Datasheet

LTC4155
27
4155fc
as explained in the Alternate Default Input Current Limit
section of Operation. Thus, the contents of these registers
may be different from the last value written by the bus
master, and reading back the contents may be useful to
determine the state of the system.
When the contents of the sub address pointer register
point to a read-only status register, the data returned is a
snapshot of the state of the LTC4155 at a particular instant
in time. If no interrupt requests are pending, the status
data is sampled when the LTC4155 acknowledges its read
address, just before the LTC4155 begins data transmis-
sion during a bus read operation. When an unmasked
interrupt event takes place, the IRQ pin is driven low and
data is latched in the three read-only status registers at
that moment. Any subsequent read operation from any
status registers will return this frozen data to facilitate
determination of the cause of the interrupt request. After
the bus master clears the LTC4155 interrupt request, the
status latches are cleared. Bus read operations will then
again return either a snapshot of the data at the read ad-
dress acknowledge, or at the time of the next interrupt
assertion, whichever comes first.
SMBus Protocol Compatibility
The SMBus specification is generally compatible with the
I
2
C bus specification, but extends beyond I
2
C to define and
standardize specific protocol formats for various types of
transactions. The LTC4155 I
2
C interface is fully compatible
with four of the protocols defined by the SMBus speci-
fication. All control and status features of the LTC4155
can be accessed using the SMBus protocols, although if
high bus utilization is a concern, certain operations can
be accomplished more efficiently by I
2
C bus operations
that do not adhere to any of the SMBus defined protocols.
SMBus Write Byte Protocol
1711 8 1811
S SLAVE
ADDRESS
WR A COMMAND
CODE
A DATA BYTE A P
The SMBus write byte protocol can be used to modify the
contents of any single control register in the LTC4155. The
transaction is initiated by the bus master with a START
condition. The SMBus slave address corresponds to the
LTC4155 write address, which is 0x09 when interpreted
as a 7-bit word (0b 000 1001), followed by WR (value
0b0). The LTC4155 will acknowledge its write address. The
SMBus command code corresponds to the sub address
pointer value and will be written to the sub address pointer
register in the LTC4155. Only the register locations with
write access (0x00 to 0x02, 0x06 to 0x07) are meaningful
values for the sub address pointer when using this protocol.
The LTC4155 will acknowledge the SMBus command code
byte. The SMBus data byte corresponds to the command
data to be written to the location pointed to by the sub
address pointer register. The LTC4155 will acknowledge
the SMBus data byte. The STOP condition at the end of the
sequence will force an update to the command registers,
causing the new command data to take immediate effect.
SMBus Read Byte Protocol
1711 8 11 7 11811
S SLAVE
ADDRESS
WR A COMMAND
CODE
A Sr SLAVE
ADDRESS
RD A DATA
BYTE
AP
The SMBus read byte protocol can be used to read the
contents of any one of the seven control or status regis-
ters with one bus transaction. The transaction is initiated
by the bus master with a START condition. The SMBus
slave address corresponds to the LTC4155 write address,
which is 0x09 when interpreted as a 7-bit word (0b 000
1001), followed by WR (value 0b0). The LTC4155 will
acknowledge its write address. The SMBus command
code corresponds to the sub address pointer value and
will be written to the sub address pointer register in the
LTC4155. The LTC4155 will acknowledge the SMBus
command code byte. The master then issues a repeat
START condition, followed by the LTC4155 slave address
(0x09) and RD (0b1). The LTC4155 will acknowledge
its read address. At this time the bus master becomes
a receiver while continuing to clock SCL. The LTC4155
becomes a slave transmitter and controls SDA to place
data on the bus. Following the single data byte, the bus
master has the option of transmitting either an ACK or
a NACK bit. According to the I
2
C specification, a master
must transmit a NACK at the end of a read transaction to
instruct the slave to terminate data transmission. Because
the LTC4155 terminates data transmission after one byte
in all cases, whether the bus master transmits an ACK or
OPERATION