AD9857 Analog Devices, AD9857 Datasheet - Page 20

• Manufacturer Part Number: AD9857
• Description: CMOS 200 MSPS 14-Bit Quadrature Digital Upconverter
• Manufacturer: Analog Devices

AD9857
Instruction Byte
The instruction byte contains the following information as shown
in Table II.
MSB
R/W
R/W—Bit 7 of the instruction byte determines whether a read or
write data transfer will occur after the instruction byte write.
Logic high indicates read operation. Logic zero indicates a write
operation.
N1, N0—Bits 6 and 5 of the instruction byte determine the
number of bytes to be transferred during the data transfer cycle
of the communications cycle. The bit decodes are shown in
Table III.
A4, A3, A2, A1, A0—Bits 4, 3, 2, 1, 0 of the instruction byte
determine which register is accessed during the data transfer
portion of the communications cycle. For multibyte transfers,
this address is the starting byte address. The remaining register
addresses are generated by the AD9857.
SERIAL INTERFACE PORT PIN DESCRIPTION
SCLK—Serial Clock. The serial clock pin is used to synchro-
nize data to and from the AD9857 and to run the internal state
machines. SCLK maximum frequency is 10 MHz.
CS—Chip Select. Active low input that allows more than one
device on the same serial communications lines. The SDO and
SDIO pins will go to a high-impedance state when this input is
high. If driven high during any communications cycle, that cycle is
suspended until CS is reactivated low. Chip Select can be tied
low in systems that maintain control of SCLK.
SDIO—Serial Data I/O. Data is always written into the AD9857
on this pin. However, this pin can be used as a bidirectional
data line. The configuration of this pin is controlled by Bit 7 of
register address 00h. The default is logic zero, which configures
the SDIO pin as bidirectional.
SDO—Serial Data Out. Data is read from this pin for protocols
that use separate lines for transmitting and receiving data. In the
case where the AD9857 operates in a single bidirectional I/O
mode, this pin does not output data and is set to a high-impedance
state.
SYNCIO—Synchronizes the I/O port state machines without
affecting the addressable registers contents. An active high input
on the SYNC I/O pin causes the current communication cycle to
abort. After SYNC I/O returns low (Logic 0) another communi-
cation cycle may begin, starting with the instruction byte write.
D6
N1
N1
0
0
1
1
Table II. Instruction Byte Information
D5
N0
Table III. N1, N2 Decode Bits
D4
A4
N0
0
1
0
1
D3
A3
Description
Transfer 1 Byte
Transfer 2 Bytes
Transfer 3 Bytes
Transfer 4 Bytes
D2
A2
D1
A1
LSB
A0
MSB/LSB Transfers
The AD9857 serial port can support both most significant bit
(MSB) first or least significant bit (LSB) first data formats. This
functionality is controlled by the Control Register 00h<6> bit.
The default value of Control Register 00h<6> is low (MSB first).
When Control Register 00h<6> is set high, the AD9857 serial
port is in LSB first format. The instruction byte must be written
in the format indicated by Control Register 00h<6>. That is, if
the AD9857 is in LSB first mode, the instruction byte must be
written from least significant bit to most significant bit.
Multibyte data transfers in MSB format can be completed by
writing an instruction byte that includes the register address of
the most significant byte. In MSB first mode, the serial port in-
ternal byte address generator decrements for each byte required
of the multibyte communication cycle. Multibyte data transfers
in LSB first format can be completed by writing an instruction
byte that includes the register address of the least significant
byte. In LSB first mode, the serial port internal byte address
generator increments for each byte required of the multibyte
communication cycle.
Notes on Serial Port Operation
The AD9857 serial port configuration bits reside in Bits 6 and 7
of register address 0h. It is important to note that the configuration
changes immediately upon writing to this register. For multibyte
transfers, writing to this register may occur during the middle of
a communication cycle. Care must be taken to compensate for this
new configuration for the remainder of the current communica-
tion cycle.
The AD9857 serial port controller address will roll from 19h to
0h for multibyte I/O operations if the MSB first mode is active.
The serial port controller address will roll from 0h to 19h for
multibyte I/O operations if the LSB first mode is active.
The system must maintain synchronization with the AD9857 or
the internal control logic will not be able to recognize further
instructions. For example, if the system sends an instruction
byte for a 2-byte write, then pulses the SCLK pin for a 3-byte
write (24 additional SCLK rising edges), communication syn-
chronization is lost. In this case, the first 16 SCLK rising edges
after the instruction cycle will properly write the first two data
bytes into the AD9857, but the next eight rising SCLK edges are
interpreted as the next instruction byte, not the final byte of the
previous communication cycle.
In the case where synchronization is lost between the system and
the AD9857, the SYNC I/O pin provides a means to re-establish
synchronization without reinitializing the entire chip. The SYNC
I/O pin enables the user to reset the AD9857 state machine to
accept the next eight SCLK rising edges to be coincident with
the instruction phase of a new communication cycle. By apply-
ing and removing a “high” signal to the SYNC I/O pin, the
AD9857 is set to once again begin performing the communica-
tion cycle in synchronization with the system. Any information
that had been written to the AD9857 registers during a valid
communication cycle prior to loss of synchronization will
remain intact.