Online course on Embedded Systems

MODULE -11

I2C Bus interface
   

How it arrived:

Almost 25 years ago, in the early 1980's Philips designed & developed a new bus standard namely I²C bus, for easy communication between Integrated Circuits (especially in TV circuits), which reside on the same circuit board.
The name I²C translates into Inter Integrated Circuits is a bi-directional 2-wire bus standard for efficient inter-IC control. So the bus is commonly known as the Inter-IC or I²C -bus.
When connecting multiple devices together, the address and data lines of each device were connected individually. This would result in a lot of traces on the PCB, and require more components. This makes the systems expensive and also susceptible to interference and disturbances by Electromagnetic Interference (EMI) and Electrostatic Discharge (ESD). The I²C bus standard is a remedy to this problem.

Introduction:

I²C is a multi-master, low-bandwidth, short distance, serial communication bus protocol. Nowadays it is not only used on single boards, but also to attach low-speed peripheral devices and components to a motherboard, embedded system, or cell-phone, as the new versions provide lot of advanced features and much higher speed. The features like simplicity & flexibility make this bus attractive for consumer and automotive electronics.

Details:

The basic design of I²C has a 7-bit address space with 16 reserved addresses, which makes the maximum number of nodes that can communicate on the same bus as 112. That means each I²C device is recognized by a unique 7-bit address. It is important to note that the maximum number of nodes is obviously limited by the address space, and also by the total bus capacitance of 400 pf.

The two bi-directional lines, which carry information between the devices connected to the bus, are known as Serial Data line (SDA) and Serial Clock line (SCL). As the name indicates the SDA line contains the data and the SCL with the clock signal for synchronization. The typical voltages used are +5 V or +3.3V.

Like the CAN & LIN protocols, the I²C also follows the master-slave communication protocol. But the I²C bus is a multi-master bus, which means more than one IC/device capable of initiating a data transfer can be connected to it. The device that initiates the communication is called MASTER, whereas the device being addressed by the Master is called as SLAVE. It is the master device who always do generation of clock signals, which means each master generates its own clock signals when transferring data on the bus.

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The real communication:

As we saw already, the active lines used for communication in I²C protocol are bi-directional. Each & every device (for example: MCU, LCD driver, ASIC, remote I/O ports, RAM, EEPROM, data converters) connected to the bus will be having a unique address. Each of these devices can act as a receiver and/or transmitter, depending on the functionality. And also each device connected to the bus is software addressable by this unique address:

As we already know the nodes or any other peripheral devices (no matter whether its master or slave) use microcontrollers to connect and communicate through the bus, this communication can be considered as inter-IC communication in general.

Let us assume that the master MCU (as always, it's the master who initiates the communication) wants to send data to one of its slaves. The step-by-step procedure will be as follows.

1. Wait until it sees no activity on the I2C bus. The SDA and SCL lines are both high. The bus is 'free'.
2. The Master MCU issues a start condition, telling that "its mine - I have started to use the bus". This      condition informs all the slave devices to listen on the serial data line for instructions/data.
3. Provide on the SCL line a clock signal. It will be used by all the ICs as the reference time at which     each bit of data on the SDA line will be correct (valid) and can be used. The data on the data line     must be valid at the time the clock line switches from 'low' to 'high' voltage.
4. The Master MCU sends the unique binary address of the target device it wants to access.
5. Master MCU puts a one-bit message (called read/write flag) on the bus telling whether it wants to     SEND or RECEIVE data from the other chip. This read/write flag is an indication to the slave node     whether the access is a read or a write operation.
6. The slave node ICs will then compare the received address with its own address. The Slave device      with the matching address responds back with an acknowledgement signal. If the address doesn't      match, they simply wait until the bus is released by the stop condition.
7. Once the master MCU receives the acknowledgement signal, it starts transmitting or receiving and      the data communication proceeds between the Master and the Slave on the data bus. Both the      master and slave can receive or transmit data depending on whether the communication is a read      or write. The transmitter sends 8-bits of data to the receiver, which replies with a 1-bit      acknowledgement. And the action of data transfer continues.
8. When the communication is complete, the master issues a stop condition indicating that everything     is done. This action free ups the bus. The stop signal is just one bit of information transferred by a     special 'wiggling' of the SDA/SCL wires of the bus.

       Notes:
1. Devices with Master capability can identify themselves to other specific Master devices and advise     their own specific address and functionality.
2. Only two devices exchange data during one 'conversation'

 

 

The trick of open-drain lines & pull-up resistors:

The bus interface in I²C is built around an input buffer and an open drain transistor. When the bus is in "idle" state, the bus lines are kept in the logic "high" state. The external pull-up resistors are used for this condition to achieve. This pull-up resistor as seen in the fig-1 is actually a small current source. If the device wants to put a signal on the bus, the chip drives its output transistor, thus pulling the bus to "low" level. Suppose the bus is already occupied by another chip by sending a "low" state to the bus. Then all other chips lose their right to access the bus. The chip does this with a built-in bus mastering technique.

Both the bus lines SDA and SCL are initially bi-directional. This means that in a particular device, these lines can be driven by the IC itself or from an external device. In order to achieve this functionality, these signals use open collector or open drain outputs.

The weak point of open-collector technique is, in a lengthy bus the speed of transmission comes down drastically due to the presence of capacitive load. The shapes of the signals alter in proportion to RC time constant. Higher the RC constant, the slower will be the transmission. At some point, the ICs will not be able to sense logic 1 and 0.

And also it can cause reflections at high speed, which creates "ghost signals" and corrupt the data, which is being transmitted.

This problem can be overcome by using an active I²C terminator. This device consists of a twin charge pump, which can be considered as a dynamic resistor (instead of the passive pull-up resistors used). The moment the state changes, it provides a large current (low dynamic resistance) to the bus. This action will charge the parasitic capacitor very quickly. Once the voltage has risen above a certain level, the high current mode cuts out and the output current drops sharply.

Different states, conditions & events on the bus:

We saw several unique states and conditions on the bus in our explanation: START, ADDRESS, ACK, DATA and STOP. Let us now try to understand some more about those states.

START:
The Start state needs to be issued on the bus before any type of transaction on the bus. The master-chip first pulls the data line (SDA) low, and next pulls the clock line (SCL) low. This condition is a signal to all the connected chips to listen to the bus to expect a transmission.
A single message can contain multiple Start conditions. The use of this is called "repeated start".

ADDRESS & DATA:

After the "start" bit, a byte (7+1) is transmitted to slaves by the master. This byte is the address, which can identify the particular slave on the bus. Bit 0 of this byte determines the slave access mode ('1' = read / '0' = write). Remember, bytes are always transmitted MSB first. The R/W bit '0' indicates the master is willing to send data to the slaves. Then the intended slave will respond back with ACK signal, indicating that its ready to receive. And the communication continues.

In the same way, a byte can be received from the slave if the R/W bit in the address was set to '1', i.e. 'read'. But now the master is not allowed to touch the SDA line. Master sends the 8 clock pulses needed to clock in a byte on the SCL line, and releases the SDA line. Instead, the slave will now take control of this SDA line for data transfer. All the master has to do now is generate a rising edge on the SCL line, read the level on SDA and generate a falling edge on the SCL line. The slave will not change the data during the time that SCL is high. This sequence has to be performed 8 times to complete the data byte.

Some of the addresses are reserved for "extended addressing mode", which use 10-bit addressing. If a standard slave node, which is not able to resolve this extended addressing receives this address, it won't do anything.

ACK:

As we know the ACK signal is send back to the master whenever the address or data byte has been transmitted onto the bus and received by the intended slave node.

The slave after sending ACK signal will drive the SDA line to low status immediately after receiving the 8th data bit transmitted by the master or after evaluating received address byte. So to signal the completion of transmission, SCL pulled to low by master, whereas SDA is pulled to low by slave.

To repeat the transmission, master drops a clock pulse on the SCL line and slave will release the SDA line after receiving the clock. With this, bus is now ready again for master to send data or to initiate a stop condition.

In the same way, the master must acknowledge the slave device upon successful reception of a byte from a slave.
SDA and the SCL line are in full control of the master. The slave will release the SDA line after sending last bit to the master and make the SDA line high. The Master will now bring the SDA line low state and put a clock pulse on the SCL line. After completion of this clock pulse, the master will again release the SDA line to allow the slave to regain control of the SDA line.

The master can stop receiving data from the slave at any time, just by sending a stop condition.

NACK:

NACK means "Not Acknowledge". Confused? Don't confuse it with "No Acknowledge". Because, "Not Acknowledge" occurs only after a master has read a byte from a slave. And "No Acknowledge" occurs after a master has written a byte to a slave. Again confused? Lets analyze this in detail.

This happens when the slave regains control of the SDA line after the ACK cycle issued by the master.

Let's assume the next bit ready to be sent to the master is a 0. The slave would pull the SDA line low immediately after the master takes the SCL line low. The master now attempts to generate a Stop condition on the bus. It releases the SCL line first and then tries to release the SDA line, which is held low by the slave. So in short, No Stop condition has been generated on the bus. This condition is called a NACK.

No ACK:

If, after transmission of the 8th bit from the master to the slave the slave does not pull the SDA line low, then this is considered a No ACK condition.
This condition may be created due to the following reasons:
1. The slave is not there (in case of an address)
2. The slave missed a pulse and got out of sync with the SCL line of the master.
3. The bus is "stuck". One of the lines could be held low permanently.

In any case the master should abort by attempting to send a stop condition on the bus.

STOP:

The stop state is sent to the bus only after the message transfer has been completed. The master-MCU first releases the SCL and then the SDA line. This condition is a true indication to all the chips and devices on the bus that the bus is idle or the bus is free and available again for another communication.
A Stop condition denotes the END of a transmission, even if it is issued in the middle of a transaction or in the middle of a byte. In this case, the chip disregards the information sent and goes to IDLE-state, waiting for a new start condition.

Modes of operation:

The I²C bus can operate in three modes, or in other words the data on the I2C bus can be transferred in three different modes.
1. Standard mode
2. Fast mode
3. High-Speed (Hs) mode

Standard mode:

1. This is the original Standard mode released in early 80's
2. It has maximum data rates of 100kbps
3. It uses 7-bit addressing, which provides 112 slave addresses.

Enhanced or Fast mode:

The fast mode added some more features to the slave devices.
1. The maximum data rate was increased to 400kbps.
2. To suppress noise spikes, Fast-mode devices were given Schmitt-triggered inputs.
3. The SCL and SDA lines of an I²C-bus slave device were made to exhibit high impedance when power was removed.

High-Speed mode:

This mode was created mainly to increase the data rate up to 36 times faster than standard mode. It provides 1.7 Mbps (with C>b = 400pF), and 3.4Mbps (with C>b = 100pF).

The major difference between High Speed (HS) mode in comparison to standard mode is, HS mode systems must include active pull up resistors on the SCL line. The other difference is, the master operating in HS -mode sends a compatibility request signal in code form to slave, if not-Acknowledge (a bit name with in the I2C frame) remains high after receiving the compatibility code, than the master assumes the slave is capable of HS-mode.

The risk of data corruption:

The operation of the bus with one master node seems to be very easy. But what happens if there are two masters connected to the bus and if both of them start communicating at the same time. Let us try to analyze this situation in detail.
When the first MCU issues a start condition and sends an address, all slaves will listen (including the second MCU which at that time is considered a slave as well). If the address does not match the address of the second MCU, it will hold back any activity until the bus becomes idle again after a stop condition.
As long as the two MCU's monitor what is going on, on the bus (start and stop) and as long as they are aware that a transaction is going on because the last issued command was not a STOP, there is no problem.
But, what will happen if one of the MCU's missed the START condition and still thinks the bus is idle, or it just came out of reset and wants to communicate.

The physical bus setup of the I²C helps to solve this problem. Since the bus structure is a wired AND (if one device pulls a line low it stays low), its possible to find whether the bus is idle or occupied.

Different versions of the I2C bus

Some of the specifications of the different versions of the I2C bus are explained below.

Version 1.0 - 1992

1. Programming of a slave address by software has been omitted.
2. The "low-speed mode" has been omitted.
3. The Fast-mode is added. Fast-mode devices are downward compatible
4. 10-bit addressing is added. This allows 1024 additional slave addresses.
5. Slope control and input filtering for Fast-mode devices is specified to improve the EMC behaviour.

Version 2.0 - 1998

1. The High-speed mode (Hs-mode) is added.
2. 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.
3. The 0.6 V at 6 mA requirement for the output stages of Fast-mode devices has been omitted.
4. The fixed input levels for new devices are replaced by bus voltage-related levels.
5. Application information for bi-directional level shifter is added.

Version 2.1 - 2000

1. After a repeated START condition in Hs-mode, it is possible to stretch the clock signal SCLH (see Section 13.2 and Figs 22, 25 and 32).
2. Some timing parameters in Hs-mode have been relaxed

Benefits and Drawbacks:

Since only two wires are required, I2C is well suited for boards with many devices connected on the bus. This helps reduce the cost and complexity of the circuit as additional devices are added to the system.

Due to the presence of only two wires, there is additional complexity in handling the overhead of addressing and acknowledgments. This can be inefficient in simple configurations and a direct-link interface such as SPI might be preferred.
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