si514 Silicon Laboratories, si514 Datasheet - Page 10

• Manufacturer Part Number: si514
• Description: Any-frequency I 2c Programmable Xo 100 Khz To 250 Mhz
• Manufacturer: Silicon Laboratories

Si 514
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
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
2. Calculate the new value of M as follows (all values are in decimal format):
*Where INT[n] rounds n down to the nearest integer (e.g., INT[3.9] = 3)
3. Using the I
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:
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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 11. Figure 2 provides a graphic
depiction of the difference between small and large frequency changes.
value (Registers 5-9).
(Registers: 5, 6, 7)
without changing the center frequency and recalibrating the VCO.
a. Mcurrent = M_Int + M_Frac/2
b. Mnew = Mcurrent x F
c. M_Intnew = INT[Mnew]*
d. M_Fracnew = (Mnew – INT[Mnew]) x 2
a. Register 5 = 0xD3 (M_Frac[7:0])
b. Register 6 = 0x65 (M_Frac[15:8])
c. Register 7 = 0x7C (M_Frac[23:16])
d. Register 8 = 0x49 (M_Int[2:0],M_Frac[28:24])
2
C port, write the new value of M_Frac[23:0] (Not all registers need to be updated.)
(2080 MHz)
F
VCO_MIN
Figure 2. Small vs Large Frequency Change Illustration
out
_new / F
-1000 ppm
F
Programming a new center frequency requires a VCO
frequency change
CENTER
Range of small
29
calibration and the output should be squelched
(Eq 2.4)
(Eq 2.2)
F
out
CENTER
_current (Eq 2.3)
+1000 ppm
Preliminary Rev. 0.9
F
CENTER
29
(Eq 2.5)
F'
Small Frequency Change
Large Frequency Change
CENTER
(2500 MHz)
F
VCO_MAX