PIC12F1822-ISLASHMF datasheet(237/398 Pages) MICROCHIP

237 / 398 page

 2010 Microchip Technology Inc.

Preliminary

DS41413A-page 237

PIC12F/LF1822/16F/LF1823

24.3

I2C

MODE OVERVIEW

The Inter-Integrated Circuit Bus (I²C) 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 I2C bus specifies two signal connections:

• Serial Clock (SCL)

• Serial Data (SDA)

Figure 24-11 shows the block diagram of the MSSP1

module when operating in I2C 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 24-11 shows a typical connection between two

processors configured as master and slave devices.

The I2C 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 sin-

gle Read/Write bit, which determines whether the mas-

ter 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 24-11:

I2C 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 I2C 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

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