This I2C tutorial shows you how the I2C protocol or more correctly written
I2C (sometimes written as IIC) stands for Inter IC Communication and
is intended for very short distance communication between ICs on a single PCB.
It gives you a fully defined protocol for data transfer between multiple
devices over two wires.
In this I2C tutorial you will learn all about the 2 wire
I2C serial protocol; How easy it is to use, how it works and when to use it.
The I2C protocol is used in a huge range of chips - just a few examples from this
site include the DS1307 (RTC), SSD1306 (OLED Display), MCP23017 (Serial
expander). The protocol allows you to connect many devices to a single set of two wires,
and then communicate individually with each device.
Warning: The protocol is designed for single board
communication it is not a long distance communication system. You can find instances
(horror stories) of people designing a multi drop inter office communication system
with I2C extenders - just don't do it - it ends in tears!
This I2C tutorial shows you how the I2C protocol works at the physical bit
level discussing single master mode (a single controlling device) which is the
most common use for I2C in a small system.
Note: You can find Master
mode soft I2C routines in the DS1307 RTC
manufacturers avoid paying royalties, or avoid patent problems, by
calling it a 2 wire protocol but it's the same I2C protocol (when you
examine the timing diagrams).
I2C is a serial protocol that can operate at different speeds 100kHz,
400kHz, and 3.4MHz. Not all chips support all speeds but 100kHz is
commonly supported. Speed is important as the data is transmitted serially,
so a faster clock allows a quicker update.
The great strength of the protocol is that it only requires two wires yet
can have many connected devices and all of these can transmit and receive data
at high speed. This saves a ton of pcb wiring
Robust ACK signalling
Unlike the SPI protocol the I2C protocol has an acknowledgement
feature that means a sending device knows that a receiver has accepted the
data. So I2C is more robust in a noisy environment.
Using I2C it is also possible to have multiple master devices makig programming the system more flexible. For SPI there
is also no concept of multiple master devices but SPI is faster.
I2C Tutorial - How I2C works
Open drain connections.
I2C works by using open drain connections. This simply refers to an
N-Channel MOSFET that has connections: Drain, Gate and Source. The top
connection is the Drain, the middle connection is the Gate (controller)
and the Lower connection is the Source.
When active (Gate voltage > Source voltage) then current flows
from drain to source. When inactive (Gate voltage < Source voltage)
then no current flows.
The open drain system simply means that multiple MOSFETS can be
connected together at the Drain terminal which is then connected to a
pull-up resistor. Now any of the MOSFETS can pull the voltage at the
drain to ground. Each device has a MOSFET used as the open drain
connection. For I2C you need two open drain connections (clock and
The two resistors above are the
pull-up resistors that allow the whole system to work - when all devices are
inactive then the "pullups" pull the signal wire to the supply voltage.
Warning: you must have one pullup resistor per signal wire
(SCL and SDA) and not more!
At any time a master device can start a transmission by pulling SDA low
while SCL is high (a unique specific condition that other I2C devices recognise
as the start of a master transmission).
The slave device listens to the next 8 serial bits of the address to see if
it matches its own address (each I2C must have a unique address built in). If
it does recognise the next 8 bits as an address from the master device. The
next bit tells the slave that it is to read or write data for the following
packet. This is followed by the ACK bit that the slave must generate to
indicate that it understood the address. The diagram further down shows this in
When the ACK signal is to be generated the master device releases the SDA
line and the open drain output is pulled high. This allows the slave device to
generate the ACK signal by pulling it low (only for that specific bit
Note: Another protocol that uses the open - drain concept is
the Dallas 1-wire protocol - but that is far slower. The advantage o the 1-wire
protocol is that it allows powering of the device through the signal wire! The
1-wire protocol also allows transmission over very large distances unlike the
I2C protocol. On this site this project (DS18B20 ) uses the 1-wire protocol.
Master and slave
The phillips I2C protocol defines the concept of master and
slave devices. A master device is simply the device that is in charge of the
bus at the present time and this device controls the clock and generates START
and STOP signals. Slaves simply listen to the bus and act on controls and data
that they are sent.
The master can send data to a slave or receive data from a
slave - slaves do not transfer data between themselves.
Multi master operation is a more complex use of I2C that lets
you have different controlling devices on the same bus. You only need to use
this mode if you have more than one microcontroller on the bus (and you want
either of them to be the bus master).
Multi master operation involves arbitration of the bus (where a
master has to fight to get control of the bus) and clock synchronisation (each
may a use a different clock e.g. because of separate crystal clocks for each
Note: Multi master is not covered in this I2C tutorial as
the more common use of I2C is to use a single bus master to control peripheral
devices e.g. serial memory, ADC, RTC etc.
Data and Clock
The I2C interface uses two bi-directional lines meaning that
any device could drive either line. In a single master system the master device
drives the clock most of the time - the master is in charge of the clock but
slaves can influence it to slow it down (See Slow Peripherals below).
The two wires must be driven as open collector/drain outputs and must be
pulled high using one resistor each (that is one resistor per I2C line i.e.for
data and clock) - this implements a 'wired NOR function' - any device pulling
the wire low causes all devices to see a low logic value - for high logic value
all devices must stop driving the wire.
Note : If you use I2C you can not put any other (non I2C) devices on the
bus as both lines are used as clock at some point (generation of START and STOP
bits toggles the data line). So you can not do something clever such as keeping
the clock line inactive and use the data line as a button press detector (to
You will often will find devices that you realise are I2C compatible but
they are labelled as using a '2 wire interface'. The manufacturer is avoiding
paying royalties by not using the words 'I2C'!
There are two wires (three if you include ground!, and four if you also
include power!) - but power and ground are taken as given i.e. they are
available on a PCB as needed so don't really count.
Standard clock speeds are 100kHz and 10kHz but the standard lets you use
clock speeds from zero to 100kHz and a fast mode is also available (400kHz -
Fast-mode). An even higher speed (3.4MHz - High-speed mode) for more demanding
applications - The mid range PIC won't be up this mode yet!
Note: The low-speed mode has been omitted (10kHz) as the standard now
specifies the basic system operating from 0 to 100kHz.
Note: Even if you run an I2C peripheral at a high speed the
overall data rate depends on how fast you can push data into the internal I2C
module and that depends on the processor speed.
A slow slave device may need to stop the bus while it gathers data or
services an interrupt etc. It can do this while holding the clock line (SCL)
low forcing the master into the wait state. The master must then wait until SCL
is released before proceeding.
Data transfer sequence
A basic Master to slave read or write sequence for I2C follows
the following order:
1. Send the START bit (S).
2. Send the slave address (ADDR). Usually 7 bits.
3. Send the Read(R)-1 / Write(W)-0 bit.
4. Wait for/Send an acknowledge bit (A).
5. Send/Receive the data byte (8 bits) (DATA).
6. Expect/Send acknowledge bit (A).
7. Send the STOP bit (P).
Note: You can use 7 bit or 10 bit addresses.
The sequence 5 and 6 can be repeated so that a multibyte block
can be read or written.
Data Transfer from master to slave
I2C Tutorial : Instruction sequence data from master to slave
A master device sends the sequence S ADDR W and then waits for an
acknowledge bit (A) from the slave which the slave will only generate if its
internal address matches the value sent by the master. If this happens then the
master sends DATA and waits for acknowledge (A) from the slave. The master
completes the byte transfer by generating a stop bit (P) (or repeated
Data transfer from slave to master
I2C Tutorial : Instruction sequence data from slave to master
A similar process happens when a master reads from the slave but in this
case, instead of W, R is sent. After the data is transmitted from the slave to
the master the master sends the acknowledge (A). If instead the master
does not want any more data it must send a not-acknowledge which indicates to
the slave that it should release the bus. This lets the master send the STOP or
repeated START signal.
Each device you use on the I2C bus must have a unique address.
For some devices e.g. serial memory you can set the lower address bits using
input pins on the device others have a fixed internal address setting e.g. a
real time clock DS1307. You can put several memory devices on the same IC bus
by using a different address for each.
Each device manufacturer is assigned a set of addresses so devices should
not conflict with each other.
Note: The maximum number of devices is limited by the number
of available addresses (and you need non-conflicting addresses) and by the
total bus capacitance (maximum 400pF).
The general call address is a reserved address which when
output by the bus master should address all devices which should respond with
an acknowledge.Its value is 0000000 (7 bits) and written by the master
0000000W. If a device does not need data from the general call it does not need
to respond to it.
0000 000 1 START byte - for slow
micros without I2C h/w
0000 001 X CBUS address - a different bus protocol
0000 010 X Reserved for different bus format
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
Most of these are not that useful for PIC microcontrollers
except perhaps the START byte and 10 bit addressing.
START (S) and STOP (P) bits
START (S) and STOP (P) bits are unique signals that can be
generated on the bus but only by a bus master.
Reception of a START bit by an I2C slave device resets its
internal bus logic. This can be done at any time so you can force a restart if
anything goes wrong even in the middle of communication.
START and STOP bits are defined as rising or falling edges on
the data line while the clock line is kept high.
I2C Tutorial : START and STOP Signal Definition
START condition (S)
SCL = 1,
SDA falling edge
STOP condition (P)
SCL = 1,
SDA rising edge
The following diagram shows the above information graphically -
these are the signals you would see on the I2C bus.
I2C Tutorial : START (S) and STOP (P) bits.
Note : In a single master system the only difference between
a slave and a master is the master's ability to generate START and STOP bits.
Both slave and master can control SDA and SCL.
Repeated START (Sr)
This seems like a confusing term at first as you ask yourself
why bother with it as it is functionally identical to the sequence :
S ADDR (R/W) DATA A P
The only difference is that for a repeated start you can repeat
the sequence starting from the stop bit (replacing the stop bit with another
S ADDR (R/W) DATA A Sr ADDR (R/W) DATA A P
and you can do this indefinitely.
Note: Reception of both S or Sr force any I2C device reset
its internal bus logic so sending S or Sr is really resetting all the bus
devices. This can be done at any time - it is a forced reset.
The main reason that the Sr bit exists is in a multi master
configuration where the current bus master does not want to release its
mastership. Using the repeated start keeps the bus busy so that no other master
can grab the bus.
Because of this when used in a Single master configuration it
is just a curiosity.
All data blocks are composed of 8 bits. The initial block has 7
address bits followed by a direction bit (Read or Write). Following blocks have
8 data bits. Acknowledge bits are squeezed in between each block.
Each data byte is transmitted MSB first including the address
To allow START and STOP bit generation by the master the data
line (SDA) must not be changed while the clock (SCL) is high - it can only be
changed when the clock is low.
The acknowledge bit (generated by the receiving device)
indicates to the transmitter that the the data transfer was ok. Note that the
clock pulse for the acknowledge bit is always created by the bus master.
The acknowledge data bit is generated by either the master or
slave depending on the data direction. For the master writing to a slave (W)
the acknowledge is generated by the slave. For the master receiving (R) data
from a slave the master generates the acknowledge bit.
ACK data master --> slave
In this case the slave generates the acknowledge signal.
When a not-acknowledge is received by the bus master the
transfer has failed and the master must generate a STOP or repeated START to
abort the sequence.
ACK data slave --> master
In this case the master generates the acknowledge signal.
Normally the master will generate an acknowledge after it has
received data but to indicate to the slave that no more data is required on the
last byte transfer the master must generate a 'not-acknowledge'. This indicates
to the slave that it should stop sending data. The master can then generate the
STOP bit (or repeated START).
The general call function is a specialised command that must be accepted by
all devices on the bus. It allows a master device to communicate to all devices
at the same time - giving them some data. Perhaps you would use this to command
a software reset in the case of a watchdog timeout in the processor.
I2C Tutorial : Specifics for the 16F88
To use the I2C mode in the 16F88 the SDA and SCL pins must be
initialised as inputs (TRIS bit = 1) so that an open drain effect is created.
By setting them as inputs they are not driving the wires and an external pull
up resistor will pull the signals high.
16F88 Slave mode
The 16F88 fully implements all slave functions except general
Full slave mode
The general call function does not really matter as it is quite
specialised commanding all devices on the bus to use some data.
A low output is generated by driving the signal line low and
changing the pin direction to an output. A high output is generated by changing
the pin direction to an input so that the external resistor pulls the signal
In slave mode this action is done for you by the SSP module
(the outputs of the register at SDA and SCL are driven low automatically -
regardless of the state of the register value).
16F88 Master mode
Basically there is very limited master mode functionality.
There are two elements that are provided:
There are two interrupts that activate on reception of either a
START or STOP condition. These two interrupts are only useful in a multi master
mode system where it is necessary for the non-master device to detect the start
and stop conditions. So for a single master system they are of no use at
16F88 Pin control
Note When the SSP module is active SDA and SCL output are
always set at zero regardless of the state of the register values. So all you
have to do is control the port direction.
In master mode (16F88) SDA and SCL must be controlled using
I2C Tutorial : Specifics for 16F877A
It does it all for you!
Full master mode.
Full slave mode.
Full general call.
Note: If you want a chip with full master and slave mode
operation look for the MSSP module in a PIC chip e.g. 16F877A - then you won't
need more software - just enough to drive the module.