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AD9991

–37–

SERIAL INTERFACE TIMING

All of the internal registers of the AD9991 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 40a. Although

many registers are fewer than 24 bits wide, all 24 bits must be

written for each register. For example, if the register is only 10

bits wide, the upper 14 bits are don’t cares and may be ﬁlled with

0s during the serial write operation. If fewer than 24 bits are writ-

ten, the register will not be updated with new data.

SDATA

D21

D22

D23

SCK

NOTES

1. SDATA BITS ARE LATCHED ON SCK RISING EDGES. SCK MAY IDLE HIGH OR LOW IN BETWEEN WRITE OPERATIONS.

2. ALL 32 BITS MUST BE WRITTEN: 8 BITS FOR ADDRESS AND 24 BITS FOR DATA.

3. IF THE REGISTER LENGTH IS <24 BITS, “DON’T CARE” BITS MUST BE USED TO COMPLETE THE 24-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 REGISTER UPDATES SECTION FOR MORE INFORMATION.

tDH

tLS

tLH

tDS

...

8-BIT ADDRESS

24-BIT DATA

234

567

Figure 40a. Serial Write Operation

SDATA

D22

D23

SCK

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.

D22

D23

...

DATA FOR STARTING

DATA FOR NEXT

...

234

567

Figure 40b. Continuous Serial Write Operation

Figure 40b shows a more efﬁcient way to write to the registers,

using the AD9991’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 will

automatically be written to the next highest register address. By

eliminating the need to write each 8-bit address, faster register

loading is achieved. Continuous write operations may be used

starting with any register location, and may be used to write to as

few as two registers, or as many as the entire register space.

REV. 0

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