AD9854
Rev. E | Page 37 of 52
INSTRUCTION BYTE
The instruction byte contains the following information:
MSB
LSB
D7
D6 D5 D4 D3 D2 D1 D0
R/W
X X X A3
A2
A1
A0
R/W—Bit 7 determines whether a read or write data transfer
occurs following the instruction byte. Logic high indicates read
operation. Logic 0 indicates a write operation.
Bit 6, Bit 5, and Bit 4 are dummy bits (don’t care).
A3, A2, A1, A0—Bit 3, Bit 2, Bit 1, and Bit 0 determine which
register is accessed during the data transfer portion of the
communication cycle (see Table 8 for register address details).
SERIAL INTERFACE PORT PIN DESCRIPTIONS
SCLK
Serial Clock (Pin 21). The serial clock pin is used to synchronize
data to and from the AD9854 and to run the internal state
machines. The SCLK maximum frequency is 10 MHz.
CS
Chip Select (Pin 22). Active low input that allows more than
one device on the same serial communication line. The SDO
and SDIO pins go to a high impedance state when this input is
high. If this pin is driven high during a communication cycle,
the cycle is suspended until CS is reactivated low. The chip select
pin can be tied low in systems that maintain control of SCLK.
SDIO
Serial Data I/O (Pin 19). Data is always written to the AD9854
on this pin. However, this pin can be used as a bidirectional
data line. The configuration of this pin is controlled by Bit 0 of
Register Address 20 hex. The default is Logic 0, which
configures the SDIO pin as bidirectional.
SDO
Serial Data Out (Pin 18). Data is read from this pin for protocols
that use separate lines for transmitting and receiving data. In
the case where the AD9854 operates in a single bidirectional
I/O mode, this pin does not output data and is set to a high
impedance state.
IO RESET
Synchronize I/O Port (Pin 17). Synchronizes the I/O port state
machines without affecting the contents of the addressable
registers. An active high input on the IO RESET pin causes the
current communication cycle to terminate. After the IO RESET
pin returns low (Logic 0), another communication cycle can begin,
starting with the instruction byte.
NOTES ON SERIAL PORT OPERATION
The AD9854 serial port configuration bits reside in Bit 1 and
Bit 0 of Register Address 20 hex. It is important to note that the
configuration changes immediately upon a valid I/O update.
For multibyte transfers, writing to this register can occur during
the middle of a communication cycle. The user must compensate
for this new configuration for the remainder of the current com-
munication cycle.
The system must maintain synchronization with the AD9854;
otherwise, the internal control logic is not able to recognize further
instructions. For example, if the system sends the instruction to
write a 2-byte register and then pulses the SCLK pin for a 3-byte
register (24 additional SCLK rising edges), communication
synchronization is lost. In this case, the first 16 SCLK rising
edges after the instruction cycle properly write the first two data
bytes into the AD9854, but the subsequent 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 AD9854, the IO RESET pin provides a means to re-
establish synchronization without reinitializing the entire chip.
Asserting the IO RESET pin (active high) resets the AD9854 serial
port state machine, terminating the current I/O operation and
forcing the device into a state in which the next eight SCLK rising
edges are understood to be an instruction byte. The IO RESET pin
must be deasserted (low) before the next instruction byte write
can begin. Any information written to the AD9854 registers
during a valid communication cycle prior to loss of synchroni-
zation remains intact.
SCLK
SDIO
t
PRE
t
DSU
t
SCLKPWH
t
SCLKPWL
t
SCLK
t
DHLD
SECOND BIT
FIRST BIT
SYMBOL
MIN
DEFINITION
CS SETUP TIME
PERIOD OF SERIAL DATA CLOCK
SERIAL DATA SETUP TIME
SERIAL DATA CLOCK PULSE WIDTH HIGH
SERIAL DATA CLOCK PULSE WIDTH LOW
SERIAL DATA HOLD TIME
t
PRE
t
SCLK
t
DSU
t
SCLKPWH
t
SCLKPWL
t
DHLD
30ns
100ns
30ns
40ns
40ns
0ns
CS
00636
-056
Figure 56. Timing Diagram for Data Write to AD9854
INSTRUCTION
CYCLE
DATA TRANSFER
INSTRUCTION
BYTE
SDIO
DATA TRANSFER
DATA BYTE 1
DATA BYTE 2
DATA BYTE 3
SDO
CS
00
636-
057
Figure 57. Timing Diagram for Read from AD9854
