SI514 Datasheet, PDF (18/36 Pages) Silicon Laboratories – ANY-FREQUENCY IC PROGRAMMABLE XO

Si514
3. All-Digital PLL Applications
The Si514 uses a high resolution divider M that enables fine frequency adjustments with resolution better than
0.026 parts per billion. Fine frequency adjustments are useful when making frequency corrections that compensate
for changing ambient conditions, long term aging or when locking the Si514 to an input clock reference. Figure 3
shows a typical implementation using a system IC such as an FPGA to control the output of the Si514 in a phase-
locked application. Refer to “AN575: An Introduction to FPGA-Based ADPLLs” for more information.
÷ ^Fin
SCL
SDA
PD
Loop Command
Filter Conversion
I²C
Master
I²C Control
Any Frequency
DSPLL
Si514
CLK_OUT
FB
÷
FPGA
Figure 3. All-Digital PLL Application Using Si514 with Dual CMOS Output
Since small frequency changes must be within ±1000 ppm of the center frequency, HS_DIV and LS_DIV remain
constant. The below expression can be used to calculate a new M₂ divider value based on a desired output
frequency shift, where ∆F_OUT is in ppm.
M₂ = M₁1 – F_OUT  10^–6
Some systems, particularly those that use feedback control, can simplify the computation by implementing an
approximate frequency change based on toggling a bit position or adding/subtracting a bit to the existing M_Frac
value. Since M ranges approximately ±10% between 65.04065041 and 78.17385866, the effect of changing
M_Frac by a single bit depends only slightly on the absolute value of M.
For M=71 near the midpoint of the range, toggling M_Frac[0] changes the output frequency by 0.026 ppb. Each
higher order bit doubles the influence such that toggling M_Frac[1] is 0.052 ppb, M_Frac[2] is 0.1 ppb, etc. Figure 4
shows this trend across multiple registers generalized to M_Frac[N]. Coarse changes greater than ±1.7 ppm are
possible but most applications require finer transitions. Toggling each bit involves incrementing or decrementing
the bit position. Writing M_Int[8:3] in register 9 completes the operation.
1.7ppm
M = 71.000000000000
6.7ppb
0.026ppb
M_Int[8:0] = 000100111 M_Frac[28:0] = 00000000000000000000000000000
M_Frac[23:16] = 00000000
M_Frac[15:8] = 00000000
M_Frac[7:0] = 00000000
Figure 4. Output Frequency Change When Toggling M_Frac[N], M=71
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Rev. 1.0