ADM1032
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direction of the data transfer, that is, whether data
is 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 writes
to the slave device. If the R/W
bit is a 1, the
master reads from the slave device.
2. 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 can 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.
3. When all data bytes are 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 device overrides the
acknowledge bit by pulling the data line high
during the low period before the ninth clock pulse.
This is known as no acknowledge. The master then
takes the data line low during the low period
before the 10th clock pulse, and high during the
10th clock pulse to assert a STOP condition.
Any number of bytes of data can 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 ADM1032, write operations contain
either one or two bytes, while 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 first be set so
that the correct data register is addressed. The first byte of
a write operation always contains a valid address that is
stored in the address pointer register. If data is 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 13. 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.
When reading data from a register, there are two
possibilities:
1. If the address pointer register value is unknown or
not the desired value, it is first necessary to set it
to the correct value before data can be read from
the desired data register. This is done by
performing a write to the ADM1032 as before, but
only the data byte containing the register read
address is sent because data is not to be written to
the register. This is shown in Figure 14.
A read operation is then performed consisting of
the serial bus address, R/W
bit set to 1, followed
by the data byte read from the data register. This is
shown in Figure 15.
2. If the address pointer register is known to be at the
desired address already, data can be read from the
corresponding data register without first writing to
the address pointer register and Figure 14 can be
omitted.
NOTES:
1. Although it is possible to read a data byte from a data
register without first writing to the address pointer register,
if the address pointer register is already at the correct value,
it is not possible to write data to a register without writing to
the address pointer register. The first data byte of a write is
always written to the address pointer register.
2. Don’t forget that some of the ADM1032 registers have
different addresses for read and write operations. The write
address of a register must be written to the address pointer
if data is to be written to that register, but it is not possible
to read data from that address. The read address of a
register must be written to the address pointer before data
can be read from that register.
