AD7747(RevPrC) Просмотр технического описания (PDF) - Analog Devices
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AD7747 Datasheet PDF : 20 Pages
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AD7747
Preliminary Technical Data
SERIAL INTERFACE
The AD7747 supports an I2C-compatible 2-wire serial interface.
The two wires on the I2C bus are called SCL (clock) and SDA
(data). These two wires carry all addressing, control, and data
information one bit at a time over the bus to all connected
peripheral devices. The SDA wire carries the data, while the
SCL wire synchronizes the sender and receiver during the data
transfer. I2C devices are classified as either master or slave
devices. A device that initiates a data transfer message is called a
master, while a device that responds to this message is called a
slave.
To control the AD7747 device on the bus, the following
protocol must be followed. First, the master initiates a data
transfer by establishing a start condition, defined by a high-to-
low transition on SDA while SCL remains high. This indicates
that the start byte follows. This 8-bit start byte is made up of a
7-bit address plus an R/W bit indicator.
All peripherals connected to the bus respond to the start
condition and shift in the next 8 bits (7-bit address + R/W bit).
The bits arrive MSB first. The peripheral that recognizes the
transmitted address responds by pulling the data line low
during the ninth clock pulse. This is known as the acknowledge
bit. All other devices withdraw from the bus at this point and
maintain an idle condition. An exception to this is the general
call address, which is described later in this document. The idle
condition is where the device monitors the SDA and SCL lines
waiting for the start condition and the correct address byte. The
R/W bit determines the direction of the data transfer. A Logic 0
LSB in the start byte means that the master writes information
to the addressed peripheral. In this case the AD7747 becomes a
slave receiver. A Logic 1 LSB in the start byte means that the
master reads information from the addressed peri-pheral. In
this case, the AD7747 becomes a slave transmitter. In all
instances, the AD7747 acts as a standard slave device on the I2C
bus.
The start byte address for the AD7747 is 0x90 for a write and
0x91 for a read.
READ OPERATION
When a read is selected in the start byte, the register that is
currently addressed by the address pointer is transmitted on to
the SDA line by the AD7747. This is then clocked out by the
master device and the AD7747 awaits an acknowledge from the
master.
If an acknowledge is received from the master, the address auto-
incrementer automatically increments the address pointer
register and outputs the next addressed register content on to
the SDA line for transmission to the master. If no acknowledge
is received, the AD7747 returns to the idle state and the address
pointer is not incremented.
The address pointers’ auto-incrementer allow block data to be
written or read from the starting address and subsequent
incremental addresses.
In continuous conversion mode, the address pointers’ auto-
incrementer should be used for reading a conversion result.
That means, the three data bytes should be read using one
multibyte read transaction rather than three separate single byte
transactions. The single byte data read transaction may result in
the data bytes from two different results being mixed. The same
applies for six data bytes if both the capacitive and the
voltage/temperature channel are enabled.
The user can also access any unique register (address) on a one-
to-one basis without having to update all the registers. The
address pointer register contents cannot be read.
If an incorrect address pointer location is accessed or, if the user
allows the auto-incrementer to exceed the required register
address, the following applies:
• In read mode, the AD7747 continues to output various
internal register contents until the master device issues a
no acknowledge, start, or stop condition. The address
pointers’ auto-incrementer’s contents are reset to point to
the status register at Address 0x00 when a stop condition is
received at the end of a read operation. This allows the
status register to be read (polled) continually without
having to constantly write to the address pointer.
• In write mode, the data for the invalid address is not
loaded into the AD7747 registers but an acknowledge is
issued by the AD7747.
WRITE OPERATION
When a write is selected, the byte following the start byte is
always the register address pointer (subaddress) byte, which
points to one of the internal registers on the AD7747. The
address pointer byte is automatically loaded into the address
pointer register and acknowledged by the AD7747. After the
address pointer byte acknowledge, a stop condition, a repeated
start condition, or another data byte can follow from the master.
A stop condition is defined by a low-to-high transition on SDA
while SCL remains high. If a stop condition is ever encountered
by the AD7747, it returns to its idle condition and the address
pointer is reset to Address 0x00.
If a data byte is transmitted after the register address pointer
byte, the AD7747 loads this byte into the register that is
currently addressed by the address pointer register, send an
acknowledge, and the address pointer auto-incrementer auto-
matically increments the address pointer register to the next
Rev. PrC | Page 10 of 20
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