2006 Microchip Technology Inc.
DS21732C-page 15
MCP3221
5.0
SERIAL COMMUNICATIONS
5.1
I2C Bus Characteristics
The following bus protocol has been defined:
Data transfer may be initiated only when the bus
is not busy.
During data transfer, the data line must remain
stable whenever the clock line is high. Changes in
the data line while the clock line is high will be
interpreted as a START or STOP condition.
Accordingly, the following bus conditions have been
5.1.1
BUS NOT BUSY (A)
Both data and clock lines remain high.
5.1.2
START DATA TRANSFER (B)
A high-to-low transition of the SDA line while the clock
(SCL) is high determines a START condition. All
commands must be preceded by a START condition.
5.1.3
STOP DATA TRANSFER (C)
A low-to-high transition of the SDA line while the clock
(SCL) is high determines a STOP condition. All
operations must be ended with a STOP condition.
5.1.4
DATA VALID (D)
The state of the data line represents valid data when,
after a START condition, the data line is stable for the
duration of the clock signal’s high period.
The data on the line must be changed during the low
period of the clock signal. There is one clock pulse per
bit of data.
Each data transfer is initiated with a START condition
and terminated with a STOP condition. The number of
data bytes transferred between the START and STOP
conditions is determined by the master device and is
unlimited.
5.1.5
ACKNOWLEDGE
Each receiving device, when addressed, is obliged to
generate an acknowledge bit after the reception of
each byte. The master device must generate an extra
clock pulse which is associated with this acknowledge
bit.
The device that acknowledges has to pull down the
SDA line during the acknowledge clock pulse in such a
way that the SDA line is stable-low during the high
period of the acknowledge-related clock pulse. Setup
and hold times must be taken into account. During
reads, a master device must signal an end of data to
the slave by not generating an acknowledge bit on the
last byte that has been clocked out of the slave (NAK).
In this case, the slave (MCP3221) will release the bus
to allow the master device to generate the STOP
condition.
The MCP3221 supports a bidirectional, 2-wire bus and
data transmission protocol. The device that sends data
onto the bus is the transmitter and the device receiving
data is the receiver. The bus has to be controlled by a
master device which generates the serial clock (SCL),
controls the bus access and generates the START and
STOP conditions, while the MCP3221 works as a slave
device. Both master and slave devices can operate as
either transmitter or receiver, but the master device
determines which mode is activated.
FIGURE 5-1:
Data Transfer Sequence on the Serial Bus.
SCL
SDA
(A)
(B)
(D)
(A)
(C)
START
CONDITION
ADDRESS OR
ACKNOWLEDGE
VALID
DATA
ALLOWED
TO CHANGE
STOP
CONDITION