Understanding I2C

Note: This post originated from Robot Electronics, I have reposted it here with some edits for clarity and as a personal reference.

As I continue to refine this document and it no longer closely resembles the original I will remove this note and replace it with a reference link at the bottom of this post.

If anyone at Robot Electronics takes issue with this then please contact me and I will be happy to remove the article.

I2C is clearly very confusing for the newcomer. I know that I personally avoided I2C devices for my first few projects but now that I have learned the ins-and-outs of I2C I thought I would share these lessons with everyone. This page attempts to de-mystify the I2C bus, lets hope I can do that!

The Physical I2C Bus

This is just two wires, called SCL and SDA. SCL is the clock line. It is used to synchronize all data transfers over the I2C bus. SDA is the data line. The SCL and SDA lines are connected to all devices on the I2C bus. There needs to be a third wire which is just the ground or 0 volts. There may also be a 5 V wire if power is being distributed to the devices.

Both SCL and SDA lines are “open drain” drivers. What this means is that the chip can drive its output low, but it cannot drive it high. For the line to be able to go high you must provide pull-up resistors to the 5 V supply. There should be a resistor from the SCL line to the 5 V line and another from the SDA line to the 5 V line. You only need one set of pull-up resistors for the whole I2C bus, not for each device, as illustrated below:

Using pull-up resistors on the I2C Bus
Using pull-up resistors on the I2C Bus

The value of the resistors is not critical. I have seen anything from 1k8 (1800 ohms) to 47k (47000 ohms) used. 1k8, 4k7 and 10k are common values, but anything in this range should work OK. I recommend 1k8 as this gives you the best performance. If the resistors are missing, the SCL and SDA lines will always be low - nearly 0 V - and the I2C bus will not work.

Masters and Slaves

The devices on the I2C bus are either masters or slaves. The master is always the device that drives the SCL clock line. The slaves are the devices that respond to the master. A slave cannot initiate a transfer over the I2C bus, only a master can do that. There can be, and usually are, multiple slaves on the I2C bus, however there is normally only one master. It is possible to have multiple masters, but it is unusual and not covered here.

On your robot, the master will be your controller and the slaves will be our modules such as the SRF08 or CMPS03. Slaves will never initiate a transfer. Both master and slave can transfer data over the I2C bus, but that transfer is always controlled by the master.

The I2C Physical Protocol

When the master (your controller) wishes to talk to a slave (our CMPS03 for example) it begins by issuing a start sequence on the I2C bus. A start sequence is one of two special sequences defined for the I2C bus, the other being the stop sequence.

The start sequence and stop sequence are special in that these are the only places where the SDA (data line) is allowed to change while the SCL (clock line) is high. When data is being transferred, SDA must remain stable and not change whilst SCL is high. The start and stop sequences mark the beginning and end of a transaction with the slave device.


Data is transferred in sequences of 8 bits. The bits are placed on the SDA line starting with the MSB (Most Significant Bit). The SCL line is then pulsed high, then low.

Remember that the chip cannot really drive the line high, it simply “lets go” of it and the resistor actually pulls it high.

For every 8 bits transferred, the device receiving the data sends back an acknowledge bit, so there are actually 9 SCL clock pulses to transfer each 8 bit byte of data. If the receiving device sends back a low ACK bit, then it has received the data and is ready to accept another byte. If it sends back a high then it is indicating it cannot accept any further data and the master should terminate the transfer by sending a stop sequence.


How Fast is it?

The standard clock speed for I2C up to 100 kHz. Philips defines faster speeds:

All of our modules are designed to work at up to 100 kHz. We have tested our modules up to 1 MHz but this needs a small delay of a few uS between each byte transferred. In practical robots, we have never had any need to use high SCL speeds. Keep SCL at or below 100 kHz and then forget about it.

I2C Device Addressing

All I2C addresses are either 7 bits or 10 bits. The use of 10 bit addresses is rare and is not covered here. All of our modules and the common chips you will use will have 7 bit addresses. This means that you can have up to 128 devices on the I2C bus, since a 7 bit number can be from 0 to 127.

When sending out the 7 bit address, we still always send 8 bits. The extra bit is used to inform the slave if the master is writing to it or reading from it. If the bit is zero the master is writing to the slave. If the bit is 1 the master is reading from the slave. The 7 bit address is placed in the upper 7 bits of the byte and the Read/Write (R/W) bit is in the LSB (Least Significant Bit).


The placement of the 7 bit address in the upper 7 bits of the byte is a source of confusion for the newcomer. It means that to write to address 21, you must actually send out 42 which is 21 moved over by 1 bit. It is probably easier to think of the I2C bus addresses as 8 bit addresses, with even addresses as write only, and the odd addresses as the read address for the same device. To take our CMPS03 for example, this is at address 0xC0 ($C0). You would uses 0xC0 to write to the CMPS03 and 0xC1 to read from it. So the read/write bit just makes it an odd/even address.

The I2C Software Protocol

The first thing that will happen is that the master will send out a start sequence. This will alert all the slave devices on the bus that a transaction is starting and they should listen in incase it is for them. Next the master will send out the device address. The slave that matches this address will continue with the transaction, any others will ignore the rest of this transaction and wait for the next.

Having addressed the slave device the master must now send out the internal location or register number inside the slave that it wishes to write to or read from. This number is obviously dependant on what the slave actually is and how many internal registers it has. Some very simple devices do not have any, but most do, including all of our modules.

Our CMPS03 has 16 locations numbered 0-15. The SRF08 has 36. Having sent the I2C address and the internal register address the master can now send the data byte (or bytes, it doesn’t have to be just one). The master can continue to send data bytes to the slave and these will normally be placed in the following registers because the slave will automatically increment the internal register address after each byte. When the master has finished writing all data to the slave, it sends a stop sequence which completes the transaction. So to write to a slave device:

As an example, you have an SRF08 at the factory default address of 0xE0. To start the SRF08 ranging you would write 0x51 to the command register at 0x00 like this:

Reading from the Slave

This is a little more complicated - but not too much more. Before reading data from the slave device, you must tell it which of its internal addresses you want to read. So a read of the slave actually starts off by writing to it. This is the same as when you want to write to it:

You send the start sequence, the I2C address of the slave with the R/W bit low (even address) and the internal register number you want to write to. Now you send another start sequence (sometimes called a restart) and the I2C address again - this time with the read bit set. You then read as many data bytes as you wish and terminate the transaction with a stop sequence. So to read the compass bearing as a byte from the CMPS03 module:

The bit sequence will look like this:


Wait a Moment

That’s almost it for simple I2C communications, but there is one more complication. When the master is reading from the slave, its the slave that places the data on the SDA line, but its the master that controls the clock. What if the slave is not ready to send the data!

With devices such as EEPROMs this is not a problem, but when the slave device is actually a microprocessor with other things to do, it can be a problem. The microprocessor on the slave device will need to go to an interrupt routine, save its working registers, find out what address the master wants to read from, get the data and place it in its transmission register. This can take many uS to happen, meanwhile the master is blissfully sending out clock pulses on the SCL line that the slave cannot respond to.

The I2C protocol provides a solution to this:

The slave is allowed to hold the SCL line low! This is called clock stretching.

When the slave gets the read command from the master it holds the clock line low. The microprocessor then gets the requested data, places it in the transmission register and releases the clock line allowing the pull-up resistor to finally pull it high.

From the master’s point of view, it will issue the first clock pulse of the read by making SCL high and then check to see if it really has gone high. If its still low then its the slave that holding it low and the master should wait until it goes high before continuing. Luckily the hardware I2C ports on most microprocessors will handle this automatically.

Sometimes however, the master I2C is just a collection of subroutines and there are a few implementations out there that completely ignore clock stretching. They work with things like EEPROM’s but not with microprocessor slaves that use clock stretching. The result is that erroneous data is read from the slave. Beware!

Example Master Code using the PIC Processor

This example shows how to implement a software I2C master, including clock stretching. It is written in C for the PIC processor, but should be applicable to most processors with minor changes to the I/O pin definitions.

It is suitable for controlling all of our I2C based robot modules. Since the SCL and SDA lines are open drain type, we use the tristate control register to control the output, keeping the output register low. The port pins still need to be read though, so they’re defined as SCL_IN and SDA_IN. This definition and the initialization is probably all you’ll need to change for a different processor.

#define SCL     TRISB4  // I2C bus 
#define SDA     TRISB1  // 
#define SCL_IN  RB4     //
#define SDA_IN  RB1     //

To initialize the ports, set the output resistors to 0 and the tristate registers to 1 which disables the outputs and allows them to be pulled high by the resistors.

SDA = SCL = 1;
SCL_IN = SDA_IN = 0;

We use a small delay routine between SDA and SCL changes to give a clear sequence on the I2C bus. This is nothing more than a subroutine call and return.

void i2c_dly(void) {}

The following 4 functions provide the primitive start, stop, read and write sequences. All I2C transactions can be built up from these.

void i2c_start(void)
    SDA = 1; //I2C start bit sequence
    SCL = 1;
    SDA = 0;
    SCL = 0;
void i2c_stop(void)
    SDA = 0; // I2C stop bit sequence 
    SCL = 1; 
    SDA = 1; 
unsigned char i2c_rx(char ack)
    char x, d = 0;
    SDA = 1;  

    for (x = 0; x < 8; x++)
        d <<= 1;

        // wait for any SCL clock stretching 
        do { SCL = 1; }
        while (SCL_IN == 0);

        if (SDA_IN) d |= 1; 
        SCL = 0; 

    if (ack) SDA = 0; 
    else SDA = 1;
    SCL = 1;

    // send (N)ACK bit 
    SCL = 0;
    SDA = 1;

    return d; 
bit i2c_tx(unsigned char d)
    char x; 
    static bit b;

    for (x = 8; x; x--)
        if(d & 0x80) SDA = 1; 
        else SDA = 0;

        SCL = 1; 
        d <<= 1;
        SCL = 0;

    SDA = 1; 
    SCL = 1; 
    b = SDA_IN; // possible ACK bit SCL = 0;

    return b; 

The 4 primitive functions above can easily be put together to form complete I2C transactions. Here’s an example to start an SRF08 ranging in cm:

i2c_start();  // send start sequence 
i2c_tx(0xE0); // SRF08 I2C address with R/W bit clear
i2c_tx(0x00); // SRF08 command register address
i2c_tx(0x51); // command to start ranging in cm
i2c_stop();   // send stop sequence

Now after waiting 65mS for the ranging to complete (I’ve left that to you) the following example shows how to read the light sensor value from register 1 and the range result from registers 2 and 3.

i2c_start();  // send start sequence 
i2c_tx(0xE0); // SRF08 I2C address with R/W bit clear
i2c_tx(0x01); // SRF08 light sensor register address
i2c_start();  // send a restart sequence 
i2c_tx(0xE1); // SRF08 I2C address with R/W bit set

// get light sensor and send acknowledge
// internal register address will increment automatically
lightsensor = i2c_rx(1);

// get the high byte of the range and send acknowledge. 
rangehigh = i2c_rx(1);

// get low byte of the range 
// note we don't acknowledge the last byte
rangelow = i2c_rx(0);

i2c_stop(); // send stop sequence

External References

The definitive specs on the I2C bus can be found on the NXP website. It is currently here but if it has moved you’ll find it easily by searching for “I2C bus specification”. I also found these references useful when compiling this tutorial so if I missed something here, perhaps you will find what you need at one of these links.