ADM1033
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10
SMBus 2.0 Fixed and Discoverable Mode
The ADM1033 also supports fixed and discoverable mode,
which is backwards compatible with SMBus 1.0 and 1.1.
Fixed and discoverable mode supports all the same
functionality as ARP-capable mode, except for assign
address in which case it powers up with a fixed address and
is not changed by the assign address call. The fixed address
is determined by the state of the LOCATION pin on powerup.
SMBus 2.0 Read and Write Operations
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 is to follow.
All slave peripherals connected to the serial bus respond to
the start condition and shift in the next 8 bits, which consist
of a 7-bit address (MSB first) plus an R/W
bit. This last bit
determines the direction of the data transfer (whether data is
written to or read from the slave device).
1. The peripheral that corresponds to the transmitted
address responds by pulling the data line low
during the low period before the 9th clock pulse,
which is known as the acknowledge bit. All other
devices on the bus 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 it.
2. Data is sent over the serial bus in sequences of 9
clock pulses − 8 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, because 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.
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 device overrides 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
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.
It is not possible to mix read and write in one operation,
because the type of operation is determined at the beginning
and cannot be changed without starting a new operation.
To write data to one of the device data registers or to read
data from it, the address pointer register (APR) must be set
so that the correct data register is addressed; then data can 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 APR. If data is to be written to the device, then the write
operation contains a second data byte, which is written to the
register selected by the APR.
As illustrated in Figure 17, 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 APR. 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.
If the ADM1033’s APR value is unknown or incorrect, it
must be set to the correct value before data can be read from
the desired data register. To do this, perform a write to the
ADM1033 as before, but send only the data byte containing
the register (See Figure 18.) A read operation is then
performed, using the serial bus address and the R/W
bit set
to 1, followed by the data byte read from the data register.
(See Figure 19.)
However, if the APR is already at the desired address, data
can be read from the corresponding data register without first
writing to the APR. In this case, see Figure 18 can be omitted.
In Figure 17 to Figure 19, the serial bus address is
determined by the state of the LOCATION pin on powerup.
Figure 17. Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register
R/W
SCL
SDA
A2
A1
A0
D7
D6
D5
D4
D3
D2
D1
D0
ACK. BY
ADM1033
START BY
MASTER
191
ACK. BY
ADM1033
9
D7
D6
D5
D4
D3
D2
D1
D0
ACK. BY
ADM1033
STOP BY
MASTER
1
9
SCL (CONTINUED)
SDA (CONTINUED)
FRAME 1
SERIAL BUS ADDRESS BYTE
FRAME 2
ADDRESS POINTER REGISTER BYTE
FRAME 3
DATA BYTE
A3A4A5A6
