Datasheet

AD9974
Rev. A | Page 41 of 52
UPDATING OF REGISTER VALUES
The internal registers of the AD9974 are updated at different
times, depending on the particular register.
Table 21 summarizes the three types of register updates. The
tables in the Complete Register Listing section also contain a
column with update type to identify when each register is updated.
• SCK Updated—Some of the registers are updated immediately
as the 28th data bit (D27) is written. These registers are
used for functions that do not require gating with the next
VD boundary, such as power-up and reset functions.
• VD Updated—Many of the registers are updated at the
next VD falling edge. By updating these values at the next
VD edge, the current field is not corrupted, and the new
register values are applied to the next field. The VD update
can be further delayed past the VD falling edge by using
UPDATE Register Address 0x17. This delays the VD-updated
register updates to any HD line in the field. Note that the
field registers are not affected by the UPDATE register.
• SCP Updated—All of the H-pattern group registers are
updated at the next SCP when they are used.
Table 21. Register Update Types
Update Type Description
SCK Updated Register is immediately updated when the 28th data bit (D27) is clocked in.
VD Updated
Register is updated at the VD falling edge. VD-updated registers can be delayed further by using the UPDATE register at
Address 0x17. Field registers are not affected by the UPDATE register.
SCP Updated Register is updated at the next SCP when the register is used.