
AD9520-1
Data Sheet
Rev. A | Page 70 of 80
Reg.
Addr.
(Hex)
Bits
Name
Description
0x01D 7
Enable
STATUS_EEPRO
M at STATUS pin
Enables the STATUS_EEPROM signal at the STATUS pin.
0: the STATUS pin is controlled by the Register 0x017[7:2] selection.
1: selects the STATUS_EEPROM signal at the STATUS pin. This bit overrides the Register 0x017[7:2] selection (default).
6
Enable XTAL OSC Enables the maintaining amplifier needed by a crystal oscillator at the PLL reference input.
0: crystal oscillator maintaining amplifier disabled (default).
1: crystal oscillator maintaining amplifier enabled.
5
Enable clock
doubler
Enables PLL reference input clock doubler.
0: doubler disabled (default).
1: doubler enabled.
4
Disable PLL
status register
Disables the PLL status register readback.
0: PLL status register enabled (default).
1: PLL status register disabled. If this bit is set, Register 0x01F is not automatically updated.
3
Enable LD pin
comparator
Enables the LD pin voltage comparator. Used with the LD pin current source lock detect mode. When th
e AD9520 is in internal
(automatic) holdover mode, this bit enables the use of the voltage on the LD pin to determine if the PLL was previously in a locked
state (
see Figure 47). Otherwise, this can be used with the REFMON and STATUS pins to monitor the voltage on the LD pin.
0: disables LD pin comparator and ignores the LD pin voltage; the automatic/internal holdover controller treats this pin as true
(high, default).
1: enables the LD pin comparator (uses the LD pin voltage to determine if the PLL was previously locked).
2
Unused
Unused.
1
Enable external
holdover
Enables the external hold control through the SYNC pin. (This bit disables the internal holdover mode.)
0: automatic holdover mode; holdover controlled by the automatic holdover circuit (default).
1: external holdover mode; holdover controlled by the SYNC pin.
0
Enable holdover Enables the internally controlled holdover function.
0: holdover disabled (default).
1: holdover enabled.
0x01E
[7:5]
Unused
Unused.
[4:3]
External zero
delay feedback
channel divider
select
Bit
4
Bit
3
Selection of Channel Divider for Use in the External Zero-Delay Path
0
Selects Channel Divider 0 (default).
0
1
Selects Channel Divider 1.
1
0
Selects Channel Divider 2.
1
Selects Channel Divider 3
2
Enable external
zero delay
Selects which zero delay mode to use.
0: enables internal zero delay mode if Register 0x01E[1] = 1 (default).
1: enables external zero delay mode if Register 0x01E[1] = 1.
1
Enable zero
delay
Enables zero delay function.
0: disables zero delay function (default).
1: enables zero delay function.
0
Unused
Unused.
0x01F
7
Unused
Unused.
6
VCO calibration
finished
(read only)
Readback register. Indicates the status of the VCO calibration.
0: VCO calibration not finished.
1: VCO calibration finished.
5
Holdover active
(read only)
Readback register. Indicates if the part is in the holdover state (
see Figure 47). Note that this is not the same as holdover enabled.
0: not in holdover state.
1: holdover state active.
4
REF2 selected
(read only)
Readback register. Indicates which PLL reference is selected as the input to the PLL.
0: REF1 selected (or differential reference if in differential mode).
1: REF2 selected.
3
VCO frequency >
threshold
(read only)
Readback register. Indicates if the VCO frequency is greater than the threshold (se
e Table 17, REF1, REF2, and VCO frequency
status monitor parameter).
0: VCO frequency is less than the threshold.
1: VCO frequency is greater than the threshold.
2
REF2 frequency >
threshold
(read only)
Readback register. Indicates if the frequency of the signal at REF2 is greater than the threshold frequency set by Register 0x01A[6].
0: REF2 frequency is less than the threshold frequency.
1: REF2 frequency is greater than the threshold frequency.
1
REF1 frequency >
threshold
(read only)
Readback register. Indicates if the frequency of the signal at REF1 is greater than the threshold frequency set by Register 0x01A[6].
0: REF1 frequency is less than the threshold frequency.
1: REF1 frequency is greater than the threshold frequency.
0
Digital lock
detect
(read only)
Readback register. Digital lock detect.
0: PLL is not locked.
1: PLL is locked.