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參數(shù)資料
| 型號(hào)： | 570BAC000304DG |
| 廠商： | SILICON LABORATORIES |
| 元件分類： | XO, clock |
| 英文描述： | CRYSTAL OSCILLATOR, CLOCK, 10 MHz - 280 MHz, LVDS OUTPUT |
| 封裝： | ROHS COMPLIANT PACKAGE-8 |
| 文件頁(yè)數(shù)： | 6/30頁(yè) |
| 文件大小： | 361K |
| 代理商： | 570BAC000304DG |
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Si570/Si571
14
Rev. 1.1
A typical frequency configuration for this example:
RFREQcurrent = 0x2EBB04CE0
Fout_current =148.35MHz
Fout_new =148.50MHz
Calculate RFREQnew to change the output frequency
from 148.35 MHz to 148.5 MHz:
Note that performing calculations with RFREQ requires
a minimum of 38-bit arithmetic precision.
3.1.2. Reconfiguring the Output Clock for Large
Changes in Output Frequency
For output frequency changes outside of ±3500 ppm
from the center frequency, it is likely that both the DCO
frequency and the output dividers need to be
reprogrammed. Note that changing the DCO frequency
outside of the ±3500 ppm window will cause the output
to momentarily stop and restart at any arbitrary point in
a clock cycle. Devices sensitive to glitches or runt
pulses may have to be reset once reconfiguration is
complete.
The process for reconfiguring the output frequency
outside of a ±3500 ppm window is shown below:
1. Using the serial port, read the current values for
RFREQ, HSDIV, and N1.
2. Calculate fXTAL for the device. Note that because of
slight variations of the internal crystal frequency from
one device to another, each device may have a
different RFREQ value or possibly even different
HSDIV or N1 values to maintain the same output
frequency. It is necessary to calculate fXTAL for each
device.
Once fXTAL has been determined, new values for
RFREQ, HSDIV, and N1 are calculated to generate a
new output frequency (fout_new). New values can be
calculated manually or with the Si57x-EVB software,
which provides a user-friendly application to help find
the optimum values.
The first step in manually calculating the frequency
configuration is to determine new frequency divider
values (HSDIV, N1). Given the desired output frequency
(fout_new), find the frequency divider values that will
keep the DCO oscillation frequency in the range of 4.85
to 5.67 GHz.
Valid values of HSDIV are 4, 5, 6, 7, 9 or 11. N1 can be
selected as 1 or any even number up to 128 (i.e. 1, 2, 4,
6, 8, 10 … 128). To help minimize the device's power
consumption, the divider values should be selected to
keep the DCO's oscillation frequency as low as
possible. The lowest value of N1 with the highest value
of HS_DIV also results in the best power savings.
Once HS_DIV and N1 have been determined, the next
step is to calculate the reference frequency multiplier
(RFREQ).
RFREQ is programmable as a 38-bit binary fractional
frequency multiplier with the first 10 most significant bits
(MSBs) representing the integer portion of the multiplier,
and the 28 least significant bits (LSBs) representing the
fractional portion.
Before entering a fractional number into the RFREQ
register, it must be converted to a 38-bit integer using a
bitwise left shift operation by 28 bits, which effectively
multiplies RFREQ by 228.
Example:
RFREQ = 46.043042064d
Multiply RFREQ by 228 = 12359584992.1
Discard the fractional portion = 12359584992
Convert to hexadecimal = 02E0B04CE0h
In the example above, the multiplication operation
requires 38-bit precision. If 38-bit arithmetic precision is
not available, then the fractional portion can be
separated from the integer and shifted to the left by 28-
bits. The result is concatenated with the integer portion
to form a full 38-bit word. An example of this operation is
shown in Figure 4.
RFREQnew
0x2EBB04CE0
148.50 MHz
148.35 MHz
--------------------------------
×
0x2EC71D666
=
fXTAL
Fout HSDIV
×
N1
×
RFREQ
---------------------------------------------------
=
fDCO_new
fout_new HSDIVnew
×
N1new
×
=
RFREQnew
fDCO_new
fXTAL
-----------------------
=
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