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2011 Microchip Technology Inc.

DS41391D-page 243

PIC16(L)F1826/27

When one device is transmitting a logical one, or letting

the line float, and a second device is transmitting a log-

ical zero, or holding the line low, the first device can

detect that the line is not a logical one. This detection,

when used on the SCLx line, is called clock stretching.

Clock stretching gives slave devices a mechanism to

control the flow of data. When this detection is used on

the SDAx line, it is called arbitration. Arbitration

ensures that there is only one master device communi-

cating at any single time.

25.3.1

CLOCK STRETCHING

When a slave device has not completed processing

data, it can delay the transfer of more data through the

process of Clock Stretching. An addressed slave

device may hold the SCLx clock line low after receiving

or sending a bit, indicating that it is not yet ready to con-

tinue. The master that is communicating with the slave

will attempt to raise the SCLx line in order to transfer

the next bit, but will detect that the clock line has not yet

been released. Because the SCLx connection is

open-drain, the slave has the ability to hold that line low

until it is ready to continue communicating.

Clock stretching allows receivers that cannot keep up

with a transmitter to control the flow of incoming data.

25.3.2

ARBITRATION

Each master device must monitor the bus for Start and

Stop bits. If the device detects that the bus is busy, it

cannot begin a new message until the bus returns to an

Idle state.

However, two master devices may try to initiate a trans-

mission on or about the same time. When this occurs,

the process of arbitration begins. Each transmitter

checks the level of the SDAx data line and compares it

to the level that it expects to find. The first transmitter to

observe that the two levels don't match, loses arbitra-

tion, and must stop transmitting on the SDAx line.

For example, if one transmitter holds the SDAx line to

a logical one (lets it float) and a second transmitter

holds it to a logical zero (pulls it low), the result is that

the SDAx line will be low. The first transmitter then

observes that the level of the line is different than

expected and concludes that another transmitter is

communicating.

The first transmitter to notice this difference is the one

that loses arbitration and must stop driving the SDAx

line. If this transmitter is also a master device, it also

must stop driving the SCLx line. It then can monitor the

lines for a Stop condition before trying to reissue its

transmission. In the meantime, the other device that

has not noticed any difference between the expected

and actual levels on the SDAx line continues with it's

original transmission. It can do so without any compli-

cations, because so far, the transmission appears

exactly as expected with no other transmitter disturbing

the message.

Slave Transmit mode can also be arbitrated, when a

master addresses multiple slaves, but this is less com-

mon.

If two master devices are sending a message to two dif-

ferent slave devices at the address stage, the master

sending the lower slave address always wins arbitra-

tion. When two master devices send messages to the

same slave address, and addresses can sometimes

refer to multiple slaves, the arbitration process must

continue into the data stage.

Arbitration usually occurs very rarely, but it is a neces-

sary process for proper multi-master support.

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