
AD9920A
Rev. B | Page 93 of 112
SERIAL INTERFACE
SERIAL INTERFACE TIMING
The internal registers of the AD9920A are accessed through a
3-wire serial interface. Each register consists of a 12-bit address
and a 28-bit data-word. Both the 12-bit address and 28-bit data-
word are written starting with the LSB. To write to each register,
a 40-bit operation is required, as shown in
Figure 113. Although
many registers are fewer than 28 bits wide, all 28 bits must be
written for each register. For example, if the register is only 20 bits
wide, the upper eight bits are don’t care bits and must be filled
with 0s during the serial write operation. If fewer than 28 data
bits are written, the register is not updated with new data.
Figure 114 shows a more efficient way to write to the registers,
using the AD9920A address auto-increment capability. Using
this method, the lowest desired address is written first, followed
by multiple 28-bit data-words. Each new 28-bit data-word is
automatically written to the next highest register address. By
eliminating the need to write each 12-bit address, faster register
loading is achieved. Continuous write operations can be used
starting with any register location.
A4
A5
A2
A3
SDATA
A0
A1
A6
A8
A9
A10
A11
D0
D1
D2
D3
D25
D26
D27
SL
A7
tLS
tDS
12-BIT ADDRESS
28-BIT DATA
5
40
6
7
8
9
10
11
12
13
14
15
16
38
39
tLH
tDH
SCK
12
34
NOTES
1. SDATA BITS ARE LATCHED ON SCK RISING EDGES. SCK CAN IDLE HIGH OR LOW BETWEEN WRITE OPERATIONS.
2. ALL 40 BITS MUST BE WRITTEN: 12 BITS FOR ADDRESS AND 28 BITS FOR DATA.
3. IF THE REGISTER LENGTH IS <28 BITS, 0s MUST BE USED TO COMPLETE THE 28-BIT DATA LENGTH.
4. NEW DATA VALUES ARE UPDATED IN THE SPECIFIED REGISTER LOCATION AT DIFFERENT TIMES, DEPENDING ON THE
PARTICULAR REGISTER WRITTEN TO. SEE THE UPDATING NEW REGISTER VALUES SECTION FOR MORE INFORMATION.
0
68
78
-11
3
Figure 113. Serial Write Operation
SDATA
A0
A1
A2
A10
A11
D0
D1
D26
D27
SCK
SL
A3
NOTES
1. MULTIPLE SEQUENTIAL REGISTERS CAN BE LOADED CONTINUOUSLY.
2. THE FIRST (LOWEST) REGISTER ADDRESS IS WRITTEN, FOLLOWED BY MULTIPLE 28-BIT DATA-WORDS.
3. THE ADDRESS AUTOMATICALLY INCREMENTS WITH EACH 28-BIT DATA-WORD (ALL 28 BITS MUST BE WRITTEN).
4. SL IS HELD LOW UNTIL THE LAST DESIRED REGISTER HAS BEEN LOADED.
D0
D1
D26
D27
D0
DATA FOR STARTING
REGISTER ADDRESS
DATA FOR NEXT
REGISTER ADDRESS
D2
D1
1
40
23
4
11
12
13
14
39
42
41
68
67
70
69
71
0
68
78
-11
4
Figure 114. Continuous Serial Write Operation