto the I2C-bus without any additional external interfacing, they allow a prototype system to be modified or upgraded simply by `clipping' or `unclipping' ICs to or from the bus. * General-purpose circuits like LCD drivers, remote I/O ports, RAM, EEPROM, or data converters * Application-oriented circuits such as digital tuning and signal processing circuits for radio and video systems, or DTMF generators for telephones with tone dialling. Here are some of the features of I2C-bus compatible ICs which are particularly attractive to designers: To exploit these similarities to the benefit of both systems designers and equipment manufacturers, as well as to maximize hardware efficiency and circuit simplicity, Philips developed a simple bi-directional 2-wire bus for efficient inter-IC control. This bus is called the Inter IC or I2C-bus. At present, Philips' IC range includes more than 150 CMOS and bipolar I2C-bus compatible types for performing functions in all three of the previously mentioned categories. All I2C-bus compatible devices incorporate an on-chip interface which allows them to communicate directly with each other via the I2C-bus. This design concept solves the many interfacing problems encountered when designing digital control circuits. * 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 * Design-time reduces as designers quickly become familiar with the frequently used functional blocks represented by I2C-bus compatible ICs Here are some of the features of the I2C-bus: * ICs can be added to or removed from a system without affecting any other circuits on the bus * Only two bus lines are required; a serial data line (SDA) and a serial clock line (SCL) * Fault diagnosis and debugging are simple; malfunctions can be immediately traced * Each device connected to the bus is software addressable by a unique address and simple master/slave relationships exist at all times; masters can operate as master-transmitters or as master-receivers * Software development time can be reduced by assembling a library of reusable software modules. * It's a true multi-master bus including collision detection and arbitration to prevent data corruption if two or more masters simultaneously initiate data transfer In addition to these advantages, the CMOS ICs in the I2C-bus compatible range offer designers special features which are particularly attractive for portable equipment and battery-backed systems. * Serial, 8-bit oriented, bi-directional data transfers can be made at up to 100 kbit/s in the Standard-mode, up to 400 kbit/s in the Fast-mode, or up to 3.4 Mbit/s in the High-speed mode They all have: * Extremely low current consumption * High noise immunity * On-chip filtering rejects spikes on the bus data line to preserve data integrity * Wide supply voltage range * Wide operating temperature range. 44 Philips Semiconductors The I2C-bus specification handbook, full pagewidth SDA SCL MICROCONTROLLER PCB83C528 PLL SYNTHESIZER TSA5512 NON-VOLATILE MEMORY PCF8582E M/S COLOUR DECODER TDA9160A STEREO / DUAL SOUND DECODER SDA TDA9840 PICTURE SIGNAL IMPROVEMENT TDA4670 SCL DTMF GENERATOR PCD3311 LINE INTERFACE HI-FI AUDIO PROCESSOR TDA9860 PCA1070 VIDEO PROCESSOR TDA4685 ADPCM PCD5032 BURST MODE CONTROLLER SINGLE-CHIP TEXT SAA52XX PCD5042 ON-SCREEN DISPLAY PCA8510 MICROCONTROLLER P80CLXXX MSB575 (a) (b) Fig.1 Two examples of I2C-bus applications: (a) a high performance highly-integrated TV set (b) DECT cordless phone base-station. 45 Philips Semiconductors The I2C-bus specification 2.2 * A system that performs a control function doesn't require high-speed data transfer Manufacturer benefits I2C-bus compatible ICs don't only assist designers, they also give a wide range of benefits to equipment manufacturers because: * Overall efficiency depends on the devices chosen and the nature of the interconnecting bus structure. * The simple 2-wire serial I2C-bus minimizes interconnections so ICs have fewer pins and there are not so many PCB tracks; result - smaller and less expensive PCBs To produce a system to satisfy these criteria, a serial bus structure is needed. Although serial buses don't have the throughput capability of parallel buses, they do require less wiring and fewer IC connecting pins. However, a bus is not merely an interconnecting wire, it embodies all the formats and procedures for communication within the system. * 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 and alignment of end-user equipment via external connections to an assembly-line Devices communicating with each other on a serial bus must have some form of protocol which avoids all possibilities of confusion, data loss and blockage of information. Fast devices must be able to communicate with slow devices. The system must not be dependent on the devices connected to it, otherwise modifications or improvements would be impossible. A procedure has also to be devised to decide which device will be in control of the bus and when. And, if different devices with different clock speeds are connected to the bus, the bus clock source must be defined. All these criteria are involved in the specification of the I2C-bus. * The availability of I2C-bus compatible ICs in SO (small outline), VSO (very small outline) as well as DIL packages reduces space requirements even more. These are just some of the benefits. In addition, I2C-bus compatible ICs increase system design flexibility by allowing simple construction of equipment variants and easy upgrading to keep designs up-to-date. In this way, an entire family of equipment can be developed around a basic model. Upgrades for new equipment, or enhanced-feature models (i.e. extended memory, remote control, etc.) can then be produced simply by clipping the appropriate ICs onto the bus. If a larger ROM is needed, it's simply a matter of selecting a micro-controller with a larger ROM from our comprehensive range. As new ICs supersede older ones, it's easy to add new features to equipment or to increase its performance by simply unclipping the outdated IC from the bus and clipping on its successor. 3 4 THE I2C-BUS CONCEPT The I2C-bus supports any IC fabrication process (NMOS, CMOS, bipolar). Two wires, serial data (SDA) and serial clock (SCL), carry information between the devices connected to the bus. Each device is recognized by a unique address (whether it's a microcontroller, LCD driver, memory or keyboard interface) and can operate as either a transmitter or receiver, depending on the function of the device. Obviously an LCD driver is only a receiver, whereas a memory can both receive and transmit data. In addition to transmitters and receivers, devices can also be considered as masters or slaves when performing data transfers (see Table 1). A master is the device which initiates a data transfer on the bus and generates the clock signals to permit that transfer. At that time, any device addressed is considered a slave. INTRODUCTION TO THE I2C-BUS SPECIFICATION For 8-bit oriented digital control applications, such as those requiring microcontrollers, certain design criteria can be established: * A complete system usually consists of at least one microcontroller and other peripheral devices such as memories and I/O expanders * The cost of connecting the various devices within the system must be minimized 46 Philips Semiconductors The I2C-bus specification Table 1 1) Suppose microcontroller A wants to send information to microcontroller B: Definition of I2C-bus terminology TERM DESCRIPTION Transmitter The device which sends data to the bus Receiver The device which receives data from the bus Master The device which initiates a transfer, generates clock signals and terminates a transfer Slave The device addressed by a master Multi-master More than one master can attempt to control the bus at the same time without corrupting the message Arbitration Procedure to ensure that, if more than one master simultaneously tries to control the bus, only one is allowed to do so and the winning message is not corrupted Synchronization * microcontroller A (master), addresses microcontroller B (slave) * microcontroller A (master-transmitter), sends data to microcontroller B (slave- receiver) * microcontroller A terminates the transfer 2) If microcontroller A wants to receive information from microcontroller B: * microcontroller A (master) addresses microcontroller B (slave) * microcontroller A (master- receiver) receives data from microcontroller B (slave- transmitter) * microcontroller A terminates the transfer. Even in this case, the master (microcontroller A) generates the timing and terminates the transfer. The possibility of connecting more than one microcontroller to the I2C-bus means that more than one master could try to initiate a data transfer at the same time. To avoid the chaos that might ensue from such an event an arbitration procedure has been developed. This procedure relies on the wired-AND connection of all I2C interfaces to the I2C-bus. Procedure to synchronize the clock signals of two or more devices The I2C-bus is a multi-master bus. This means that more than one device capable of controlling the bus can be connected to it. As masters are usually micro-controllers, let's consider the case of a data transfer between two microcontrollers connected to the I2C-bus (see Fig.2). If two or more masters try to put information onto the bus, the first to produce a `one' when the other produces a `zero' will lose the arbitration. The clock signals during arbitration are a synchronized combination of the clocks generated by the masters using the wired-AND connection to the SCL line (for more detailed information concerning arbitration see Section 8). This highlights the master-slave and receiver-transmitter relationships to be found on the I2C-bus. It should be noted that these relationships are not permanent, but only depend on the direction of data transfer at that time. The transfer of data would proceed as follows: LCD DRIVER MICRO CONTROLLER A STATIC RAM OR EEPROM SDA SCL GATE ARRAY MICRO CONTROLLER B ADC MBC645 Fig.2 Example of an I2C-bus configuration using two microcontrollers. 47 Philips Semiconductors The I2C-bus specification Generation of clock signals on the I2C-bus is always the responsibility of master devices; each master generates its own clock signals when transferring data on the bus. Bus clock signals from a master can only be altered when they are stretched by a slow-slave device holding-down the clock line, or by another master when arbitration occurs. 5 6 BIT TRANSFER Due to the variety of different technology devices (CMOS, NMOS, bipolar) which can be connected to the I2C-bus, the levels of the logical `0' (LOW) and `1' (HIGH) are not fixed and depend on the associated level of VDD (see Section 15 for electrical specifications). One clock pulse is generated for each data bit transferred. GENERAL CHARACTERISTICS 6.1 Both SDA and SCL are bi-directional lines, connected to a positive supply voltage via a current-source or pull-up resistor (see Fig.3). When the bus is free, both lines are HIGH. The output stages of devices connected to the bus must have an open-drain or open-collector to perform the wired-AND function. Data on the I2C-bus can be transferred at rates of up to 100 kbit/s in the Standard-mode, up to 400 kbit/s in the Fast-mode, or up to 3.4 Mbit/s in the High-speed mode. The number of interfaces connected to the bus is solely dependent on the bus capacitance limit of 400 pF. For information on High-speed mode master devices, see Section 13. Data validity The data on the SDA line must be stable during the HIGH period of the clock. The HIGH or LOW state of the data line can only change when the clock signal on the SCL line is LOW (see Fig.4). VDD pull-up resistors Rp Rp SDA (Serial Data Line) SCL (Serial Clock Line) SCLK SCLK SCLKN1 OUT DATAN1 OUT SCLKN2 OUT DATAN2 OUT SCLK IN DATA IN SCLK IN DATA IN DEVICE 1 DEVICE 2 MBC631 Fig.3 Connection of Standard- and Fast-mode devices to the I2C-bus. 48 Philips Semiconductors The I2C-bus specification handbook, full pagewidth SDA SCL data line stable; data valid change of data allowed MBC621 Fig.4 Bit transfer on the I2C-bus. 6.2 START and STOP conditions The bus stays busy if a repeated START (Sr) is generated instead of a STOP condition. In this respect, the START (S) and repeated START (Sr) conditions are functionally identical (see Fig. 10). For the remainder of this document, therefore, the S symbol will be used as a generic term to represent both the START and repeated START conditions, unless Sr is particularly relevant. Within the procedure of the I2C-bus, unique situations arise which are defined as START (S) and STOP (P) conditions (see Fig.5). A HIGH to LOW transition on the SDA line while SCL is HIGH is one such unique case. This situation indicates a START condition. Detection of START and STOP conditions by devices connected to the bus is easy if they incorporate the necessary interfacing hardware. However, microcontrollers with no such interface have to sample the SDA line at least twice per clock period to sense the transition. A LOW to HIGH transition on the SDA line while SCL is HIGH defines a STOP condition. START and STOP conditions are always generated by the master. The bus is considered to be busy after the START condition. The bus is considered to be free again a certain time after the STOP condition. This bus free situation is specified in Section 15. handbook, full pagewidth SDA SDA SCL SCL S P START condition STOP condition Fig.5 START and STOP conditions. 49 MBC622 Philips Semiconductors The I2C-bus specification 7 7.1 during the HIGH period of this clock pulse (see Fig.7) . Of course, set-up and hold times (specified in Section 15) must also be taken into account. TRANSFERRING DATA Byte format Every byte put on the SDA line must be 8-bits long. The number of bytes that can be transmitted per transfer is unrestricted. Each byte has to be followed by an acknowledge bit. Data is transferred with the most significant bit (MSB) first (see Fig.6) . If a slave can't receive or transmit another complete byte of data until it has performed some other function, for example servicing an internal interrupt, it can hold the clock line SCL LOW to force the master into a wait state. Data transfer then continues when the slave is ready for another byte of data and releases clock line SCL. Usually, a receiver which has been addressed is obliged to generate an acknowledge after each byte has been received, except when the message starts with a CBUS address (see Section 10.1.3). When a slave doesn't acknowledge the slave address (for example, it's unable to receive or transmit because it's performing some real-time function), the data line must be left HIGH by the slave. The master can then generate either a STOP condition to abort the transfer, or a repeated START condition to start a new transfer. If a slave-receiver does acknowledge the slave address but, some time later in the transfer cannot receive any more data bytes, the master must again abort the transfer. This is indicated by the slave generating the not-acknowledge on the first byte to follow. The slave leaves the data line HIGH and the master generates a STOP or a repeated START condition. In some cases, it's permitted to use a different format from the I2C-bus format (for CBUS compatible devices for example). A message which starts with such an address can be terminated by generation of a STOP condition, even during the transmission of a byte. In this case, no acknowledge is generated (see Section 10.1.3). 7.2 Acknowledge If a master-receiver is involved in a transfer, it must signal the end of data to the slave- transmitter by not generating an acknowledge on the last byte that was clocked out of the slave. The slave-transmitter must release the data line to allow the master to generate a STOP or repeated START condition. Data transfer with acknowledge is obligatory. The acknowledge-related clock pulse is generated by the master. The transmitter releases the SDA line (HIGH) during the acknowledge clock pulse. The receiver must pull down the SDA line during the acknowledge clock pulse so that it remains stable LOW handbook, full pagewidth P SDA acknowledgement signal from slave MSB acknowledgement signal from receiver Sr byte complete, interrupt within slave clock line held low while interrupts are serviced SCL S or Sr 1 2 7 8 9 1 ACK START or repeated START condition 2 3-8 9 ACK Sr or P STOP or repeated START condition MSC608 Fig.6 Data transfer on the I2C-bus. 50 Philips Semiconductors The I2C-bus specification handbook, full pagewidth DATA OUTPUT BY TRANSMITTER not acknowledge DATA OUTPUT BY RECEIVER acknowledge SCL FROM MASTER 1 2 8 9 S clock pulse for acknowledgement START condition MBC602 Fig.7 Acknowledge on the I2C-bus. 8 8.1 that a HIGH to LOW transition on the SCL line will cause the devices concerned to start counting off their LOW period and, once a device clock has gone LOW, it will hold the SCL line in that state until the clock HIGH state is reached (see Fig.8). However, the LOW to HIGH transition of this clock may not change the state of the SCL line if another clock is still within its LOW period. The SCL line will therefore be held LOW by the device with the longest LOW period. Devices with shorter LOW periods enter a HIGH wait-state during this time. ARBITRATION AND CLOCK GENERATION Synchronization All masters generate their own clock on the SCL line to transfer messages on the I2C-bus. Data is only valid during the HIGH period of the clock. A defined clock is therefore needed for the bit-by-bit arbitration procedure to take place. Clock synchronization is performed using the wired-AND connection of I2C interfaces to the SCL line. This means wait state start counting HIGH period CLK 1 CLK 2 counter reset SCL MBC632 Fig.8 Clock synchronization during the arbitration procedure. 51 Philips Semiconductors The I2C-bus specification When all devices concerned have counted off their LOW period, the clock line will be released and go HIGH. There will then be no difference between the device clocks and the state of the SCL line, and all the devices will start counting their HIGH periods. The first device to complete its HIGH period will again pull the SCL line LOW. to address the same device, arbitration continues with comparison of the data-bits if they are master-transmitter, or acknowledge-bits if they are master-receiver. Because address and data information on the I2C-bus is determined by the winning master, no information is lost during the arbitration process. In this way, a synchronized SCL clock is generated with its LOW period determined by the device with the longest clock LOW period, and its HIGH period determined by the one with the shortest clock HIGH period. A master that loses the arbitration can generate clock pulses until the end of the byte in which it loses the arbitration. 8.2 As an Hs-mode master has a unique 8-bit master code, it will always finish the arbitration during the first byte (see Section 13). Arbitration A master may start a transfer only if the bus is free. Two or more masters may generate a START condition within the minimum hold time (tHD;STA) of the START condition which results in a defined START condition to the bus. If a master also incorporates a slave function and it loses arbitration during the addressing stage, it's possible that the winning master is trying to address it. The losing master must therefore switch over immediately to its slave mode. Arbitration takes place on the SDA line, while the SCL line is at the HIGH level, in such a way that the master which transmits a HIGH level, while another master is transmitting a LOW level will switch off its DATA output stage because the level on the bus doesn't correspond to its own level. Figure 9 shows the arbitration procedure for two masters. Of course, more may be involved (depending on how many masters are connected to the bus). The moment there is a difference between the internal data level of the master generating DATA 1 and the actual level on the SDA line, its data output is switched off, which means that a HIGH output level is then connected to the bus. This will not affect the data transfer initiated by the winning master. Arbitration can continue for many bits. Its first stage is comparison of the address bits (addressing information is given in Sections 10 and 14). If the masters are each trying master 1 loses arbitration DATA 1 SDA handbook, full pagewidth DATA 1 DATA 2 SDA SCL S MSC609 Fig.9 Arbitration procedure of two masters. 52 Philips Semiconductors The I2C-bus specification Since control of the I2C-bus is decided solely on the address or master code and data sent by competing masters, there is no central master, nor any order of priority on the bus. then hold the SCL line LOW after reception and acknowledgment of a byte to force the master into a wait state until the slave is ready for the next byte transfer in a type of handshake procedure (see Fig.6). Special attention must be paid if, during a serial transfer, the arbitration procedure is still in progress at the moment when a repeated START condition or a STOP condition is transmitted to the I2C-bus. If it's possible for such a situation to occur, the masters involved must send this repeated START condition or STOP condition at the same position in the format frame. In other words, arbitration isn't allowed between: On the bit level, a device such as a microcontroller with or without limited hardware for the I2C-bus, can slow down the bus clock by extending each clock LOW period. The speed of any master is thereby adapted to the internal operating rate of this device. In Hs-mode, this handshake feature can only be used on byte level (see Section 13). * A repeated START condition and a data bit * A STOP condition and a data bit 9 * A repeated START condition and a STOP condition. Data transfers follow the format shown in Fig.10. After the START condition (S), a slave address is sent. This address is 7 bits long followed by an eighth bit which is a data direction bit (R/W) - a `zero' indicates a transmission (WRITE), a `one' indicates a request for data (READ). A data transfer is always terminated by a STOP condition (P) generated by the master. However, if a master still wishes to communicate on the bus, it can generate a repeated START condition (Sr) and address another slave without first generating a STOP condition. Various combinations of read/write formats are then possible within such a transfer. Slaves are not involved in the arbitration procedure. 8.3 Use of the clock synchronizing mechanism as a handshake In addition to being used during the arbitration procedure, the clock synchronization mechanism can be used to enable receivers to cope with fast data transfers, on either a byte level or a bit level. FORMATS WITH 7-BIT ADDRESSES On the byte level, a device may be able to receive bytes of data at a fast rate, but needs more time to store a received byte or prepare another byte to be transmitted. Slaves can handbook, full pagewidth SDA SCL 1-7 8 9 1-7 8 9 1-7 8 9 P S START condition ADDRESS R/W ACK DATA ACK DATA ACK STOP condition MBC604 Fig.10 A complete data transfer. 53 Philips Semiconductors The I2C-bus specification Possible data transfer formats are: NOTES: * Master-transmitter transmits to slave-receiver. The transfer direction is not changed (see Fig.11). 1. Combined formats can be used, for example, to control a serial memory. During the first data byte, the internal memory location has to be written. After the START condition and slave address is repeated, data can be transferred. * Master reads slave immediately after first byte (see Fig.12). At the moment of the first acknowledge, the master- transmitter becomes a master- receiver and the slave-receiver becomes a slave-transmitter. This first acknowledge is still generated by the slave. The STOP condition is generated by the master, which has previously sent a not-acknowledge (A). 2. All decisions on auto-increment or decrement of previously accessed memory locations etc. are taken by the designer of the device. 3. Each byte is followed by an acknowledgment bit as indicated by the A or A blocks in the sequence. * Combined format (see Fig.13). During a change of direction within a transfer, the START condition and the slave address are both repeated, but with the R/W bit reversed. If a master receiver sends a repeated START condition, it has previously sent a not-acknowledge (A). 4. I2C-bus compatible devices must reset their bus logic on receipt of a START or repeated START condition such that they all anticipate the sending of a slave address, even if these START conditions are not positioned according to the proper format. 5. A START condition immediately followed by a STOP condition (void message) is an illegal format. handbook, full pagewidth ,,,,,, ,,, ,,, ,, ,,,,,, ,,, ,,, ,, ,, S SLAVE ADDRESS R/W A DATA A DATA A/A P data transferred (n bytes + acknowledge) '0' (write) from master to slave A = acknowledge (SDA LOW) A = not acknowledge (SDA HIGH) S = START condition from slave to master MBC605 P = STOP condition Fig.11 A master-transmitter addressing a slave receiver with a 7-bit address. The transfer direction is not changed. handbook, full pagewidth ,,,,,, ,, ,, ,,,,,, ,,,, 1 S MBC606 SLAVE ADDRESS R/W A (read) DATA A DATA A P data transferred (n bytes + acknowledge) Fig.12 A master reads a slave immediately after the first byte. 54 Philips Semiconductors The I2C-bus specification ,,,,, handbook, full pagewidth S SLAVE ADDRESS R/W A read or write ,,,,, , DATA A/A Sr SLAVE ADDRESS (n bytes + ack.) * R/W transfer direction of data and acknowledge bits depends on R/W bits. DATA A/A P (n bytes + ack.) * read or write * not shaded because A Sr = repeated START condition direction of transfer may change at this point. MBC607 Fig.13 Combined format. 10 7-BIT ADDRESSING The addressing procedure for the I2C-bus is such that the first byte after the START condition usually determines which slave will be selected by the master. The exception is the `general call' address which can address all devices. When this address is used, all devices should, in theory, respond with an acknowledge. However, devices can be made to ignore this address. The second byte of the general call address then defines the action to be taken. This procedure is explained in more detail in Section 10.1.1. For information on 10-bit addressing, see Section 14 10.1 MSB handbook, halfpage LSB R/W slave address MBC608 Fig.14 The first byte after the START procedure. A slave address can be made-up of a fixed and a programmable part. Since it's likely that there will be several identical devices in a system, the programmable part of the slave address enables the maximum possible number of such devices to be connected to the I2C-bus. The number of programmable address bits of a device depends on the number of pins available. For example, if a device has 4 fixed and 3 programmable address bits, a total of 8 identical devices can be connected to the same bus. Definition of bits in the first byte The first seven bits of the first byte make up the slave address (see Fig.14). The eighth bit is the LSB (least significant bit). It determines the direction of the message. A `zero' in the least significant position of the first byte means that the master will write information to a selected slave. A `one' in this position means that the master will read information from the slave. When an address is sent, each device in a system compares the first seven bits after the START condition with its address. If they match, the device considers itself addressed by the master as a slave-receiver or slave-transmitter, depending on the R/W bit. The I2C-bus committee coordinates allocation of I2C addresses. Further information can be obtained from the Philips representatives listed on the back cover. Two groups of eight addresses (0000XXX and 1111XXX) are reserved for the purposes shown in Table 2. The bit combination 11110XX of the slave address is reserved for 10-bit addressing (see Section 14). 55 Philips Semiconductors The I2C-bus specification Table 2 Definition of bits in the first byte SLAVE R/W BIT ADDRESS LSB DESCRIPTION 0 0000 000 0 General call address 0000 000 1 START byte(1) 0000 001 X CBUS address(2) 0000 010 X Reserved for different bus format(3) 0000 011 X Reserved for future purposes 0000 1XX X Hs-mode master code 1111 1XX X Reserved for future purposes 1111 0XX X 10-bit slave addressing 0 0 0 0 0 A X X X X X X X B A second byte MBC623 Fig.15 General call address format. When bit B is a `zero'; the second byte has the following definition: * 00000110 (H`06'). Reset and write programmable part of slave address by hardware. On receiving this 2-byte sequence, all devices designed to respond to the general call address will reset and take in the programmable part of their address. Pre-cautions have to be taken to ensure that a device is not pulling down the SDA or SCL line after applying the supply voltage, since these low levels would block the bus. No device is allowed to acknowledge at the reception of the START byte. 2. The CBUS address has been reserved to enable the inter-mixing of CBUS compatible and I2C-bus compatible devices in the same system. I2C-bus compatible devices are not allowed to respond on reception of this address. * 00000100 (H`04'). Write programmable part of slave address by hardware. All devices which define the programmable part of their address by hardware (and which respond to the general call address) will latch this programmable part at the reception of this two byte sequence. The device will not reset. 3. The address reserved for a different bus format is included to enable I2C and other protocols to be mixed. Only I2C-bus compatible devices that can work with such formats and protocols are allowed to respond to this address. 10.1.1 0 first byte (general call address) Notes 1. 0 * 00000000 (H`00'). This code is not allowed to be used as the second byte. GENERAL CALL ADDRESS Sequences of programming procedure are published in the appropriate device data sheets. The general call address is for addressing every device connected to the I2C-bus. However, if a device doesn't need any of the data supplied within the general call structure, it can ignore this address by not issuing an acknowledgment. If a device does require data from a general call address, it will acknowledge this address and behave as a slave- receiver. The second and following bytes will be acknowledged by every slave- receiver capable of handling this data. A slave which cannot process one of these bytes must ignore it by not-acknowledging. The meaning of the general call address is always specified in the second byte (see Fig.15). The remaining codes have not been fixed and devices must ignore them. When bit B is a `one'; the 2-byte sequence is a `hardware general call'. This means that the sequence is transmitted by a hardware master device, such as a keyboard scanner, which cannot be programmed to transmit a desired slave address. Since a hardware master doesn't know in advance to which device the message has to be transferred, it can only generate this hardware general call and its own address - identifying itself to the system (see Fig.16). There are two cases to consider: The seven bits remaining in the second byte contain the address of the hardware master. This address is recognized by an intelligent device (e.g. a microcontroller) connected to the bus which will then direct the information from the hardware master. If the hardware master can also act as a slave, the slave address is identical to the master address. * When the least significant bit B is a `zero'. * When the least significant bit B is a `one'. 56 Philips Semiconductors The I2C-bus specification ,,,,,,,, ,,,, handbook, full pagewidth S 00000000 A MASTER ADDRESS 1 A DATA A DATA A P (B) general call address second byte (n bytes + ack.) MBC624 Fig.16 Data transfer from a hardware master-transmitter. an interface, it must constantly monitor the bus via software. Obviously, the more times the microcontroller monitors, or polls the bus, the less time it can spend carrying out its intended function. In some systems, an alternative could be that the hardware master transmitter is set in the slave-receiver mode after the system reset. In this way, a system configuring master can tell the hardware mastertransmitter (which is now in slave-receiver mode) to which address data must be sent (see Fig.17). After this programming procedure, the hardware master remains in the master-transmitter mode. 10.1.2 There is therefore a speed difference between fast hardware devices and a relatively slow microcontroller which relies on software polling. In this case, data transfer can be preceded by a start procedure which is much longer than normal (see Fig.18). The start procedure consists of: START BYTE Microcontrollers can be connected to the I2C-bus in two ways. A microcontroller with an on-chip hardware I2C-bus interface can be programmed to be only interrupted by requests from the bus. When the device doesn't have such * A START condition (S) * A START byte (00000001) * An acknowledge clock pulse (ACK) * A repeated START condition (Sr). handbook, full pagewidth ,,,,,, ,,,,,, ,, ,,,,,,,,,,,,,, ,,,,,,, ,, ,, ,, ,,,,,,, ,,,, ,, S SLAVE ADDR. H/W MASTER R/W A DUMP ADDR. FOR H/W MASTER X A P write (a) S DUMP ADDR. FROM H/W MASTER R/W A DATA A DATA A/A P MBC609 write (n bytes + ack.) (b) Fig.17 Data transfer by a hardware-transmitter capable of dumping data directly to slave devices. (a) Configuring master sends dump address to hardware master (b) Hardware master dumps data to selected slave. 57 Philips Semiconductors The I2C-bus specification dummy acknowledge (HIGH) SDA 1 SCL 2 7 8 9 ACK S start byte 00000001 Sr MBC633 Fig.18 START byte procedure. After the START condition S has been transmitted by a master which requires bus access, the START byte (00000001) is transmitted. Another microcontroller can therefore sample the SDA line at a low sampling rate until one of the seven zeros in the START byte is detected. After detection of this LOW level on the SDA line, the microcontroller can switch to a higher sampling rate to find the repeated START condition Sr which is then used for synchronization. 10.1.3 CBUS COMPATIBILITY CBUS receivers can be connected to the Standard-mode I2C-bus. However, a third bus line called DLEN must then be connected and the acknowledge bit omitted. Normally, I2C transmissions are sequences of 8-bit bytes; CBUS compatible devices have different formats. In a mixed bus structure, I2C-bus devices must not respond to the CBUS message. For this reason, a special CBUS address (0000001X) to which no I2C-bus compatible device will respond, has been reserved. After transmission of the CBUS address, the DLEN line can be made active and a CBUS-format transmission sent (see Fig.19). After the STOP condition, all devices are again ready to accept data. A hardware receiver will reset on receipt of the repeated START condition Sr and will therefore ignore the START byte. An acknowledge-related clock pulse is generated after the START byte. This is present only to conform with the byte handling format used on the bus. No device is allowed to acknowledge the START byte. Master-transmitters can send CBUS formats after sending the CBUS address. The transmission is ended by a STOP condition, recognized by all devices. NOTE: If the CBUS configuration is known, and expansion with CBUS compatible devices isn't foreseen, the designer is allowed to adapt the hold time to the specific requirements of the device(s) used. 58 Philips Semiconductors The I2C-bus specification SDA SCL DLEN S START condition P CBUS address R/W bit ACK related clock pulse n - data bits CBUS load pulse STOP condition MBC634 Fig.19 Data format of transmissions with CBUS transmitter/receiver. apparent that more address combinations were required to prevent problems with the allocation of slave addresses for new devices. This problem was resolved with the new 10-bit addressing scheme, which allowed about a tenfold increase in available addresses. 11 EXTENSIONS TO THE STANDARD-MODE I2C-BUS SPECIFICATION The Standard-mode I2C-bus specification, with its data transfer rate of up to 100 kbit/s and 7-bit addressing, has been in existence since the beginning of the 1980's. This concept rapidly grew in popularity and is today accepted worldwide as a de facto standard with several hundred different compatible ICs on offer from Philips Semiconductors and other suppliers. To meet the demands for higher speeds, as well as make available more slave address for the growing number of new devices, the Standard-mode I2C-bus specification was upgraded over the years and today is available with the following extensions: New slave devices with a Fast- or Hs-mode I2C-bus interface can have a 7- or a 10-bit slave address. If possible, a 7-bit address is preferred as it is the cheapest hardware solution and results in the shortest message length. Devices with 7- and 10-bit addresses can be mixed in the same I2C-bus system regardless of whether it is an F/S- or Hs-mode system. Both existing and future masters can generate either 7- or 10-bit addresses. * Fast-mode, with a bit rate up to 400 kbit/s. 12 FAST-MODE * High-speed mode (Hs-mode), with a bit rate up to 3.4 Mbit/s. With the Fast-mode I2C-bus specification, the protocol, format, logic levels and maximum capacitive load for the SDA and SCL lines quoted in the Standard-mode I2C-bus specification are unchanged. New devices with an I2C-bus interface must meet at least the minimum requirements of the Fast- or Hs-mode specification (see Section 13). * 10-bit addressing, which allows the use of up to 1024 additional slave addresses. There are two main reasons for extending the regular I2C-bus specification: Fast-mode devices can receive and transmit at up to 400 kbit/s. The minimum requirement is that they can synchronize with a 400 kbit/s transfer; they can then prolong the LOW period of the SCL signal to slow down the transfer. Fast-mode devices are downward-compatible and can communicate with Standard-mode devices in a 0 to 100 kbit/s I2C-bus system. As Standard-mode devices, however, are not upward compatible, they should not be incorporated in a Fast-mode I2C-bus system as they cannot follow the higher transfer rate and unpredictable states would occur. * Many of today's applications need to transfer large amounts of serial data and require bit rates far in excess of 100 kbit/s (Standard-mode), or even 400 kbit/s (Fast-mode). As a result of continuing improvements in semiconductor technologies, I2C-bus devices are now available with bit rates of up to 3.4 Mbit/s (Hs-mode) without any noticeable increases in the manufacturing cost of the interface circuitry. * As most of the 112 addresses available with the 7-bit addressing scheme were soon allocated, it became 59 Philips Semiconductors The I2C-bus specification The Fast-mode I2C-bus specification has the following additional features compared with the Standard-mode: current-source of one master is enabled at any one time, and only during Hs-mode. * No arbitration or clock synchronization is performed during Hs-mode transfer in multi-master systems, which speeds-up bit handling capabilities. The arbitration procedure always finishes after a preceding master code transmission in F/S-mode. * The maximum bit rate is increased to 400 kbit/s. * Timing of the serial data (SDA) and serial clock (SCL) signals has been adapted. There is no need for compatibility with other bus systems such as CBUS because they cannot operate at the increased bit rate. * Hs-mode master devices generate a serial clock signal with a HIGH to LOW ratio of 1 to 2. This relieves the timing requirements for set-up and hold times. * The inputs of Fast-mode devices incorporate spike suppression and a Schmitt trigger at the SDA and SCL inputs. * As an option, Hs-mode master devices can have a built-in bridge(1). During Hs-mode transfer, the high speed data (SDAH) and high-speed serial clock (SCLH) lines of Hs-mode devices are separated by this bridge from the SDA and SCL lines of F/S-mode devices. This reduces the capacitive load of the SDAH and SCLH lines resulting in faster rise and fall times. * The output buffers of Fast-mode devices incorporate slope control of the falling edges of the SDA and SCL signals. * If the power supply to a Fast-mode device is switched off, the SDA and SCL I/O pins must be floating so that they don't obstruct the bus lines. * The external pull-up devices connected to the bus lines must be adapted to accommodate the shorter maximum permissible rise time for the Fast-mode I2C-bus. For bus loads up to 200 pF, the pull-up device for each bus line can be a resistor; for bus loads between 200 pF and 400 pF, the pull-up device can be a current source (3 mA max.) or a switched resistor circuit (see Fig.43). * The only difference between Hs-mode slave devices and F/S-mode slave devices is the speed at which they operate. Hs-mode slaves have open-drain output buffers on the SCLH and SDAH outputs. Optional pull-down transistors on the SCLH pin can be used to stretch the LOW level of the SCLH signal, although this is only allowed after the acknowledge bit in Hs-mode transfers. * The inputs of Hs-mode devices incorporate spike suppression and a Schmitt trigger at the SDAH and SCLH inputs. 13 Hs-MODE High-speed mode (Hs-mode) devices offer a quantum leap in I2C-bus transfer speeds. Hs-mode devices can transfer information at bit rates of up to 3.4 Mbit/s, yet they remain fully downward compatible with Fast- or Standard-mode (F/S-mode) devices for bi-directional communication in a mixed-speed bus system. With the exception that arbitration and clock synchronization is not performed during the Hs-mode transfer, the same serial bus protocol and data format is maintained as with the F/S-mode system. Depending on the application, new devices may have a Fast or Hs-mode I2C-bus interface, although Hs-mode devices are preferred as they can be designed-in to a greater number of applications. 13.1 * The output buffers of Hs-mode devices incorporate slope control of the falling edges of the SDAH and SCLH signals. Figure 20 shows the physical I2C-bus configuration in a system with only Hs-mode devices. Pins SDA and SCL on the master devices are only used in mixed-speed bus systems and are not connected in an Hs-mode only system. In such cases, these pins can be used for other functions. Optional series resistors Rs protect the I/O stages of the I2C-bus devices from high-voltage spikes on the bus lines and minimize ringing and interference. High speed transfer Pull-up resistors Rp maintain the SDAH and SCLH lines at a HIGH level when the bus is free and ensure the signals are pulled up from a LOW to a HIGH level within the required rise time. For higher capacitive bus-line loads (>100 pF), the resistor Rp can be replaced by external current source pull-ups to meet the rise time requirements. Unless proceeded by an acknowledge bit, the rise time of the SCLH clock pulses in Hs-mode transfers is shortened by the internal current-source pull-up circuit MCS of the active master. To achieve a bit transfer of up to 3.4 Mbit/s the following improvements have been made to the regular I2C-bus specification: * Hs-mode master devices have an open-drain output buffer for the SDAH signal and a combination of an open-drain pull-down and current-source pull-up circuit on the SCLH output(1). This current-source circuit shortens the rise time of the SCLH signal. Only the (1) Patent application pending. 60 Philips Semiconductors The I2C-bus specification VDD handbook, full pagewidth Rp Rp SDAH SCLH Rs Rs Rs Rs Rs Rs Rs (1) SDAH (2) SCLH (2) SDAH SCLH SDAH SCLH (2) (2) (2) (2) SDA (1) SCL (1) SDAH SCLH (2) (2) MCS (4) (4) VSS VSS SLAVE VSS SLAVE Rs SDA (1) SCL MCS (3) (3) VDD VDD VSS MASTER/SLAVE MASTER/SLAVE MSC612 (1) SDA and SCL are not used here but may be used for other functions. (2) To input filter. (3) Only the active master can enable its current-source pull-up circuit (4) Dotted transistors are optional open-drain outputs which can stretch the serial clock signal SCLH. Fig.20 I2C-bus configuration with Hs-mode devices only. 13.2 Serial data transfer format in Hs-mode master code for an Hs-mode master device is software programmable and is chosen by the System Designer. Serial data transfer format in Hs-mode meets the Standard-mode I2C-bus specification. Hs-mode can only commence after the following conditions (all of which are in F/S-mode): Arbitration and clock synchronization only take place during the transmission of the master code and not-acknowledge bit (A), after which one winning master remains active. The master code indicates to other devices that an Hs-mode transfer is to begin and the connected devices must meet the Hs-mode specification. As no device is allowed to acknowledge the master code, the master code is followed by a not-acknowledge (A). 1. START condition (S) 2. 8-bit master code (00001XXX) 3. not-acknowledge bit (A) Figures 21 and 22 show this in more detail. This master code has two main functions: After the not-acknowledge bit (A), and the SCLH line has been pulled-up to a HIGH level, the active master switches to Hs-mode and enables (at time tH, see Fig.22) the current-source pull-up circuit for the SCLH signal. As other devices can delay the serial transfer before tH by stretching the LOW period of the SCLH signal, the active master will enable its current-source pull-up circuit when all devices have released the SCLH line and the SCLH signal has reached a HIGH level, thus speeding up the last part of the rise time of the SCLH signal. * It allows arbitration and synchronization between competing masters at F/S-mode speeds, resulting in one winning master. * It indicates the beginning of an Hs-mode transfer. Hs-mode master codes are reserved 8-bit codes, which are not used for slave addressing or other purposes. Furthermore, as each master has its own unique master code, up to eight Hs-mode masters can be present on the one I2C-bus system (although master code 0000 1000 should be reserved for test and diagnostic purposes). The The active master then sends a repeated START condition (Sr) followed by a 7-bit slave address (or 10-bit slave 61 Philips Semiconductors The I2C-bus specification address, see Section 14) with a R/W bit address, and receives an acknowledge bit (A) from the selected slave. SCLH signal reaches a HIGH level, and so speeds up the last part of the SCLH signal's rise time. After each acknowledge bit (A) or not-acknowledge bit (A) the active master disables its current-source pull-up circuit. This enables other devices to delay the serial transfer by stretching the LOW period of the SCLH signal. The active master re-enables its current-source pull-up circuit again when all devices have released and the Data transfer continues in Hs-mode after the next repeated START (Sr), and only switches back to F/S-mode after a STOP condition (P). To reduce the overhead of the master code, it's possible that a master links a number of Hs-mode transfers, separated by repeated START conditions (Sr). ,,,,, ,,,,, ,,,,,,,,,, handbook, full pagewidth Hs-mode (current-source for SCLH enabled) F/S-mode S MASTER CODE A Sr SLAVE ADD. R/W A DATA ,, ,, ,,,, ,,,, F/S-mode A/A P (n bytes + ack.) Hs-mode continues Sr SLAVE ADD. MSC616 Fig.21 Data transfer format in Hs-mode. handbook, full pagewidth 8-bit Master code 00001xxx S A t1 tH SDAH SCLH 1 6 2 to 5 7 8 9 F/S mode R/W 7-bit SLA Sr n x (8-bit DATA A + A/A) 8 9 Sr P SDAH SCLH 1 2 to 5 6 7 8 9 1 2 to 5 6 7 If P then F/S mode Hs-mode If Sr (dotted lines) then Hs-mode tH tFS = MCS current source pull-up = Rp resistor pull-up Fig.22 A complete Hs-mode transfer. 62 MSC618 Philips Semiconductors The I2C-bus specification 13.3 The non-active, or loosing masters: Switching from F/S- to Hs-mode and back After reset and initialization, Hs-mode devices must be in Fast-mode (which is in effect F/S-mode as Fast-mode is downward compatible with Standard-mode). Each Hs-mode device can switch from Fast- to Hs-mode and back and is controlled by the serial transfer on the I2C-bus. 1. Adapt their SDAH and SCLH input filters according to the spike suppression requirement in Hs-mode. Before time t1 in Fig.22, each connected device operates in Fast-mode. Between times t1 and tH (this time interval can be stretched by any device) each connected device must recognized the "S 00001XXX A" sequence and has to switch its internal circuit from the Fast-mode setting to the Hs-mode setting. Between times t1 and tH the connected master and slave devices perform this switching by the following actions. 1. Adapt their SDAH and SCLH input filters according to the spike suppression requirement in Hs-mode. 2. Wait for a STOP condition to detect when the bus is free again. All slaves: 2. Adapt the set-up and hold times according to the Hs-mode requirements. This requirement may already be fulfilled by the adaptation of the input filters. 3. Adapt the slope control of their SDAH output stages, if necessary. For slave devices, slope control is applicable for the SDAH output stage only and, depending on circuit tolerances, both the Fast- and Hs-mode requirements may be fulfilled without switching its internal circuit. The active (winning) master: 1. Adapts its SDAH and SCLH input filters according to the spike suppression requirement in Hs-mode. 2. Adapts the set-up and hold times according to the Hs-mode requirements. At time tFS in Fig.22, each connected device must recognize the STOP condition (P) and switch its internal circuit from the Hs-mode setting back to the Fast-mode setting as present before time t1. This must be completed within the minimum bus free time as specified in Table 5 according to the Fast-mode specification. 3. Adapts the slope control of its SDAH and SCLH output stages according to the Hs-mode requirement. 4. Switches to the Hs-mode bit-rate, which is required after time tH. 5. Enables the current source pull-up circuit of its SCLH output stage at time tH. 63 Philips Semiconductors The I2C-bus specification 13.4 Hs-mode devices at lower speed modes F/S-mode and communicate at F/S-mode speeds with their current-source disabled. The SDAH and SCLH pins are used to connect to the F/S-mode bus system, allowing the SDA and SCL pins (if present) on the Hs-mode master device to be used for other functions. Hs-mode devices are fully downwards compatible, and can be connected to an F/S-mode I2C-bus system (see Fig.23). As no master code will be transmitted in such a configuration, all Hs-mode master devices stay in VDD handbook, full pagewidth Rp Rp SDA SCL Rs Rs Rs Rs Rs Rs Rs (1) SDAH (2) SCLH SDAH (2) (2) SCLH SDAH (2) (4) (2) SCLH SDA Rs Rs SCL (2) SDA (2) SCL (2) SDA (2) SCL (2) (3) (4) Rs (1) (4) VDD VSS VSS Hs-mode SLAVE Hs-mode SLAVE VSS VSS Hs-mode MASTER/SLAVE F/S-mode MASTER/SLAVE VSS F/S-mode SLAVE MSC613 (1) Bridge not used. SDA and SCL may have an alternative function. (2) To input filter. (3) The current-source pull-up circuit stays disabled. (4) Dotted transistors are optional open-drain outputs which can stretch the serial clock signal SCL. Fig.23 Hs-mode devices at F/S-mode speed. 13.5 Mixed speed modes on one serial bus system a high impedance between the drain and source of each switched on transistor. In the latter case, the transistors will act as a level shifter as SDAH and SCLH will be pulled-up to VDD1 and SDA and SCL will be pulled-up to VDD2 If a system has a combination of Hs-, Fast- and/or Standard-mode devices, it's possible, by using an interconnection bridge, to have different bit rates between different devices (see Figs 24 and 25). During F/S-mode speed, a bridge on one of the Hs-mode masters connects the SDAH and SCLH lines to the corresponding SDA and SCL lines thus permitting Hs-mode devices to communicate with F/S-mode devices at slower speeds. Arbitration and synchronization is possible during the total F/S-mode transfer between all connected devices as described in Section 8. During Hs-mode transfer, however, the bridge opens to separate the two bus sections and allows Hs-mode devices to communicate with each other at 3.4 Mbit/s. Arbitration between Hs-mode devices and F/S-mode devices is only performed during the master code (00001XXX), and normally won by one Hs-mode master as no slave address has four leading zeros. Other masters can win the arbitration only if they send a reserved 8-bit code (00000XXX). In such cases, the bridge remains closed and the transfer proceeds in F/S-mode. Table 3 gives the possible communication speeds in such a system. One bridge is required to connect/disconnect an Hs-mode section to/from an F/S-mode section at the appropriate time. This bridge includes a level shift function that allows devices with different supply voltages to be connected. For example F/S-mode devices with a VDD2 of 5 V can be connected to Hs-mode devices with a VDD1 of 3 V or less (i.e. where VDD2 VDD1), provided SDA and SCL pins are 5 V tolerant. This bridge is incorporated in Hs-mode master devices and is completely controlled by the serial signals SDAH, SCLH, SDA and SCL. Such a bridge can be implemented in any IC as an autonomous circuit. TR1, TR2 and TR3 are N-channel transistors. TR1 and TR2 have a transfer gate function, and TR3 is an opendrain pull-down stage. If TR1 or TR2 are switched on they transfer a LOW level in both directions, otherwise when both the drain and source rise to a HIGH level there will be 64 Philips Semiconductors The I2C-bus specification VDD2 VDD1 Rp Rp Rp BRIDGE SDAH Rs SDAH TR1 SDA SCLH Rs SCLH TR2 SCL (1) (1) Rs SDAH SCLH (2) (2) Rs Rs Rs SDAH SCLH (2) (2) Rs SDAH SCLH (2) (2) Rs SDA VSS SCL (2) (4) SDA (2) (2) MCS (4) VSS Hs-mode SLAVE Hs-mode SLAVE Rs SCL (2) Rs SDA (2) Rs SCL (2) MCS (3) (4) (3) VDD VSS Rs TR3 Rp VDD VSS VSS Hs-mode MASTER/SLAVE VSS Hs-mode MASTER/SLAVE F/S-mode MASTER/SLAVE VSS F/S-mode SLAVE MSC614 (1) Bridge not used. SDA and SCL may have an alternative function. (2) To input filter. (3) Only the active master can enable its current-source pull-up circuit. (4) Dotted transistors are optional open-drain outputs which can stretch the serial clock signal SCL or SCLH. Fig.24 Bus system with transfer at Hs- and F/S-mode speeds. Table 3 Communication bit-rates in a mixed speed bus system SERIAL BUS SYSTEM CONFIGURATION TRANSFER BETWEEN Hs + FAST + STANDARD Hs + FAST Hs + STANDARD FAST + STANDARD Hs <-> Hs 0 to 3.4 Mbit/s 0 to 3.4 Mbit/s 0 to 3.4 Mbit/s - Hs <-> Fast 0 to 100 kbit/s 0 to 400 kbit/s - - Hs <-> Standard 0 to 100 kbit/s - 0 to 100 kbit/s - Fast <-> Standard 0 to 100 kbit/s - - 0 to 100 kbit/s Fast <-> Fast 0 to 100 kbit/s 0 to 400 kbit/s - 0 to 100 kbit/s Standard <-> Standard 0 to 100 kbit/s - 0 to 100 kbit/s 0 to 100 kbit/s 13.5.1 F/S-MODE TRANSFER IN A MIXED-SPEED BUS 13.5.2 SYSTEM HS-MODE TRANSFER IN A MIXED-SPEED BUS SYSTEM The bridge shown in Fig.24 interconnects corresponding serial bus lines, forming one serial bus system. As no master code (00001XXX) is transmitted, the current-source pull-up circuits stay disabled and all output stages are open-drain. All devices, including Hs-mode devices, communicate with each other according the protocol, format and speed of the F/S-mode I2C-bus specification. Figure 25 shows the timing diagram of a complete Hs-mode transfer, which is invoked by a START condition, a master code, and a not-acknowledge A (at F/S-mode speed). Although this timing diagram is split in two parts, it should be viewed as one timing diagram were time point tH is a common point for both parts. 65 Philips Semiconductors The I2C-bus specification handbook, full pagewidth 8-bit Master code 00001xxx S A t1 tH SDAH SCLH 1 2 to 5 6 7 8 9 1 2 to 5 6 7 8 9 SDA SCL F/S mode R/W 7-bit SLA Sr n x (8-bit DATA A + A/A) 8 9 Sr P SDAH SCLH 1 2 to 5 6 7 8 9 1 2 to 5 6 7 P SDA t2 SCL If P then F/S mode Hs-mode If Sr (dotted lines) then Hs-mode tH tFS MSC611 = MCS current source pull-up = Rp resistor pull-up Fig.25 A complete Hs-mode transfer in a mixed-speed bus system. The master code is recognized by the bridge in the active or non-active master (see Fig.24). The bridge performs the following actions: 2. When both SCLH and SCL become HIGH (tH in Fig.25), transistor TR2 opens to separate the SCLH and SCL lines. TR2 must be opened before SCLH goes LOW after Sr. 1. Between t1 and tH (see Fig.25), transistor TR1 opens to separate the SDAH and SDA lines, after which transistor TR3 closes to pull-down the SDA line to VSS. Hs-mode transfer starts after tH with a repeated START condition (Sr). During Hs-mode transfer, the SCL line stays at a HIGH and the SDA line at a LOW steady-state level, and so is prepared for the transfer of a STOP condition (P). 66 Philips Semiconductors The I2C-bus specification seven bits of the first byte following a START (S) or repeated START (Sr) condition as explained in Section 10.1. The 10-bit addressing does not affect the existing 7-bit addressing. Devices with 7-bit and 10-bit addresses can be connected to the same I2C-bus, and both 7-bit and 10-bit addressing can be used in F/S-mode and Hs-mode systems. After each acknowledge (A) or not-acknowledge bit (A) the active master disables its current-source pull-up circuit. This enables other devices to delay the serial transfer by stretching the LOW period of the SCLH signal. The active master re-enables its current-source pull-up circuit again when all devices are released and the SCLH signal reaches a HIGH level, and so speeds up the last part of the SCLH signal's rise time. In irregular situations, F/S-mode devices can close the bridge (TR1 and TR2 closed, TR3 open) at any time by pulling down the SCL line for at least 1 s, e.g. to recover from a bus hang-up. Although there are eight possible combinations of the reserved address bits 1111XXX, only the four combinations 11110XX are used for 10-bit addressing. The remaining four combinations 11111XX are reserved for future I2C-bus enhancements. Hs-mode finishes with a STOP condition and brings the bus system back into the F/S-mode. The active master disables its current-source MCS when the STOP condition (P) at SDAH is detected (tFS in Fig.25). The bridge also recognizes this STOP condition and takes the following actions: 14.1 The 10-bit slave address is formed from the first two bytes following a START condition (S) or a repeated START condition (Sr). 1. Transistor TR2 closes after tFS to connect SCLH with SCL; both of which are HIGH at this time. Transistor TR3 opens after tFS, which releases the SDA line and allows it to be pulled HIGH by the pull-up resister Rp. This is the STOP condition for the F/S-mode devices. TR3 must open fast enough to ensure the bus free time between the STOP condition and the earliest next START condition is according to the Fast-mode specification (see tBUF in Table 5). The first seven bits of the first byte are the combination 11110XX of which the last two bits (XX) are the two most-significant bits (MSBs) of the 10-bit address; the eighth bit of the first byte is the R/W bit that determines the direction of the message. A `zero' in the least significant position of the first byte means that the master will write information to a selected slave. A `one' in this position means that the master will read information from the slave. If the R/W bit is `zero', then the second byte contains the remaining 8 bits (XXXXXXXX) of the 10-bit address. If the R/W bit is `one', then the next byte contains data transmitted from a slave to a master. 2. When SDA reaches a HIGH (t2 in Fig.25) transistor TR1 closes to connect SDAH with SDA. (Note: interconnections are made when all lines are HIGH, thus preventing spikes on the bus lines). TR1 and TR2 must be closed within the minimum bus free time according to the Fast-mode specification (see tBUF in Table 5). 13.5.3 Definition of bits in the first two bytes 14.2 Formats with 10-bit addresses Various combinations of read/write formats are possible within a transfer that includes 10-bit addressing. Possible data transfer formats are: TIMING REQUIREMENTS FOR THE BRIDGE IN A MIXED-SPEED BUS SYSTEM * Master-transmitter transmits to slave-receiver with a 10-bit slave address. The transfer direction is not changed (see Fig.26). When a 10-bit address follows a START condition, each slave compares the first seven bits of the first byte of the slave address (11110XX) with its own address and tests if the eighth bit (R/W direction bit) is 0. It is possible that more than one device will find a match and generate an acknowledge (A1). All slaves that found a match will compare the eight bits of the second byte of the slave address (XXXXXXXX) with their own addresses, but only one slave will find a match and generate an acknowledge (A2). The matching slave will remain addressed by the master until it receives a STOP It can be seen from Fig.25 that the actions of the bridge at t1, tH and tFS must be so fast that it does not affect the SDAH and SCLH lines. Furthermore the bridge must meet the related timing requirements of the Fast-mode specification for the SDA and SCL lines. 14 10-BIT ADDRESSING This section describes 10-bit addressing and can be disregarded if only 7-bit addressing is used. 10-bit addressing is compatible with, and can be combined with, 7-bit addressing. Using 10 bits for addressing exploits the reserved combination 1111XXX for the first 67 Philips Semiconductors The I2C-bus specification * Combined format. A master transmits data to one slave and then transmits data to another slave (Fig.29). The same master occupies the bus all the time. condition (P) or a repeated START condition (Sr) followed by a different slave address. * Master-receiver reads slave- transmitter with a 10-bit slave address. The transfer direction is changed after the second R/W bit (Fig.27). Up to and including acknowledge bit A2, the procedure is the same as that described for a master-transmitter addressing a slave-receiver. After the repeated START condition (Sr), a matching slave remembers that it was addressed before. This slave then checks if the first seven bits of the first byte of the slave address following Sr are the same as they were after the START condition (S), and tests if the eighth (R/W) bit is 1. If there is a match, the slave considers that it has been addressed as a transmitter and generates acknowledge A3. The slave-transmitter remains addressed until it receives a STOP condition (P) or until it receives another repeated START condition (Sr) followed by a different slave address. After a repeated START condition (Sr), all the other slave devices will also compare the first seven bits of the first byte of the slave address (11110XX) with their own addresses and test the eighth (R/W) bit. However, none of them will be addressed because R/W = 1 (for 10-bit devices), or the 11110XX slave address (for 7-bit devices) does not match. * Combined format. 10-bit and 7-bit addressing combined in one serial transfer (Fig.30). After each START condition (S), or each repeated START condition (Sr), a 10-bit or 7-bit slave address can be transmitted. Figure 27 shows how a master-transmits data to a slave with a 7-bit address and then transmits data to a second slave with a 10-bit address. The same master occupies the bus all the time. NOTES: 1. Combined formats can be used, for example, to control a serial memory. During the first data byte, the internal memory location has to be written. After the START condition and slave address is repeated, data can be transferred. 2. All decisions on auto-increment or decrement of previously accessed memory locations etc. are taken by the designer of the device. 3. Each byte is followed by an acknowledgment bit as indicated by the A or blocks in the sequence. 4. I2C-bus compatible devices must reset their bus logic on receipt of a START or repeated START condition such that they all anticipate the sending of a slave address. * Combined format. A master transmits data to a slave and then reads data from the same slave (Fig.28). The same master occupies the bus all the time. The transfer direction is changed after the second R/W bit. handbook, full pagewidth ,,,,, ,,,, ,, ,, ,, ,,,,, ,,,, ,, ,,,, 1 1 1 1 0 X X S 0 SLAVE ADDRESS SLAVE ADDRESS R/W A1 A2 DATA 1st 7 BITS 2nd BYTE A DATA A/A P MBC613 (write) Fig.26 A master-transmitter addresses a slave-receiver with a 10-bit address. ,,,,,,,, ,,,,, , ,, handbook, full pagewidth1 1 1 1 0 X X 0 SLAVE ADDRESS SLAVE ADDRESS S R/W A1 A2 1st 7 BITS 2nd BYTE 1 1 1 1 0 X X 1 SLAVE ADDRESS Sr R/W A3 DATA A 1st 7 BITS (write) (read) Fig.27 A master-receiver addresses a slave-transmitter with a 10-bit address. 68 DATA A P MBC614 Philips Semiconductors The I2C-bus specification ,,,,,,,,,,,, ,,,,,,,,,,,, ,,,,,,, ,, ,,,,,,, ,, handbook, full pagewidth 1 1 1 1 0 X X 0 SLAVE ADDRESS S R/W A 1st 7 BITS SLAVE ADDRESS 2nd BYTE DATA A A DATA A/A (write) 1 1 1 1 0 X X 1 Sr SLAVE ADDRESS R/W A 1st 7 BITS DATA A DATA A P MBC615 (read) Fig.28 Combined format. A master addresses a slave with a 10-bit address, then transmits data to this slave and reads data from this slave. ,,,,, ,,,, ,, ,, ,,,,,,,,, ,,,, ,,,,, ,,,, ,, ,, ,, ,,,,, ,,,,,,,,,, handbook, full pagewidth 1 1 1 1 0 X X 0 SLAVE ADDRESS S R/W A 1st 7 BITS SLAVE ADDRESS 2nd BYTE DATA A A DATA A/A (write) 1 1 1 1 0 X X 0 SLAVE ADDRESS Sr R/W A 1st 7 BITS SLAVE ADDRESS A 2nd BYTE DATA A DATA A/A (write) P MBC616 Fig.29 Combined format. A master transmits data to two slaves, both with 10-bit addresses. ,,,, ,, ,, ,,,,,,,, ,,,,, ,,,, ,, ,, , ,,,,, ,,,,,,,,, dbook, full pagewidth 0 7 - BIT S R/W A SLAVE ADDRESS DATA A DATA A/A (write) 1 1 1 1 0 X X 0 2nd BYTE OF 10-BIT 1st 7 BITS OF 10-BIT Sr R/W A SLAVE ADDRESS A SLAVE ADDRESS (write) DATA A DATA A/A P MBC617 Fig.30 Combined format. A master transmits data to two slaves, one with a 7-bit address, and one with a 10-bit address. 69 Philips Semiconductors The I2C-bus specification 14.3 General call address and start byte with 10-bit addressing 15 ELECTRICAL SPECIFICATIONS AND TIMING FOR I/O STAGES AND BUS LINES The 10-bit addressing procedure for the I2C-bus is such that the first two bytes after the START condition (S) usually determine which slave will be selected by the master. The exception is the "general call" address 00000000 (H`00'). Slave devices with 10-bit addressing will react to a "general call" in the same way as slave devices with 7-bit addressing (see Section 10.1.1). 15.1 Standard- and Fast-mode devices The I/O levels, I/O current, spike suppression, output slope control and pin capacitance for F/S-mode I2C-bus devices are given in Table 4. The I2C-bus timing characteristics, bus-line capacitance and noise margin are given in Table 5. Figure 31 shows the timing definitions for the I2C-bus. Hardware masters can transmit their 10-bit address after a `general call'. In this case, the `general call' address byte is followed by two successive bytes containing the 10-bit address of the master-transmitter. The format is as shown in Fig.10 where the first DATA byte contains the eight least-significant bits of the master address. The minimum HIGH and LOW periods of the SCL clock specified in Table 5 determine the maximum bit transfer rates of 100 kbit/s for Standard-mode devices and 400 kbit/s for Fast-mode devices. Standard-mode and Fast-mode I2C-bus devices must be able to follow transfers at their own maximum bit rates, either by being able to transmit or receive at that speed or by applying the clock synchronization procedure described in Section 8 which will force the master into a wait state and stretch the LOW period of the SCL signal. Of course, in the latter case the bit transfer rate is reduced. The START byte 00000001 (H`01') can precede the 10-bit addressing in the same way as for 7-bit addressing (see Section 10.1.2). 70 Philips Semiconductors The I2C-bus specification Table 4 Characteristics of the SDA and SCL I/O stages for F/S-mode I2C-bus devices STANDARD-MODE PARAMETER FAST-MODE SYMBOL UNIT MIN. LOW level input voltage: fixed input levels VDD-related input levels VIL HIGH level input voltage: fixed input levels VDD-related input levels VIH Hysteresis of Schmitt trigger inputs: VDD > 2 V VDD < 2 V Vhys MAX. MIN. MAX. -0.5 -0.5 1.5 0.3VDD n/a -0.5 n/a 0.3VDD (1) V V 3.0 0.7VDD (2) n/a 0.7VDD(1) n/a (2) (2) V V n/a n/a n/a n/a 0.05VDD 0.1VDD - - V V LOW level output voltage (open drain or open collector) at 3 mA sink current: VDD > 2 V VDD < 2 V VOL1 VOL3 0 n/a 0.4 n/a 0 0 0.4 0.2VDD V V Output fall time from VIHmin to VILmax with a bus capacitance from 10 pF to 400 pF tof - 250(4) 20 + 0.1Cb(3) 250(4) ns Pulse width of spikes which must be suppressed by the input filter tSP n/a n/a 0 50 ns Input current each I/O pin with an input voltage between 0.1VDD and 0.9VDDmax Ii -10 10 -10(5) 10(5) A Capacitance for each I/O pin Ci - 10 - 10 pF Notes 1. Devices that use non-standard supply voltages which do not conform to the intended I2C-bus system levels must relate their input levels to the VDD voltage to which the pull-up resistors Rp are connected. 2. Maximum VIH = VDDmax + 0.5 V. 3. Cb = capacitance of one bus line in pF. 4. The maximum tf for the SDA and SCL bus lines quoted in Table 5 (300 ns) is longer than the specified maximum tof for the output stages (250 ns). This allows series protection resistors (Rs) to be connected between the SDA/SCL pins and the SDA/SCL bus lines as shown in Fig.36 without exceeding the maximum specified tf. 5. I/O pins of Fast-mode devices must not obstruct the SDA and SCL lines if VDD is switched off. n/a = not applicable 71 Philips Semiconductors The I2C-bus specification Table 5 Characteristics of the SDA and SCL bus lines for F/S-mode I2C-bus devices(1) STANDARD-MODE PARAMETER FAST-MODE SYMBOL UNIT MIN. MAX. MIN. MAX. SCL clock frequency fSCL 0 100 0 400 kHz Hold time (repeated) START condition. After this period, the first clock pulse is generated tHD;STA 4.0 - 0.6 - s LOW period of the SCL clock tLOW 4.7 - 1.3 - s HIGH period of the SCL clock tHIGH 4.0 - 0.6 - s Set-up time for a repeated START condition tSU;STA 4.7 - 0.6 - s Data hold time: tHD;DAT for CBUS compatible masters (see NOTE, Section 10.1.3) for I2C-bus devices 5.0 0(2) - 3.45(3) - 0(2) - 0.9(3) s s Data set-up time 250 - 100(4) tSU;DAT - ns (5) Rise time of both SDA and SCL signals tr - 1000 20 + 0.1Cb 300 ns Fall time of both SDA and SCL signals tf - 300 20 + 0.1Cb(5) 300 ns Set-up time for STOP condition tSU;STO 4.0 - 0.6 - s Bus free time between a STOP and START condition tBUF 4.7 - 1.3 - s Capacitive load for each bus line Cb - 400 - 400 pF Noise margin at the LOW level for each connected device (including hysteresis) VnL 0.1VDD - 0.1VDD - V Noise margin at the HIGH level for each connected device (including hysteresis) VnH 0.2VDD - 0.2VDD - V Notes 1. All values referred to VIHmin and VILmax levels (see Table 4). 2. A device must internally provide a hold time of at least 300 ns for the SDA signal (referred to the VIHmin of the SCL signal) to bridge the undefined region of the falling edge of SCL. 3. The maximum tHD;DAT has only to be met if the device does not stretch the LOW period (tLOW) of the SCL signal. 4. A Fast-mode I2C-bus device can be used in a Standard-mode I2C-bus system, but the requirement tSU;DAT 250 ns must then be met. This will automatically be the case if the device does not stretch the LOW period of the SCL signal. If such a device does stretch the LOW period of the SCL signal, it must output the next data bit to the SDA line tr max + tSU;DAT = 1000 + 250 = 1250 ns (according to the Standard-mode I2C-bus specification) before the SCL line is released. 5. Cb = total capacitance of one bus line in pF. If mixed with Hs-mode devices, faster fall-times according to Table 6 are allowed. n/a = not applicable 72 Philips Semiconductors The I2C-bus specification handbook, full pagewidth SDA tLOW tf tr tSU;DAT tf tHD;STA tSP tr tBUF SCL S tHD;STA tHD;DAT tHIGH tSU;STA Sr tSU;STO P S MSC610 Fig.31 Definition of timing for F/S-mode devices on the I2C-bus. 73 Philips Semiconductors The I2C-bus specification 15.2 Hs-mode devices With an internally generated SCLH signal with LOW and HIGH level periods of 200 ns and 100 ns respectively, an Hs-mode master can fulfil the timing requirements for the external SCLH clock pulses (taking the rise and fall times into account) for the maximum bit rate of 3.4 Mbit/s. So a basic frequency of 10 MHz, or a multiple of 10 MHz, can be used by an Hs-mode master to generate the SCLH signal. There are no limits for maximum HIGH and LOW periods of the SCLH clock, and there is no limit for a lowest bit rate. The I/O levels, I/O current, spike suppression, output slope control and pin capacitance for I2C-bus Hs-mode devices are given in Table 6. The noise margin for HIGH and LOW levels on the bus lines are the same as specified for F/S-mode I2C-bus devices. Figure 32 shows all timing parameters for the Hs-mode timing.The "normal" START condition S does not exist in Hs-mode. Timing parameters for Address bits, R/W bit, Acknowledge bit and DATA bits are all the same. Only the rising edge of the first SCLH clock signal after an acknowledge bit has an larger value because the external Rp has to pull-up SCLH without the help of the internal current-source. Timing parameters are independent for capacitive load up to 100 pF for each bus line allowing the maximum possible bit rate of 3.4 Mbit/s. At a higher capacitive load on the bus lines, the bit rate decreases gradually. The timing parameters for a capacitive bus load of 400 pF are specified in Table 7, allowing a maximum bit rate of 1.7 Mbit/s. For capacitive bus loads between 100 pF and 400 pF, the timing parameters must be interpolated linearly. Rise and fall times are in accordance with the maximum propagation time of the transmission lines SDAH and SCLH to prevent reflections of the open ends. The Hs-mode timing parameters for the bus lines are specified in Table 7. The minimum HIGH and LOW periods and the maximum rise and fall times of the SCLH clock signal determine the highest bit rate. 74 Philips Semiconductors The I2C-bus specification Table 6 Characteristics of the SDAH, SCLH, SDA and SCL I/O stages for Hs-mode I2C-bus devices Hs-MODE PARAMETER SYMBOL UNIT MIN. MAX. VIL -0.5 HIGH level input voltage VIH VDD + Hysteresis of Schmitt trigger inputs Vhys 0.7VDD(1) 0.1VDD(1) - V LOW level output voltage (open drain) at 3 mA sink current at SDAH, SDA and SCLH for: VDD > 2 V VDD < 2 V VOL 0 0 0.4 0.2VDD V V On resistance of the transfer gate, for both current directions at VOL level between SDA and SDAH or SCL and SCLH at 3 mA RonL - 50 On resistance of the transfer gate between SDA and SDAH or SCL and SCLH if both are at VDD level RonH(2) 50 - k Pull-up current of the SCLH current-source. Applies for SCLH output levels between 0.3VDD and 0.7VDD ICS 3 12 mA Output rise time (current-source enabled) and fall time at SCLH with a capacitive load from 10 to 100 pF trCL, tfCL 10 40 ns Output rise time (current-source enabled) and fall time trCL(3), tfCL(3) at SCLH with an external pull-up current source of 3 mA and a capacitive load of 400 pF 20 80 ns Output fall time at SDAH with a capacitive load from 10 to 100 pF tfDA 20 80 ns Output fall time at SDAH with a capacitive load of 400 pF tfDA(3) 40 160 ns Pulse width of spikes at SDAH and SCLH that must be suppressed by the input filters tSP 0 10 ns Input current each I/O pin with an input voltage between Ii(4) 0.1VDD and 0.9VDD - 10 A Capacitance for each I/O pin - 10 pF LOW level input voltage Ci 0.3VDD(1) 0.5(2) V V Notes 1. Devices that use non-standard supply voltages which do not conform to the intended I2C-bus system levels must relate their input levels to the VDD voltage to which the pull-up resistors Rp are connected. 2. Devices that offer the level shift function must tolerate a maximum input voltage of 5.5 V at SDA and SCL. 3. For capacitive bus loads between 100 and 400 pF, the rise and fall time values must be linearly interpolated. 4. SDAH and SCLH I/O stages of Hs-mode slave devices must have floating outputs if their supply voltage has been switched off. Due to the current-source output circuit, which normally has a clipping diode to VDD, this requirement is not mandatory for the SCLH or the SDAH I/O stage of Hs-mode master devices. This means that the supply voltage of Hs-mode master devices cannot be switched off without affecting the SDAH and SCLH lines. 75 Philips Semiconductors The I2C-bus specification Table 7 Characteristics of the SDAH, SCLH, SDA and SCL bus lines for Hs-mode I2C-bus devices(1) PARAMETER Cb = 400 pF(2) Cb = 100 pF MAX. SYMBOL MIN. MAX. MIN. UNIT MAX. SCLH clock frequency fSCLH 0 3.4 0 1.7 MHz Set-up time (repeated) START condition tSU;STA 160 - 320 - ns Hold time (repeated) START condition tHD;STA 160 - 320 - ns LOW period of the SCLH clock tLOW 160 - 320 - ns HIGH period of the SCLH clock tHIGH 60 - 120 - ns Data set-up time tSU;DAT 10 - 10 - ns Data hold time tHD;DAT 0(3) 70 0(3) 150 ns Rise time of SCLH signal trCL 10 40 20 80 ns Rise time of SCLH signal after acknowledge bit trCL1 20 80 40 160 ns Fall time of SCLH signal tfCL 10 40 20 80 ns Rise time of SDAH signal trDA 20 80 40 160 ns Fall time of SDAH signal tfDA 20 80 40 160 ns Set-up time for STOP condition tSU;STO 160 - 320 - ns Capacitive load for SDAH and SCLH lines Cb(2) - 100 - 400 pF Capacitive load for SDAH + SDA line and SCLH + SCL line Cb - 400 - 400 pF Noise margin at the LOW level for each connected device (including hysteresis) VnL 0.1VDD - 0.1VDD - V Noise margin at the HIGH level for each connected device (including hysteresis) VnH 0.2VDD - 0.2VDD - V Notes 1. All values referred to VIHmin and VILmax levels (see Table 6). 2. For bus line loads Cb between 100 and 400 pF the timing parameters must be linearly interpolated. 3. A device must internally provide a Data hold time to bridge the undefined part between VIH and VIL of the falling edge of the SCLH signal. An input circuit with a threshold as low as possible for the falling edge of the SCLH signal minimizes this hold time. 76 Philips Semiconductors The I2C-bus specification handbook, full pagewidth Sr Sr trDA tfDA P SDAH tHD;DAT tSU;STA tHD;STA tSU;STO tSU;DAT SCLH tfCL trCL1 trCL tHIGH tLOW tLOW trCL1 (1) (1) tHIGH MGK871 = MCS current source pull-up = Rp resistor pull-up (1) Rising edge of the first SCLH clock pulse after an acknowledge bit. Fig.32 Definition of timing for Hs-mode devices on the I2C-bus. must be connected to one common supply line of 5 V 10% and must have pull-up resistors connected to their SDA and SCL pins as shown in Fig.35. 16 ELECTRICAL CONNECTIONS OF I2C-BUS DEVICES TO THE BUS LINES The electrical specifications for the I/Os of I2C-bus devices and the characteristics of the bus lines connected to them are given in Section 15. New Fast- and Hs-mode devices must have supply voltage related input levels as specified in Tables 4 and 6. Input levels are defined in such a way that: I2C-bus devices with fixed input levels of 1.5 V and 3 V can each have their own appropriate supply voltage. Pull-up resistors must be connected to a 5 V 10% supply (Fig.33). I2C-bus devices with input levels related to VDD must have one common supply line to which the pull-up resistor is also connected (Fig.34). * The noise margin on the LOW level is 0.1VDD * The noise margin on the HIGH level is 0.2VDD * As shown in Fig.36, series resistors (RS) of e.g. 300 can be used for protection against high-voltage spikes on the SDA and SCL lines (resulting from the flash-over of a TV picture tube, for example). When devices with fixed input levels are mixed with devices with input levels related to VDD, the latter devices 77 Philips Semiconductors The I2C-bus specification VDD2 - 4 are device dependent (e.g. 12 V) handbook, full pagewidth V DD1 = 5 V Rp Rp 10 % NMOS V DD2 V DD3 V DD4 BiCMOS CMOS BIPOLAR SDA SCL MBC610 Fig.33 Fixed input level devices connected to the I2C-bus. V DD = e.g. 3 V handbook, full pagewidth Rp Rp CMOS CMOS CMOS CMOS SDA SCL MBC625 Fig.34 Devices with wide supply voltage range connected to the I2C-bus. V DD2,3 are device dependent (e.g. 12 V) handbook, full pagewidth V DD1 = 5V Rp Rp CMOS 10 % CMOS V DD2 V DD3 NMOS BIPOLAR SDA SCL MBC626 Fig.35 Devices with input levels related to VDD (supply VDD1) mixed with fixed input level devices (supply VDD2,3) on the I2C-bus. 78 Philips Semiconductors The I2C-bus specification handbook, full pagewidth Rs V DD V DD I2 C DEVICE I2 C DEVICE Rs Rs Rp Rp Rs SDA SCL MBC627 Fig.36 Series resistors (Rs) for protection against high-voltage spikes. 16.1 Maximum and minimum values of resistors Rp and Rs for Standard-mode I2C-bus devices Rp min is shown in Fig.37. The required noise margin of 0.1VDD for the LOW level, limits the maximum value of Rs. Rs max as a function of Rp is shown in Fig.38. For Standard-mode I2C-bus systems, the values of resistors Rp and Rs in Fig.33 depend on the following parameters: The bus capacitance is the total capacitance of wire, connections and pins. This capacitance limits the maximum value of Rp due to the specified rise time. Fig.39 shows Rp max as a function of bus capacitance. * Supply voltage * Bus capacitance The maximum HIGH level input current of each input/output connection has a specified maximum value of 10 A. Due to the required noise margin of 0.2 VDD for the HIGH level, this input current limits the maximum value of Rp. This limit depends on VDD. The total HIGH level input current is shown as a function of Rp max in Fig.40. * Number of connected devices (input current + leakage current). The supply voltage limits the minimum value of resistor Rp due to the specified minimum sink current of 3 mA at VOLmax = 0.4 V for the output stages. VDD as a function of 79 Philips Semiconductors The I2C-bus specification 6 handbook, halfpage minimum value R p (k ) 5 RS = 0 4 MBC635 20 maximum value R p (k) 16 MBC628 12 3 RS = 0 8 max. R S 2 max. R S @ V DD = 5 V 4 1 0 0 0 0 4 8 12 100 16 Fig.37 Minimum value of Rp as a function of supply voltage with the value os Rs as a parameter. MBC630 20 maximum value R p (k ) 16 10 5V V DD = 2.5 V 400 Fig.39 Maximum value of Rp as a function of bus capacitance for a Standard-mode I2C-bus. MBC629 8 300 bus capacitance (pF) V DD (V) Rp (k ) 200 12 6 VDD= 15 V 15 V 8 4 10 V 10 V 4 5V 2 2.5 V 0 0 0 0 400 800 1200 1600 40 80 120 160 200 total high level input current (A) maximum value R s () Fig.40 Total HIGH level input current as a function of the maximum value of Rp with supply voltage as a parameter. Fig.38 Maximum value of Rs as a function of the value of Rp with supply voltage as a parameter. 80 Philips Semiconductors The I2C-bus specification 17 APPLICATION INFORMATION 17.1 17.2 Slope-controlled output stages of Fast-mode I2C-bus devices The supply voltage (VDD) and the maximum output LOW level determine the minimum value of pull-up resistor Rp (see Section 16.1). For example, with a supply voltage of VDD = 5 V 10% and VOLmax = 0.4 V at 3 mA, Rp min = (5.5 - 0.4)/0.003 = 1.7 k. As shown in Fig.33, this value of Rp limits the maximum bus capacitance to about 200 pF to meet the maximum tr requirement of 300 ns. If the bus has a higher capacitance than this, a switched pull-up circuit as shown in Fig.43 can be used. The electrical specifications for the I/Os of I2C-bus devices and the characteristics of the bus lines connected to them are given in Section 15. Figures 41 and 42 show examples of output stages with slope control in CMOS and bipolar technology. The slope of the falling edge is defined by a Miller capacitor (C1) and a resistor (R1). The typical values for C1 and R1 are indicated on the diagrams. The wide tolerance for output fall time tof given in Table 4 means that the design is not critical. The fall time is only slightly influenced by the external bus load (Cb) and external pull-up resistor (Rp). However, the rise time (tr) specified in Table 5 is mainly determined by the bus load capacitance and the value of the pull-up resistor. VDD to input circuit P1 R1 50 k C1 2 pF N1 The switched pull-up circuit in Fig.43 is for a supply voltage of VDD = 5 V 10% and a maximum capacitive load of 400 pF. Since it is controlled by the bus levels, it needs no additional switching control signals. During the rising/falling edges, the bilateral switch in the HCT4066 switches pull-up resistor Rp2 on/off at bus levels between 0.8 V and 2.0 V. Combined resistors Rp1 and Rp2 can pull-up the bus line within the maximum specified rise time (tr) of 300 ns. Series resistors Rs are optional. They protect the I/O stages of the I2C-bus devices from high-voltage spikes on the bus lines, and minimize crosstalk and undershoot of the bus line signals. The maximum value of Rs is determined by the maximum permitted voltage drop across this resistor when the bus line is switched to the LOW level in order to switch off Rp2. VDD Rp I/O SDA or SCL bus line Cb N2 VSS Switched pull-up circuit for Fast-mode I2C-bus devices VSS MBC618 Fig.41 Slope-controlled output stage in CMOS technology. nE nZ R1 20 k to input circuit C1 Rp I/O GND R p2 100 1.7 k N R p1 SDA or SCL bus line Rs I/O SDA or SCL bus line Cb T2 Rs I/O 5 pF T1 100 Cb 400 pF max. N VSS MBC620 GND VSS FAST - MODE I 2 C BUS DEVICES MBC619 Fig.42 Slope-controlled output stage in bipolar technology. Fig.43 Switched pull-up circuit. 81 10 % N 1.3 k VDD 5V VCC P Vp VDD nY 1/4 HCT4066 Philips Semiconductors The I2C-bus specification 17.3 Wiring pattern of the bus lines In general, the wiring must be so chosen that crosstalk and interference to/from the bus lines is minimized. The bus lines are most susceptible to crosstalk and interference at the HIGH level because of the relatively high impedance of the pull-up devices. MBC612 7.5 maximum value R p (k) 6.0 handbook, halfpage If the length of the bus lines on a PCB or ribbon cable exceeds 10 cm and includes the VDD and VSS lines, the wiring pattern must be: 4.5 SDA 3.0 RS = 0 VDD max. R S @ V DD = 5 V 1.5 VSS SCL 0 If only the VSS line is included, the wiring pattern must be: 0 200 300 400 bus capacitance (pF) SDA Fig.44 Maximum value of Rp as a function of bus capacitance for meeting the tr max requirement for a Fast-mode I2C-bus. VSS SCL These wiring patterns also result in identical capacitive loads for the SDA and SCL lines. The VSS and VDD lines can be omitted if a PCB with a VSS and/or VDD layer is used. 17.5 Maximum and minimum values of resistors Rp and Rs for Hs-mode I2C-bus devices The maximum and minimum values for resistors Rp and Rs connected to an Hs-mode I2C-bus can be calculated from the data in Tables 6 and 7. Many combinations of these values are possible, owing to different rise and fall times, bus line loads, supply voltages, mixed speed systems and level shifting. Because of this, no further graphs are included in this specification. If the bus lines are twisted-pairs, each bus line must be twisted with a VSS return. Alternatively, the SCL line can be twisted with a VSS return, and the SDA line twisted with a VDD return. In the latter case, capacitors must be used to decouple the VDD line to the VSS line at both ends of the twisted pairs. If the bus lines are shielded (shield connected to VSS), interference will be minimized. However, the shielded cable must have low capacitive coupling between the SDA and SCL lines to minimize crosstalk. 17.4 100 18 BI-DIRECTIONAL LEVEL SHIFTER FOR F/S-MODE I2C-BUS SYSTEMS Present technology processes for integrated circuits with clearances of 0.5 m and less limit the maximum supply voltage and consequently the logic levels for the digital I/O signals. To interface these lower voltage circuits with existing 5 V, devices a level shifter is needed. For bi-directional bus systems like the I2C-bus, such a level shifter must also be bi-directional, without the need of a direction control signal(1). The simplest way to solve this problem is by connecting a discrete MOS-FET to each bus line. Maximum and minimum values of resistors Rp and Rs for Fast-mode I2C-bus devices The maximum and minimum values for resistors Rp and Rs connected to a Fast-mode I2C-bus can be determined from Figs 37, 38 and 40 in Section 16.1. Because a Fast-mode I2C-bus has faster rise times (tr) the maximum value of Rp as a function of bus capacitance is less than that shown in Fig.39 The replacement graph for Fig.39 showing the maximum value of Rp as a function of bus capacitance (Cb) for a Fast-mode I2C-bus is given in Fig.44. (1) US 5,689,196 granted; corresponding patent applications pending. 82 Philips Semiconductors The I2C-bus specification interconnect two sections of an I2C-bus system, with each section having a different supply voltage and different logic levels. Such a configuration is shown in Fig.45. The left "low-voltage" section has pull-up resistors and devices connected to a 3.3 V supply voltage, the right "high-voltage" section has pull-up resistors and devices connected to a 5 V supply voltage. The devices of each section have I/Os with supply voltage related logic input levels and an open drain output configuration. In spite of its surprising simplicity, such a solution not only fulfils the requirement of bi-directional level shifting without a direction control signal, it also: * isolates a powered-down bus section from the rest of the bus system * protects the "lower voltage" side against high voltage spikes from the "higher-voltage" side. The bi-directional level shifter can be used for both Standard-mode (up to100 kbit/s) or in Fast-mode (up to 400 kbit/s) I2C-bus systems. It is not intended for Hs-mode systems, which may have a bridge with a level shifting possibility (see Section 13.5) 18.1 The level shifter for each bus line is identical and consists of one discrete N-channel enhancement MOS-FET; TR1 for the serial data line SDA and TR2 for the serial clock line SCL. The gates (g) have to be connected to the lowest supply voltage VDD1, the sources (s) to the bus lines of the "lower-voltage" section, and the drains (d) to the bus lines of the "higher-voltage" section. Many MOS-FETs have the substrate internally connected with its source, if this is not the case, an external connection should be made. Each MOS-FET has an integral diode (n-p junction) between the drain and substrate. Connecting devices with different logic levels Section 16 described how different voltage devices could be connected to the same bus by using pull-up resistors to the supply voltage line. Although this is the simplest solution, the lower voltage devices must be 5 V tolerant, which can make them more expensive to manufacture. By using a bi-directional level shifter, however, it's possible to VDD2 = 5 V VDD1 = 3.3 V handbook, full pagewidth Rp SDA1 g Rp TR1 s Rp SCL1 3.3 V DEVICE SDA2 d g s TR2 SCL2 d 3.3 V DEVICE Rp 5 V DEVICE 5 V DEVICE MGK879 Fig.45 Bi-directional level shifter circuit connecting two different voltage sections in an I2C-bus system. 83 Philips Semiconductors The I2C-bus specification 18.1.1 3. A 5 V device pulls down the bus line to a LOW level. The drain-substrate diode of the MOS-FET the "lower-voltage" section is pulled down until VGS passes the threshold and the MOS-FET starts to conduct. The bus line of the "lower-voltage" section is then further pulled down to a LOW level by the 5 V device via the conducting MOS-FET. So the bus lines of both sections go LOW to the same voltage level. OPERATION OF THE LEVEL SHIFTER The following three states should be considered during the operation of the level shifter: 1. No device is pulling down the bus line. The bus line of the "lower-voltage" section is pulled up by its pull-up resistors Rp to 3.3 V. The gate and the source of the MOS-FET are both at 3.3 V, so its VGS is below the threshold voltage and the MOS-FET is not conducting. This allows the bus line at the "higher-voltage" section to be pulled up by its pull-up resistor Rp to 5 V. So the bus lines of both sections are HIGH, but at a different voltage level. The three states show that the logic levels are transferred in both directions of the bus system, independent of the driving section. State 1 performs the level shift function. States 2 and 3 perform a "wired AND" function between the bus lines of both sections as required by the I2C-bus specification. 2. A 3.3 V device pulls down the bus line to a LOW level. The source of the MOS-FET also becomes LOW, while the gate stay at 3.3 V. VGS rises above the threshold and the MOS-FET starts to conduct. The bus line of the "higher-voltage" section is then also pulled down to a LOW level by the 3.3 V device via the conducting MOS-FET. So the bus lines of both sections go LOW to the same voltage level. Supply voltages other than 3.3 V for VDD1 and 5 V for VDD2 can also be applied, e.g. 2 V for VDD1 and 10 V for VDD2 is feasible. In normal operation VDD2 must be equal to or higher than VDD1 (VDD2 is allowed to fall below VDD1 during switching power on/off). 84 Philips Semiconductors The I2C-bus specification 19 DEVELOPMENT TOOLS AVAILABLE FROM PHILIPS Table 8 I2C evaluation boards PRODUCT DESCRIPTION I2C-bus OM4151/ S87C00KSD evaluation board with microcontroller, LCD, LED, Par. I/O, SRAM, EEPROM, Clock, DTMF generator, AD/DA conversion. OM5500 Demo kit for the PCF2166 LCD driver and PCD3756A telecom microcontroller Table 9 Development tools for 80C51-based systems PRODUCT PDS51 DESCRIPTION A board-level, full featured, in-circuit emulator: RS232 interface to PC, universal motherboard, controlled via terminal emulation Table 10 Development tools for 68000-based systems PRODUCT DESCRIPTION OM4160/2 Microcore-2 demonstration/evaluation board with SCC68070 OM4160/4 Microcore-4 demonstration/evaluation board with 90CE201 OM4160/5 Microcore-5 demonstration/evaluation board with 90CE301 Table 11 I2C analyzers PRODUCT DESCRIPTION I2C-bus OM1022 PC analyzer with multi-master capability. Hardware and software (runs on IBM or compatible PC) to experiment with and analyze the behaviour of the I2C-bus (includes documentation) OM4777 Similar to OM1022 but for single-master systems only PF8681 I2C-bus analyzer support package for the PM3580 logic analyzer family 85 Philips Semiconductors The I2C-bus specification 20 SUPPORT LITERATURE Table 12 Data handbooks TITLE ORDERING CODE IC01: Semiconductors for Radio, Audio and CD/DVD Systems 9397 750 02453 IC02: Semiconductors for Television and Video Systems 9397 750 01989 IC03: Semiconductors for Wired Telecom Systems (parts a & b) 9397 750 00839, 9397 750 00811 IC12: I2C Peripherals 9397 750 01647 IC14: 8048-based 8-bit microcontrollers 9398 652 40011 IC17: Semiconductors for wireless communications 9397 750 01002 IC18: Semiconductors for in-car electronics 9397 750 00418 IC19: ICs for data communications 9397 750 00138 IC20: 80C51-based 8-bit microcontrollers + Application notes and Development tools 9397 750 00963 IC22: Multimedia ICs 9397 750 02183 Table 13 Brochures/leaflets/lab. reports/books etc. TITLE Can you make the distance... with extender IC) I2C-bus ORDERING CODE (information about the P82B715 I2C-bus 9397 750 00008 I2C-bus multi-master & single-master controller kits 9397 750 00953 Desktop video (CD-ROM) 9397 750 00644 80C51 core instructions quick reference 9398 510 76011 80C51 microcontroller selection guide 9397 750 01587 OM5027 I2C-bus evaluation board for low-voltage, low-power ICs & software 9398 706 98011 P90CL301 I2C driver routines AN94078 User manual of Microsoft Pascal I2C-bus driver (MICDRV4.OBJ) ETV/IR8833 C routines for the PCF8584 AN95068 Using the PCF8584 with non-specified timings and other frequently asked questions AN96040 User's guide to I2C-bus control programs ETV8835 The I2C-bus from theory to practice (book and disk) Author: D. Paret Publisher: Wiley ISBN: 0-471-96268-6 Bi-directional level shifter for I2C-bus and other systems AN97055 OM5500 demo kit for the PCF2166 LCD driver and PCD3756A telecom microcontroller 9397 750 00954 For more information about Philips Semiconductors and how we can help in your I2C-bus design, contact your nearest Philips Semiconductors national organization from the address list of the back of this book, or visit our worldwide web site at http://www.semiconductors.philips.com 86