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ADM1025A Datasheet(PDF) 10 Page - ON Semiconductor
ONSEMI [ON Semiconductor]
ADM1025A Datasheet(HTML) 10 Page - ON Semiconductor
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Preliminary Technical Data
Rev. P5 | Page 10 of 21| www.onsemi.com
If ADD is left open-circuit, the default address will be 0101110.
ADD is sampled only after power-up, so any changes made will
have no effect, unless power is cycled.
The facility to make hardwired changes to A1 and A0 allows the
user to avoid conflicts with other devices sharing the same
serial bus if, for example, more than one ADM1025/
ADM1025A is used in a system. However, as previously
mentioned, the ADD pin may also function as a reset output or
interrupt output. Use of these functions may restrict the
addresses that can be set. See the sections on RST and INT for
The serial bus protocol operates as follows.
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 an address/data stream will follow. All
slave peripherals connected to the serial bus respond to the
START condition and shift in the next eight bits, consisting
of a 7-bit address (MSB first) plus an R/W bit, which
determines the direction of the data transfer, i.e., whether
data will be written to or read from the slave device.
The peripheral whose address corresponds to the
transmitted address responds by pulling the data line low
during the low period before the ninth clock pulse, known
as the Acknowledge Bit. All other devices on the bus now
remain idle while the selected device waits for data to be
read from or written to it. If the R/W bit is a 0, the master
will write to the slave device. If the R/W bit is a 1, the
master will read from the slave device.
Data is sent over the serial bus in sequences of nine clock
pulses, eight bits of data followed by an Acknowledge Bit
from the slave device. Transitions on the data line must
occur during the low period of the clock signal and remain
stable during the high period, since a low-to-high
transition when the clock is high may be interpreted as a
STOP signal. The number of data bytes that can be
transmitted over the serial bus in a single READ or
WRITE operation is limited only by what the master and
slave devices can handle.
When all data bytes have been read or written, STOP
conditions are established. In WRITE mode, the master will
pull the data line high during the 10th clock pulse to assert a
STOP condition. In READ mode, the master device will
override the Acknowledge Bit by pulling the data line high
during the low period before the 9th clock pulse. This is
known as No Acknowledge. The master will then take the
data line low during the low period before the 10th clock
pulse, then high during the 10th clock pulse to assert a STOP
Any number of bytes of data may be transferred over the serial
bus in one operation, but it is not possible to mix read and write
in one operation because the type of operation is determined at
the beginning and cannot subsequently be changed without
starting a new operation.
In the case of the ADM1025/ADM1025A, write operations
contain either one or two bytes, and read operations contain
one byte and perform the following functions.
To write data to one of the device data registers or read data
from it, the Address Pointer Register must be set so that the
correct data register is addressed; data can then be written into
that register or read from it. The first byte of a write operation
always contains an address that is stored in the Address Pointer
Register. If data is to be written to the device, the write
operation contains a second data byte that is written to the
register selected by the Address Pointer Register.
This is illustrated in Figure 11. The device address is sent over
the bus followed by R/W set to 0. This is followed by two data
bytes. The first data byte is the address of the internal data
register to be written to, which is stored in the Address Pointer
Register. The second data byte is the data to be written to the
internal data register.
Figure 11. Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register
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