Datasheet

AD9785/AD9787/AD9788
Rev. A | Page 22 of 64
There are two phases to a communication cycle with the
AD9785/AD9787/AD9788. Phase 1 is the instruction cycle,
which is the writing of an instruction byte into the AD9785/
AD9787/AD9788, coincident with the first eight SCLK rising
edges. The instruction byte provides the AD9785/AD9787/
AD9788 serial port controller with information regarding the
data transfer cycle, which is Phase 2 of the communication
cycle. The instruction byte defines whether the upcoming data
transfer is read or write and the serial address of the register
being accessed.
The first eight SCLK rising edges of each communication cycle
are used to write the instruction byte into the AD9785/AD9787/
AD9788. The remaining SCLK edges are for Phase 2 of the
communication cycle. Phase 2 is the actual data transfer
between the AD9785/AD9787/AD9788 and the system
controller. The number of bytes transferred during Phase 2 of
the communication cycle is a function of the register being
accessed.
For example, when accessing the frequency tuning word (FTW)
register, which is four bytes wide, Phase 2 requires that four
bytes be transferred. If accessing the amplitude scale factor (ASF)
register, which is three bytes wide, Phase 2 requires that three
bytes be transferred. After transferring all data bytes per the
instruction byte, the communication cycle is completed.
At the completion of any communication cycle, the AD9785/
AD9787/AD9788 serial port controller expects the next eight
rising SCLK edges to be the instruction byte of the next
communication cycle.
All data input is registered on the rising edge of SCLK. All data
is driven out of the AD9785/AD9787/AD9788 on the falling
edge of SCLK.
Figure 43 through Figure 46 are useful in understanding the
general operation of the AD9785/AD9787/AD9788 serial port.
R/W N1 N0 A4 A3 A2 A1 A0 D7 D6
N
D5
N
D0
0
D1
0
D2
0
D3
0
D7 D6
N
D5
N
D0
0
D1
0
D2
0
D3
0
INSTRUCTION CYCLE DATA TRANSFER CYCLE
SPI_CSB
SCLK
SPI_SDIO
SPI_SDO
07098-006
Figure 43. Serial Register Interface Timing, MSB First
A0 A1 A2 A3 A4 N0 N1 R/W D0
0
D1
0
D2
0
D7
N
D6
N
D5
N
D4
N
D0
0
D1
0
D2
0
D7
N
D6
N
D5
N
D4
N
INSTRUCTION CYCLE DATA TRANSFER CYCLE
SPI_CSB
SCLK
SPI_SDIO
SPI_SDO
07098-007
Figure 44. Serial Register Interface Timing, LSB First
INSTRUCTION BIT 6INSTRUCTION BIT 7
SPI_CSB
SCLK
SPI_SDIO
t
DS
t
DS
t
DH
t
PWH
t
PWL
t
SCLK
07098-008
Figure 45. SPI Register Write Timing
DATA BIT n–1DATA BIT n
SPI_CSB
SCLK
SPI_SDIO
SPI_SDO
t
DV
07098-009
Figure 46. SPI Register Read Timing Instruction Byte