ADM1026
Serial Bus Interface
The peripheral whose address corresponds to the trans-
mitted address responds by pulling the data line low during
the low period before the ninth clock pulse, known as the
acknowledge bit, and holding it low during the high period
of this clock pulse. All other devices on the bus remain idle
while the selected device waits for data to be read from or
Control of the ADM1026 is carried out via the serial system
management bus (SMBus). The ADM1026 is connected to this
bus as a slave device, under the control of a master device.
The ADM1026 has a 7-bit serial bus slave address. When the
device is powered on, it does so with a default serial bus address.
The 5 MSBs of the address are set to 01011, and the 2 LSBs are
determined by the logical states of Pin 15 ADD/NTESTOUT.
This pin is a three-state input that can be grounded, connected
to VCC, or left open-circuit to give three different addresses.
written to it. If the R/ bit is 0, the master writes to the
W
slave device. If the R/ bit is 1, the master reads from the
W
slave device.
2. Data is sent over the serial bus in sequences of nine clock
pulses, 8 bits of data followed by an acknowledge bit from
the slave device. Data transitions on the data line must
occur during the low period of the clock signal and re-
main stable during the high period, because a low-to-high
transition when the clock is high may be interpreted as
a stop signal.
Table 5. Address Pin Truth Table
ADD Pin
A1
0
±
A0
0
0
GND
No Connect
VCC
0
±
If ADD is left open-circuit, the default address is 0101110
(5Ch). ADD is sampled only at power-up on the first valid
SMBus transaction, so any changes made while the power is on
(and the address is locked) have no effect.
If the operation is a write operation, the first data byte after
the slave address is a command byte. This tells the slave
device what to expect next. It may be an instruction telling
the slave device to expect a block write, or it may simply be
a register address that tells the slave where subsequent data
is to be written.
The facility to make hardwired changes to device addresses
allows the user to avoid conflicts with other devices sharing the
same serial bus, for example if more than one ADM1026 is used
in a system.
Because data can flow in only one direction as defined by
the R/ bit, it is not possible to send a command to a slave
W
General SMBus Timing
device during a read operation. Before doing a read oper-
ation, it may first be necessary to do a write operation to
tell the slave what type of read operation to expect and/or
the address from which data is to be read.
Figure 17 and Figure 18 show timing diagrams for general read
and write operations using the SMBus. The SMBus specification
defines specific conditions for different types of read and write
operations, which are discussed later in this section.
3. When all data bytes have been read or written, stop
conditions are established. In write mode, the master pulls
the data line high during the 10th clock pulse to assert a
stop condition. In read mode, the master
The general SMBus protocol1 operates as follows:
1. The master initiates data transfer by establishing a start
condition, defined as a high-to-low transition on the serial
data line (SDA) while the serial clock line SCL remains
high. This indicates that a data stream follows. All slave
peripherals connected to the serial bus respond to the start
condition and shift in the next 8 bits, consisting of a 7-bit
device releases the SDA line during the low period before
the ninth clock pulse, but the slave device does not pull
it low (called No Acknowledge). The master takes the data
line low during the low period before the 10th clock pulse,
then high during the 10th clock pulse to assert a stop
condition.
slave address (MSB first) and an R/ bit, which determine
W
the direction of the data transfer, that is, whether data is
written to or read from the slave device
(0 = write, 1 = read).
± If it is required to perform several read or write operations in succession, the
master can send a repeat start condition instead of a stop condition to begin
a new operation.
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