
AD9882A
Rev. 0 | Page 33 of 40
change only when SCL is low. If SDA changes state while SCL is
high, the serial interface interprets that action as a start or stop
sequence.
The five components to serial bus operation are
Start signal
Slave address byte
Base register address byte
Data byte to read or write
Stop signal
When the serial interface is inactive (SCL and SDA are high),
communications are initiated by sending a start signal. The start
signal is a high-to-low transition on SDA while SCL is high.
This signal alerts all slaved devices that a data transfer sequence
is coming.
The first eight bits of data transferred after a start signal com-
prise a 7-bit slave address (the first seven bits) and a single R/W
bit (the eighth bit). The R/W bit indicates the direction of data
transfer: read from (1) or write to (0) the slave device. If the
transmitted slave address matches the address of the device
(set by the state of the SA input pin listed in
Table 52), the
AD9882A acknowledges by bringing SDA low on the ninth
SCL pulse. If the addresses do not match, the AD9882A does
not acknowledge.
Table 52. Serial Port Addresses
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
A6
(MSB)
A5
A4
A3
A2
A1
A0
(LSB)
1
0
1
0
1
0
1
0
1
Data Transfer via Serial Interface
For each byte of data read or written, the MSB is the first bit of
the sequence. If the AD9882A does not acknowledge the master
device during a write sequence, the SDA remains high so the
master can gen-erate a stop signal. If the master device does not
acknowledge the AD9882A during a read sequence, the
AD9882A interprets this as end of data. The SDA remains
high so the master can generate a stop signal.
Writing data to specific control registers of the AD9882A
requires that the 8-bit address of the control register of interest
be written after the slave address has been established. This
control register address is the base address for subsequent write
operations. The base address autoincrements by one for each
byte of data written after the data byte intended for the base
address. If there are more bytes transferred than there are avail-
able addresses, the address does not increment and remains at
its maximum value of 0x1E. Any base address higher than 0x1E
does not produce an acknowledge signal.
Data is read from the control registers of the AD9882A in a
similar manner. Reading requires two data transfer operations:
The base address must be written with the R/W bit of the
slave address byte low to set up a sequential read operation.
Reading (the R/W bit of the slave address byte high) begins
at the previously established base address. The address of
the read register autoincrements after each byte is
transferred.
To terminate a read/write sequence to the AD9882A, a stop
signal must be sent. A stop signal comprises a low-to-high
transition of SDA while SCL is high. The timing for the
read/write is shown in
Figure 18, and a typical byte transfer is
A repeated start signal occurs when the master device driving
the serial interface generates a start signal without first gener-
ating a stop signal to terminate the current communication.
This is used to change the mode of communication (read, write)
between the slave and master without releasing the serial
interface lines.
SDA
SCL
tDHO
tDSU
tSTASU
tSTAH
tBUFF
tDAL
tDAH
tSTOSU
05123-018
Figure 18. Serial Port Read/Write Timing
BIT 7
SDA
SCL
ACK
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
05123-019
Figure 19. Serial Interface, Typical Byte Transfer