LTC4258
21
4258fb
APPLICATIO S I FOR ATIO
WUUU
SERIAL DIGITAL INTERFACE
The LTC4258 communicates with a host (master) using
the standard 2-wire interface as described in the SMBus
Specification Version 2.0 (available at http://smbus.org).
The SMBus is an extension of the I
2
C bus, and the
LTC4258 is also compatible with the I
2
C bus standard. The
Timing Diagrams (Figures 5 through 9) show the timing
relationship of the signals on the bus. The two bus lines,
SDA and SCL, must be high when the bus is not in use.
External pull-up resistors or current sources, such as the
LTC1694 SMBus accelerator, are required on these lines.
If the SDA and SCL pull-ups are absent, not connected to
the same positive supply as the LTC4258’s V
DD
pin, or are
not activated when the power is applied to the LTC4258, it
is possible for the LTC4258 to see a START condition on
the I
2
C bus. The interrupt pin (INT) is only updated
between I
2
C transactions. Therefore if the LTC4258 sees
a START condition when it powers up because the SCL and
SDA lines were left floating, it will not assert an interrupt
(pull INT low) until it sees a STOP condition on the bus. In
a typical application the I
2
C bus will immediately have
traffic and the LTC4258 will see a STOP so soon after
power up that this momentary condition will go unnoticed.
Isolating the Serial Digital Interface
IEEE 802.3af requires that network segments be electri-
cally isolated from the chassis ground of each network
interface device. However, the network segments are not
required to be isolated from each other provided that the
segments are connected to devices residing within a
single building on a single power distribution system.
For simple devices such as small powered Ethernet
switches, the requirement can be met by using an iso-
lated power supply to power the entire device. This
implementation can only be used if the device has no
electrically conducting ports other than twisted-pair
Ethernet. In this case, the SDAIN and SDAOUT pins of the
LTC4258 can be connected together to act as a standard
I
2
C/SMBus SDA pin.
If the device is part of a larger system, contains serial
ports, or must be referenced to protective ground for
some other reason, the Power over Ethernet subsystem
including the LTC4258s must be electrically isolated
from the rest of the system. The LTC4258 includes
separate pins (SDAIN and SDAOUT) for the input and
output functions of the bidirectional data line. This eases
the use of optocouplers to isolate the data path between
the LTC4258s and the system controller. Figure 17
shows one possible implementation of an isolated inter-
face. The SDAOUT pin of the LTC4258 is designed to
drive the inputs of an optocoupler directly, but a standard
I
2
C device typically cannot. U1 is used to buffer I
2
C
signals into the optocouplers from the system controller
side. Schmitt triggers must be used to prevent extra
edges on transitions of SDA and SCL.
Bus Addresses and Protocols
The LTC4258 is a read-write slave device. The master can
communicate with the LTC4258 using the Write Byte,
Read Byte and Receive Byte protocols. The LTC4258’s
primary serial bus address is (010A
3
A
2
A
1
A
0
)b, as desig-
nated by pins AD3-AD0. All LTC4258s also respond to the
address (0110000)b, allowing the host to write the same
command into all of the LTC4258s on a bus in a single
transaction. If the LTC4258 is asserting (pulling low) the
INT pin, it will also acknowledge the Alert Response
Address (0001100)b using the receive byte protocol.
The START and STOP Conditions
When the bus is idle, both SCL and SDA must be high. A
bus master (typically the host controller) signals the
beginning of communication with a slave device (like the
LTC4258) by transmitting a START condition. A START
condition is generated by transitioning SDA from high to
low while SCL is high. A REPEATED START condition is
functionally the same as a START condition, but used to
extend the protocol for a change in data transmission
direction. A STOP condition is not used to set up a
REPEATED START condition, for this would clear any data
already latched in. When the master has finished commu-
nicating with the slave, it issues a STOP condition. A STOP
condition is generated by transitioning SDA from low to
high while SCL is high. The bus is then free for communi-
cation with another SMBus or I
2
C device.
