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Si570/Si571
46.043042064
Multiply the fractional portion by 2 28
.043042064 x 2 28 = 11554016.077
Convert integer portion to a 10-bit binary number
46 = 00 0010 1110b
Truncate the remaining fractional portion
= 11554016
Convert to a 28-bit binary number (pad 0s on the left)
0000 1011 0000 0100 1100 1110 0000
Concatenate the two results
00 0010 1110 0000 1011 0000 0100 1100 1110 0000b
Convert to Hex
02E0B04CE0h
Figure 4. Example of RFREQ Decimal to Hexadecimal Conversion
f XTAL = --------------------------------------- = 114.285 MHz
RFREQ new = ----------------------- = 45.11746948
3.1.2.1. Writing the New Frequency Configuration
Once the new values for RFREQ, HSDIV, and N1 are
determined, they can be written directly into the device
from the serial port using the following procedure:
1. Freeze the DCO (bit 4 of Register 137)
2. Write the new frequency configuration (RFREQ,
HSDIV, and N1) to addresses 7–12 for all Si571
devices and Si570 devices with 20 ppm and 50 ppm
temperature stability; or addresses 13–18 for Si570
devices with 7 ppm temperature stability.
3. Unfreeze the DCO and assert the NewFreq bit (bit 6
of Register 135) within the maximum Unfreeze to
NewFreq Timeout specified in Table 11,
“Programming Constraints and Timing,” on page 12.
The process of freezing and unfreezing the DCO will
cause the output clock to momentarily stop and start at
any arbitrary point during a clock cycle. This process
can take up to 10 ms. Circuitry that is sensitive to
glitches or runt pulses may have to be reset after the
new frequency configuration is written.
Example:
An Si570 generating 156.25 MHz must be re-configured
to generate a 161.1328125 MHz clock (156.25 MHz x
66/64). This frequency change is greater than
±3500 ppm.
f out = 156.25 MHz
Read the current values for RFREQ, HS_DIV, N1:
RFREQ current = 0x2BC011EB8h = 11744124600d,
11744124600d x 2 28 = 43.7502734363d
HS_DIV = 4
N1 = 8
Calculate f XTAL , f DCO_current
f DCO_current = f out ? HSDV ? N1 = 5.000000000 GHz
f DCO_current
RFREQ current
Given f out_new = 161.1328125 MHz, choose output
dividers that will keep f DCO within the range of 4.85 to
5.67 GHz. In this case, keeping the same output
dividers will still keep f DCO within its range limits:
f DCO_new = f out_new ? HSDV new ? N1 new
= 161.1328125 MHz ? 4 ? 8 = 5.156250000 GHz
Calculate the new value of RFREQ given the new DCO
frequency:
f DCO_new
f XTAL
= 0x2D1E127AD
16
Rev. 1.4
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