Datasheet

AD9549
Rev. D | Page 62 of 76
Register 0x01AC to Register 0x01AD—Phase
Table 66.
Bits Bit Name Description
[7:0] DDS phase word Allows user to vary the phase of the DDS output. See the Direct Digital Synthesizer section. Register
0x01AC is the least significant byte of the phase offset word (POW). Note that a momentary phase
discontinuity may occur as the phase passes through 45° intervals. Active only when the loop is not closed.
Register 0x01AD—Phase (Continued)
Table 67.
Bits Bit Name Description
[15:8] DDS phase word Allows user to vary the phase of the DDS output. See the Direct Digital Synthesizer section. Register
0x01AC is the least significant byte of the phase offset word (POW). Note that a momentary phase
discontinuity may occur as the phase passes through 45° intervals. Active only when the loop is not closed.
REFERENCE SELECTOR/HOLDOVER (REGISTER 0x01C0 TO REGISTER 0x01C3)
Register 0x01C0—Automatic Control
Table 68.
Bits Bit Name Description
4 Holdover mode This bit determines which frequency tuning word (FTW) is used in holdover mode.
0 = use last FTW at time of holdover.
1 = use averaged FTW at time of holdover, which is the recommended setting. The number of averages
used is set in Register 0x01C2.
3 Reserved Reserved.
2 Automatic selector Setting this bit permits state machine to switch the active reference clock input.
1 Automatic recover Setting this bit permits state machine to leave holdover mode.
0 Automatic holdover Setting this bit permits state machine to enter holdover (free-run) mode.
Register 0x01C1—Override
Table 69.
Bits Bit Name Description
4 Enable line card mode Enables line card mode of reference switch MUX, which eliminates the possibility of a runt pulse during
switchover. See the Use of Line Card Mode to Eliminate Runt Pulses section.
3 Enable ref input
override
Setting this bit disables automatic reference switchover, and allows user to switch references manually
via Bit 2 of this register. Setting this bit overrides the REFSELECT pin.
2 REF_AB This bit selects the input when Bit 3 of this register is set.
0 = REFA.
1 Enable holdover
override
Setting this bit disables automatic holdover and allows user to enter/exit holdover manually via Bit 0
(see the description for Bit 0). Setting this bit overrides the HOLDOVER pin.
0 Holdover on/off This bit controls the status of holdover when Bit 1 of this register is set.
Register 0x01C2—Averaging Window
Table 70.
Bits Bit Name Description
[3:0] FTW windowed
average size
This register sets the number of FTWs (frequency tuning words) that are used for calculating the average
FTW. Bit 4 in Register 0x01C0 enables this feature. An average size of at least 32,000 is recommended for
most applications. The number of averages equals 2
(FTW Windowed Average Size [3:0])
. These samples are taken at
the rate of (fs/2
PIO
).