SI570 Datasheet, PDF (12/26 Pages) Silicon Laboratories – ANY-RATE I2C PROGRAMMABLE XO/VCXO

Si570/Si571
3. Functional Description
The Si570 XO and the Si571 VCXO are low-jitter,
programmable oscillators ideally suited for applications
requiring multiple frequencies. The Si57x can be
programmed to generate any output clock rate between
10 and 1.4 GHz with <1 ppb resolution. Output jitter
performance exceeds the strict requirements of high-
speed communication systems including OC-48/OC-
192 and 10 Gigabit Ethernet.
The Si57x employs Silicon Laboratories’ third-
generation digital signal processing based phase-
locked loop (DSPLL^®) technology providing excellent
jitter performance, digital programmability, and stability
while requiring minimal external components. At the
core of the Si57x is a digitally-controlled oscillator
(DCO) based on DSPLL technology that is driven by a
digital frequency control word and produces a low-jitter
output clock. (See "1. Detailed Block Diagrams" on
page 4.)
3.1. Frequency Programming Summary
The output frequency is determined by programming
the output dividers (HS_DIV and N1) and the fine
frequency control value (RFREQ). The value
programmed into RFREQ is a high-resolution 38-bit
value that adjusts the DCO frequency in a range from
4.85 to 5.67 GHz. The output of the DCO is divided
down by HS_DIV and N1 to produce the desired output
frequency. The 38-bit length of RFREQ provides an
output frequency resolution of better than 1 ppb.
3.2. Frequency Programming Details
Programming consists of the following basic steps:
deriving the actual crystal frequency, choosing new
output dividers (HS_DIV & N1), calculating a new
frequency multiplier (RFREQ), and writing the new
frequency set into the device (HS_DIV, N1, and
RFREQ).
3.2.1. Selecting the Correct Output Dividers
By listing all of the combinations of HS_DIV and N1,
one can choose the output divider set with the lowest
power within the allowed internal oscillator frequency
range as specified in Table 12. The sets of dividers
should be sorted to minimize f_osc for power dissipation
and to minimize N1 divider's power consumption.
Silicon Laboratories’ Si57x software automatically
provides this optimization and returns the smallest
HS_DIV x N1 combination with the highest HS_DIV
value.
3.2.2. Calculating the Reference Frequency Multi-
plier (RFREQ)
RFREQ is a binary representation of the reference
frequency multiplier and is 38 bits in length. To convert
from a decimal number to the binary number RFREQ
must be broken into two parts: the integer portion and
the fractional portion. The first 10 most-significant-bits
(MSBs) of RFREQ represent the integer portion, and
the lower 28 least-significant-bits (LSB's) represent the
fractional portion. The integer portion can be converted
directly from decimal to binary (e.g. decimal
43 = hexadecimal 02Bh--the leading nibble only
occupies two bits of RFREQ). The fractional portion
should be made into an integer by multiplying by 2²⁸
and truncating (or rounding) the result as follows:
(e.g. 0.54587216*2^28 = 146531442.18730496; then,
truncate to 146531442). The truncated value can then
be converted to binary (e.g. decimal
146531442 = hexadecimal 8BBE472h). The resulting
binary RFREQ for 43.54587216 is 02B8BBE472h
(02Bh concatenated with 8BBE472h).
3.2.3. Programming Procedure
The following steps must be followed to set a new
output frequency:
1. Read the frequency configuration (RFREQ, HS_DIV,
and N1) from the device after power-up or reset.
2. Calculate the actual nominal crystal frequency
(f_XTAL) as: (f_XTAL = f₀ x HS_DIV x N1)/RFREQ
where f₀ is the nominal output frequency.
3. Choose new output frequency (f₁).
4. Choose the output dividers (HS_DIV and N1) for the
new output frequency by ensuring the DCO
oscillation frequency (f_osc) is within the allowed
internal oscillator frequency (See Table 12) where:
f_osc = f₁ x HS_DIV x N1.
5. Calculate the new crystal frequency multiplication
ratio (RFREQ₁) as: f_osc = f_XTAL x RFREQ.
6. Freeze the DCO (bit 5 of Register 137).
7. Write the frequency configuration (RFREQ, HS_DIV,
and N1).
8. Unfreeze the DCO and assert the NewFreq bit (bit 6
of Register 135) within the maximum delay specified
in Table 12, “Programming Constraints,” on page 11.
3.2.4. Programming Procedure Example
The Si57x-EVB software can be used to generate
examples as needed.
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Rev. 0.3