參數(shù)資料
型號(hào)： 514SCCXXXXXXAAG
廠商： SILICON LABORATORIES
元件分類： XO, clock
英文描述： 125 MHz, OTHER CLOCK GENERATOR, PDSO6
封裝： 5 X 7 MM, ROHS COMPILANT PACKAGE-6
文件頁數(shù)： 2/32頁
文件大?。?/td> 267K
代理商： 514SCCXXXXXXAAG
Si514
10
Preliminary Rev. 0.9
LP2 values, in addition to updating the value of M, which requires the VCO to be recalibrated. Refer to section
depiction of the difference between small and large frequency changes.
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 s 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 Fout_new / Fout_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])
d. Register 8 = 0x49 (M_Int[2:0],M_Frac[28:24])
FVCO_MIN
(2080 MHz)
FVCO_MAX
(2500 MHz)
Range of small
frequency change
Programming a new center frequency requires a VCO
calibration and the output should be squelched
FCENTER
F'CENTER
Small Frequency Change
Large Frequency Change
FCENTER
+1000 ppm
FCENTER
-1000 ppm
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