Datasheet

INA3221
www.ti.com SBOS576A –MAY 2012–REVISED JUNE 2013
BUS OVERVIEW
The INA3221 offers compatibility with both I
2
C and SMBus interfaces. The I
2
C and SMBus protocols are
essentially compatible with one another.
The I
2
C interface is used throughout this data sheet as the primary example, with the SMBus protocol specified
only when a difference between the two systems is discussed. Two bidirectional lines, the serial clock (SCL) and
data signal line (SDA), connect the INA3221 to the bus. Both SCL and SDA are open-drain connections.
The device that initiates a data transfer is called a master, and the devices controlled by the master are slaves.
The bus must be controlled by the master device that generates the SCL, controls the bus access, and
generates start and stop conditions.
To address a specific device, the master initiates a start condition by pulling SDA from a high to a low logic level
while SCL is high. All slaves on the bus shift in the slave address byte on the SCL rising edge, with the last bit
indicating whether a read or write operation is intended. During the ninth clock pulse, the slave being addressed
responds to the master by generating an Acknowledge bit and pulling SDA low.
Data transfer is then initiated and eight bits of data are sent, followed by an Acknowledge bit. During data
transfer, SDA must remain stable while SCL is high. Any change in SDA while SCL is high is interpreted as a
start or stop condition.
Once all data are transferred, the master generates a stop condition, indicated by pulling SDA from low to high
while SCL is high. The INA3221 includes a 28-ms timeout on the interface to prevent locking up the bus.
Serial Bus Address
To communicate with the INA3221, the master must first address slave devices with a slave address byte. This
byte consists of seven address bits and a direction bit to indicate whether the intended action is a read or write
operation.
The INA3221 has one address pin, A0. Table 7 describes the pin logic levels for each of the four possible
addresses. The state of the A0 pin is sampled on every bus communication and should be set before any activity
on the interface occurs.
Table 7. INA3221 Address Pins and Slave Addresses
A0 SLAVE ADDRESS
GND 1000000
V
S+
1000001
SDA 1000010
SCL 1000011
Serial Interface
The INA3221 only operates as a slave device on the I
2
C bus and SMBus. Bus connections are made via the
open-drain I/O lines, SDA and SCL. The SDA and SCL pins feature integrated spike-suppression filters and
Schmitt triggers to minimize the effects of input spikes and bus noise. While there is spike suppression integrated
into the digital I/O lines, proper layout should be used to minimize the amount of coupling into the communication
lines. This noise introduction could occur from capacitively coupling signal edges between the two
communication lines themselves or from other switching noise sources present in the system. Routing traces in
parallel with ground between layers on a printed circuit board (PCB) typically reduces the effects of coupling
between the communication lines. Shielding communication lines in general is recommended to reduce the
possibility of unintended noise coupling into the digital I/O lines that could be incorrectly interpreted as start or
stop commands.
The INA3221 supports a transmission protocol for Fast (1 kHz to 400 kHz) and High-speed (1 kHz to 3.4 MHz)
modes. All data bytes are transmitted MSB first.
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