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AD9549
Rev. D | Page 62 of 76
Register 0x01AC to Register 0x01AD—Phase
Table 66.
Bits
Bit Name
Description
[7:0]
DDS phase word
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
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
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(FTWWindowed Average Size [3:0]). These samples are taken at
the rate of (fs/2PIO).