Datasheet

MCP19111
DS22331A-page 148  2013 Microchip Technology Inc.
27.2.1 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 SCL clock line low after receiving
or sending a bit, indicating that it is not yet ready to
continue. The master that is communicating with the
slave will attempt to raise the SCL line in order to
transfer the next bit, but will detect that the clock line
has not yet been released. Because the SCL
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.
27.2.2 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
transmission on or about the same time. When this
occurs, the process of arbitration begins. Each
transmitter checks the level of the SDA 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 arbitration, and must stop transmitting on the
SDA line.
For example, if one transmitter holds the SDA 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
SDA 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 SDA
line. If this transmitter is also a master device, it also
must stop driving the SCL 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 SDA line continues with it's
original transmission. It can do so without any
complications, 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
common.
If two master devices are sending a message to two
different slave devices at the address stage, the master
sending the lower slave address always wins
arbitration. 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.
27.3 I
2
C MODE OPERATION
All MSSP I
2
C communication is byte oriented and
shifted out MSb first. Six SFR registers and two
interrupt flags interface the module with the PIC
microcontroller and user software. Two pins, SDA and
SCL, are exercised by the module to communicate
with other external I
2
C devices.
27.3.1 BYTE FORMAT
All communication in I
2
C is done in 9-bit segments. A
byte is sent from a Master to a Slave or vice-versa, fol-
lowed by an Acknowledge bit sent back. After the 8
th
falling edge of the SCL line, the device outputting data
on the SDA changes that pin to an input and reads in
an acknowledge value on the next clock pulse.
The clock signal, SCL, is provided by the master. Data
is valid to change while the SCL signal is low, and
sampled on the rising edge of the clock. Changes on
the SDA line while the SCL line is high define special
conditions on the bus, explained below.
27.3.2 DEFINITION OF I
2
C TERMINOLOGY
There is language and terminology in the description
of I
2
C communication that have definitions specific to
I
2
C. That word usage is defined below and may be
used in the rest of this document without explanation.
This table was adapted from the Philips I
2
C
specification.
27.3.3 SDA AND SCL PINS
On the MCP19111, the SCL and SDA pins are always
open-drain. These pins should be set by the user to
inputs by setting the appropriate TRIS bits.
Note: Data is tied to output zero when an I
2
C
mode is enabled.