SI514 Datasheet, PDF (13/36 Pages) Silicon Laboratories – ANY-FREQUENCY IC PROGRAMMABLE XO

Si514
LP2 values, in addition to updating the value of M, which requires the VCO to be recalibrated. Refer to section
"2.3. Programming a Large Frequency Change (> ±1000 ppm)" on page 14. Figure 2 provides a graphic
depiction of the difference between small and large frequency changes.
Range of small
frequency change
F_CENTER
-1000 ppm
F_CENTER
+1000 ppm
Small Frequency Change
Large Frequency Change
F_{VCO_MIN}
(2080 MHz)
F_CENTER
F'_CENTER
Programming a new center frequency requires a VCO
calibration and the output should be squelched
F_{VCO_MAX}
(2500 MHz)
Figure 2. Small vs. Large Frequency Change Illustration
2.2. Programming a Small Frequency Change (sub ±1000 ppm)
The value of the feedback multiplier, M is the only parameter that needs to be updated for output frequency
changes less than ±1000 ppm from the center frequency (recalibrating the VCO is NOT required). This enables
the output to remain continuous during the change. For example, the output frequency can be swept continuously
between 148.5 MHz and 148.352 MHz (i.e., –0.997 ppm) with no output discontinuities or glitches by changing M
in either multiple steps or in a single step. For small frequency changes, each update of M requires 100 µs to settle.
Note: It is not possible to implement a frequency change ≥ ±1000 ppm using multiple small frequency changes
without changing the center frequency and recalibrating the VCO.
Use the following procedure to make small frequency changes:
1. If the current value of M is already known, then skip to step 2; else, using the serial port, read the current M
value (Registers 5-9).
2. Calculate the new value of M as follows (all values are in decimal format):
a. Mcurrent = M_Int + M_Frac/2²⁹ (Eq 2.2)
b. Mnew = Mcurrent x F_out_new / F_out_current (Eq 2.3)
c. M_Intnew = INT[Mnew]*
(Eq 2.4)
d. M_Fracnew = (Mnew – INT[Mnew]) x 2²⁹ (Eq 2.5)
*Where INT[n] rounds n down to the nearest integer (e.g., INT[3.9] = 3)
3. Using the I²C port, write the new value of M_Frac[23:0] (Not all registers need to be updated.)
(Registers: 5, 6, 7)
4. If necessary, write new value of M_Int[2:0] and M_Frac[28:24] register. (Register 8)
5. Write M_Int[8:3]. (Register 9) Frequency changes take effect when M_Int[8:3] is written.
Example 2.1:
An Si514 generating a 148.5 MHz clock must be reconfigured “on-the-fly” to generate a 148.352 MHz clock. This
represents a change of –0.996.633 ppm which is within the ±1000 ppm window.
1. Read the current value of M:
a. Register 5 = 0xD3 (M_Frac[7:0])
b. Register 6 = 0x65 (M_Frac[15:8])
c. Register 7 = 0x7C (M_Frac[23:16])
Rev. 1.0
13