AD9949
Rev. B | Page 12 of 36
SERIAL INTERFACE TIMING
The AD9949’s internal registers are accessed through a 3-wire
serial interface. Each register consists of an 8-bit address and a
24-bit data-word. Both the 8-bit address and 24-bit data-word
are written starting with the LSB. To write to each register, a
32-bit operation is required, as shown in Figure 14. Although
many registers are less than 24 bits wide, all 24 bits must be
written for each register. If the register is only 16 bits wide, then
the upper eight bits may be filled with zeros during the serial
write operation. If fewer than 24 bits are written, the register
will not be updated with new data.
Figure 15 shows a more efficient way to write to the registers by
using the AD9949’s address auto-increment capability. Using
this method, the lowest desired address is written first, followed
by multiple 24-bit data-words. Each new 24-bit data-word is
written automatically to the next highest register address. By
eliminating the need to write each 8-bit address, faster register
loading is achieved. Address auto-increment may be used start-
ing with any register location and may be used to write to as few
as two registers or as many as the entire register space.
SDAT
A0 A1 A2 A4 A5 A6 A7
D0
D1 D2 D3 D21 D22 D23
SCK
SL
A3
NOTES
1. INDIVIDUAL SDATA BITS ARE LATCHED ON SCK RISING EDGES.
2. ALL 32 BITS MUST BE WRITTEN: 8 BITS FOR ADDRESS AND 24 BITS FOR DATA.
3. IF THE REGISTER LENGTH IS <24 BITS, THEN DON’T CARE BITS MUST BE USED TO COMPLETE THE 24-BIT DATA LENGTH.
4. NEW DATA IS UPDATED AT EITHER THE SL RISING EDGE OR AT THE HD FALLING EDGE AFTER THE NEXT VD FALLING EDGE.
5. VD/HD UPDATE POSITION MAY BE DELAYED TO ANY HD FALLING EDGE IN THE FIELD USING THE UPDATE REGISTER.
VD
SL UPDATED VD/HD UPDATED
HD
...
...
...
...
...
8-BIT ADDRESS
24-BIT DATA
1 322 3 4 5 6 7 8 9 10 11 12 30 31
03751-015
t
LS
t
DS
t
DH
t
LH
Figure 14. Serial Write Operation
SDATA
A0 A1 A2 A4 A5 A6 A7 D0 D1 D22 D23
SCK
SL
A3
NOTES
1. MULTIPLE SEQUENTIAL REGISTERS MAY BE LOADED CONTINUOUSLY.
2. THE FIRST (LOWEST ADDRESS) REGISTER ADDRESS IS WRITTEN, FOLLOWED BY MULTIPLE 24-BIT DATA-WORDS.
3. THE ADDRESS WILL AUTOMATICALLY INCREMENT WITH EACH 24-BIT DATA-WORD (ALL 24 BITS MUST BE WRITTEN).
4. SL IS HELD LOW UNTIL THE LAST DESIRED REGISTER HAS BEEN LOADED.
5. NEW DATA IS UPDATED AT EITHER THE SL RISING EDGE OR AT THE HD FALLING EDGE AFTER THE NEXT VD FALLING EDGE.
D0 D1 D22 D23
D0
...
...
...
DATA FOR STARTING
REGISTER ADDRESS
DATA FOR NEXT
REGISTER ADDRESS
D2D1
...
...
...
...
...
...
1 322345678910 31
3433 5655
58
57
59
03751-016
Figure 15. Continuous Serial Write Operation
