Datasheet

PIC12(L)F1822/16(L)F1823
DS40001413E-page 228  2010-2015 Microchip Technology Inc.
25.3 I
2
C MODE OVERVIEW
The Inter-Integrated Circuit Bus (I
2
C) is a multi-master
serial data communication bus. Devices communicate
in a master/slave environment where the master
devices initiate the communication. A Slave device is
controlled through addressing.
The I
2
C bus specifies two signal connections:
• Serial Clock (SCL)
• Serial Data (SDA)
Figure 25-11 shows the block diagram of the MSSP1
module when operating in I
2
C mode.
Both the SCL and SDA connections are bidirectional
open-drain lines, each requiring pull-up resistors for the
supply voltage. Pulling the line to ground is considered
a logical zero and letting the line float is considered a
logical one.
Figure 25-11 shows a typical connection between two
processors configured as master and slave devices.
The I
2
C bus can operate with one or more master
devices and one or more slave devices.
There are four potential modes of operation for a given
device:
• Master Transmit mode
(master is transmitting data to a slave)
• Master Receive mode
(master is receiving data from a slave)
•Slave Transmit mode
(slave is transmitting data to a master)
• Slave Receive mode
(slave is receiving data from the master)
To begin communication, a master device starts out in
Master Transmit mode. The master device sends out a
Start bit followed by the address byte of the slave it
intends to communicate with. This is followed by a
single Read/Write bit, which determines whether the
master intends to transmit to or receive data from the
slave device.
If the requested slave exists on the bus, it will respond
with an Acknowledge bit, otherwise known as an ACK
.
The master then continues in either Transmit mode or
Receive mode and the slave continues in the comple-
ment, either in Receive mode or Transmit mode,
respectively.
A Start bit is indicated by a high-to-low transition of the
SDA line while the SCL line is held high. Address and
data bytes are sent out, Most Significant bit (MSb) first.
The Read/Write bit is sent out as a logical one when the
master intends to read data from the slave, and is sent
out as a logical zero when it intends to write data to the
slave.
FIGURE 25-11: I
2
C MASTER/
SLAVE CONNECTION
The Acknowledge bit (ACK) is an active-low signal,
which holds the SDA line low to indicate to the transmit-
ter that the slave device has received the transmitted
data and is ready to receive more.
The transition of a data bit is always performed while
the SCL line is held low. Transitions that occur while the
SCL line is held high are used to indicate Start and Stop
bits.
If the master intends to write to the slave, then it repeat-
edly sends out a byte of data, with the slave responding
after each byte with an ACK
bit. In this example, the
master device is in Master Transmit mode and the
slave is in Slave Receive mode.
If the master intends to read from the slave, then it
repeatedly receives a byte of data from the slave, and
responds after each byte with an ACK
bit. In this exam-
ple, the master device is in Master Receive mode and
the slave is Slave Transmit mode.
On the last byte of data communicated, the master
device may end the transmission by sending a Stop bit.
If the master device is in Receive mode, it sends the
Stop bit in place of the last ACK
bit. A Stop bit is indi-
cated by a low-to-high transition of the SDA line while
the SCL line is held high.
In some cases, the master may want to maintain con-
trol of the bus and re-initiate another transmission. If
so, the master device may send another Start bit in
place of the Stop bit or last ACK
bit when it is in receive
mode.
The I
2
C bus specifies three message protocols;
• Single message where a master writes data to a
slave.
• Single message where a master reads data from
a slave.
• Combined message where a master initiates a
minimum of two writes, or two reads, or a
combination of writes and reads, to one or more
slaves.
Master
SCL
SDA
SCL
SDA
Slave
VDD
VDD