SNOSAZ8J – SEPTEMBER 2008 – REVISED SEPTEMBER 2011
Table 20. EN_PLL2_XTAL: External Crystal Option
EN_PLL2_XTAL
Oscillator Amplifier State
0
OFF
1
ON
EN_Fout: Fout Power Down Bit
The EN_Fout bit allows the Fout port to be enabled or disabled. By default EN_Fout = 0.
CLK Global Enable: Clock Global enable bit
In addition to the external GOE pin, an internal Register 13 bit (b18) can be used to globally enable/disable the
clock outputs via the uWire programming interface. The default value is 1. When CLK Global Enable = 1, the
active output clocks are enabled. The active output clocks are disabled if this bit is 0.
POWERDOWN Bit -- Device Power Down
This bit can power down the entire device. Enabling this bit powers down the entire device and all functional
blocks, regardless of the state of any of the other bits or pins.
Table 21. Power Down Bit Values
POWERDOWN Bit
Mode
0
Normal Operation
1
Entire device powered down
EN_PLL2 REF2X: PLL2 Frequency Doubler control bit
When FOSCin is below 50 MHz, the PLL2 frequency doubler can be enabled by setting EN_PLL2_REF2X = 1. The
default value is 0. When EN_PLL2_REF2X = 1, the signal at the OSCin port bypasses the PLL2_R counter and
is passed through a frequency doubler circuit. The output of this circuit is then input to the PLL2 phase
comparator block. This feature allows the phase comparison frequency to be increased for lower frequency
OSCin sources (< 50 MHz), and can be used with either VXCOs or crystals. For instance, when using a pullable
crystal of 12.288 MHz to drive the OSCin port, the PLL2 phase comparison frequency is 24.576 MHz when
EN_PLL2_REF2X = 1. A higher PLL phase comparison frequency reduces PLL2 in-band phase noise and RMS
jitter. The PLL in-band phase noise can be reduced by approximately 2 to 3 dB. The on-chip loop filter typically is
enabled to reduce PLL2 reference spurs when EN_PLL2_REF2X is enabled. Suggested values in this case are:
R3 = 600
Ω, C3 = 50 pF, R4 = 10 kΩ, C4 = 60 pF.
PLL2 Internal Loop Filter Component Values
Internal loop filter components are available for PLL2, enabling the user to implement either 3rd or 4th order loop
filters without requiring external components. The user may select from a fixed set of values for both the resistors
and capacitors. Internal loop filter resistance values for R3 and R4 can be set individually according to
Table 22Table 22. PLL2 Internal Loop Filter Resistor Values, PLL2_R3_LF
PLL2_R3_LF [2:0]
RESISTANCE
b2
b1
b0
0
< 600
Ω
0
1
10 k
Ω
0
1
0
20 k
Ω
0
1
30 k
Ω
1
0
40 k
Ω
1
0
1
Invalid
1
0
Invalid
1
Invalid
38
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