AD9741/AD9743/AD9745/AD9746/AD9747 Data Sheet
Rev. A | Page 18 of 28
THEORY OF OPERATION
The AD9741/AD9743/AD9745/AD9746/AD9747 combine
many features to make them very attractive for wired and
wireless communications systems. The dual DAC architecture
facilitates easy interfacing to common quadrature modulators
when designing single sideband transmitters. In addition, the
speed and performance of the devices allow wider bandwidths
and more carriers to be synthesized than in previously available
products.
All features and options are software programmable through
the SPI port.
SERIAL PERIPHERAL INTERFACE
AD9747
SPI
PORT
SDO
SDIO
SCLK
CSB
06569-013
Figure 22. SPI Port
The SPI port is a flexible, synchronous serial communications
port allowing easy interfacing to many industry-standard
microcontrollers and microprocessors. The port is compatible
with most synchronous transfer formats including both the
Motorola SPI and Intel
® SSR protocols.
The interface allows read and write access to all registers that
configure the AD9741/AD9743/AD9745/AD9746/AD9747.
Single or multiple byte transfers are supported as well as MSB-
first or LSB-first transfer formats. Serial data input/output can
be accomplished through a single bidirectional pin (SDIO) or
through two unidirectional pins (SDIO/SDO).
The serial port configuration is controlled by Register 0x00,
Bits<7:6>. It is important to note that any change made to the
serial port configuration occurs immediately upon writing to
the last bit of this byte. Therefore, it is possible with a multibyte
transfer to write to this register and change the configuration in
the middle of a communication cycle. Care must be taken to
compensate for the new configuration within the remaining
bytes of the current communication cycle.
Use of a single-byte transfer when changing the serial port
configuration is recommended to prevent unexpected device
behavior.
GENERAL OPERATION OF THE SERIAL INTERFACE
There are two phases to any communication cycle with the
AD9741/AD9743/AD9745/AD9746/AD9747: Phase 1 and
Phase 2. Phase 1 is the instruction cycle, which writes an
instruction byte into the device. This byte provides the serial
port controller with information regarding Phase 2 of the
communication cycle: the data transfer cycle.
The Phase 1 instruction byte defines whether the upcoming
data transfer is read or write, the number of bytes in the data
transfer, and a reference register address for the first byte of the
data transfer. A logic high on the CSB pin followed by a logic
low resets the SPI port to its initial state and defines the start
of the instruction cycle. From this point, the next eight rising
SCLK edges define the eight bits of the instruction byte for the
current communication cycle.
The remaining SCLK edges are for Phase 2 of the communication
cycle, which is the data transfer between the serial port control-
ler and the system controller. Phase 2 can be a transfer of 1, 2, 3,
or 4 data bytes as determined by the instruction byte. Using
multibyte transfers is usually preferred although single-byte
data transfers are useful to reduce CPU overhead or when only
a single register access is required.
All serial port data is transferred to and from the device in syn-
chronization with the SCLK pin. Input data is always latched
on the rising edge of SCLK whereas output data is always valid
after the falling edge of SCLK. Register contents change imme-
diately upon writing to the last bit of each transfer byte.
When synchronization is lost, the device has the ability to
asynchronously terminate an I/O operation whenever the CSB
pin is taken to logic high. Any unwritten register content data is
lost if the I/O operation is aborted. Taking CSB low then resets the
serial port controller and restarts the communication cycle.
INSTRUCTION BYTE
The instruction byte contains the information shown in the
following bit map.
MSB LSB
B7 B6 B5 B4 B3 B2 B1 B0
R/W N1 N0 A4 A3 A2 A1 A0
Bit 7, R/W, determines whether a read or a write data transfer
occurs after the instruction byte write. Logic high indicates a
read operation. Logic 0 indicates a write operation.
Bits<6:5>, N1 and N0, determine the number of bytes to be
transferred during the data transfer cycle. The bits decode as
shown in Table 13.
Table 13. Byte Transfer Count
N1 N0 Description
0 0 Transfer one byte
1 0 Transfer three bytes
1 1 Transfer four bytes
Bits<4:0>, A4, A3, A2, A1, and A0, determine which register is
accessed during the data transfer of the communications cycle.
For multibyte transfers, this address is a starting or ending
address depending on the current data transfer mode. For MSB-
first format, the specified address is an ending address or the
most significant address in the current cycle. Remaining
register addresses for multiple byte data transfers are generated
