1997 Microchip Technology Inc.
DS30390E-page 89
PIC16C7X
11.4
I2C Overview
This section provides an overview of the Inter-Inte-
the operation of the SSP module in I2C mode.
The I2C bus is a two-wire serial interface developed by
the Philips Corporation. The original specication, or
standard mode, was for data transfers of up to 100
Kbps. The enhanced specication (fast mode) is also
supported. This device will communicate with both
standard and fast mode devices if attached to the same
bus. The clock will determine the data rate.
The I2C interface employs a comprehensive protocol to
ensure reliable transmission and reception of data.
When transmitting data, one device is the 鈥渕aster鈥�
which initiates transfer on the bus and generates the
clock signals to permit that transfer, while the other
device(s) acts as the 鈥渟lave.鈥� All portions of the slave
protocol are implemented in the SSP module鈥檚 hard-
ware, except general call support, while portions of the
master protocol need to be addressed in the
I2C bus terminology. For additional information on the
I2C interface specication, refer to the Philips docu-
ment 鈥�
The I2C bus and how to use it.鈥� #939839340011,
which can be obtained from the Philips Corporation.
In the I2C interface protocol each device has an
address. When a master wishes to initiate a data trans-
fer, it rst transmits the address of the device that it
wishes to 鈥渢alk鈥� to. All devices 鈥渓(f膩)isten鈥� to see if this is
their address. Within this address, a bit species if the
master wishes to read-from/write-to the slave device.
The master and slave are always in opposite modes
(transmitter/receiver) of operation during a data trans-
fer. That is they can be thought of as operating in either
of these two relations:
Master-transmitter and Slave-receiver
Slave-transmitter and Master-receiver
In both cases the master generates the clock signal.
The output stages of the clock (SCL) and data (SDA)
lines must have an open-drain or open-collector in
order to perform the wired-AND function of the bus.
External pull-up resistors are used to ensure a high
level when no device is pulling the line down. The num-
ber of devices that may be attached to the I2C bus is
limited only by the maximum bus loading specication
of 400 pF.
11.4.1
INITIATING AND TERMINATING DATA
TRANSFER
During times of no data transfer (idle time), both the
clock line (SCL) and the data line (SDA) are pulled high
through the external pull-up resistors. The START and
STOP conditions determine the start and stop of data
transmission. The START condition is dened as a high
to low transition of the SDA when the SCL is high. The
STOP condition is dened as a low to high transition of
START and STOP conditions. The master generates
these conditions for starting and terminating data trans-
fer. Due to the denition of the START and STOP con-
ditions, when data is being transmitted, the SDA line
can only change state when the SCL line is low.
FIGURE 11-14: START AND STOP
CONDITIONS
SDA
SCL
S
P
Start
Condition
Change
of Data
Allowed
Change
of Data
Allowed
Stop
Condition
TABLE 11-3:
I2C BUS TERMINOLOGY
Term
Description
Transmitter
The device that sends the data to the bus.
Receiver
The device that receives the data from the bus.
Master
The device which initiates the transfer, generates the clock and terminates the transfer.
Slave
The device addressed by a master.
Multi-master
More than one master device in a system. These masters can attempt to control the bus at the
same time without corrupting the message.
Arbitration
Procedure that ensures that only one of the master devices will control the bus. This ensure that
the transfer data does not get corrupted.
Synchronization
Procedure where the clock signals of two or more devices are synchronized.
Applicable Devices
72 73 73A 74 74A 76 77